Class-7th SCIENCE

STUDY NOTES & WORKSHEET

LIGHT

LIGHT

1. INRODUCTION

Light is a form of energy. Light enables us to see object from which it comes (or from which it is reflected)

1.1 Sources of Light

An object which gives out light is called a source of light. The various sources of light around us are: Sun, Stars, Electric bulb, Tube-light, Kerosene oil lamp, Candle, Torch and Firefly. The most important source of light for us is the sun.

1.2 Natural and Man-Made Sources of Light

All the sources of light can be divided into two groups: (i) Natural sources of light: Those sources of light which occur in nature as

such are called natural sources of light. The sun is the best natural source of light. Nearly all the natural light we receive on the earth comes from the sun. The stars, meteors and firefly are the other natural sources of light.

(ii) Man-made sources of light: Those sources of light which have been made by man are called man-made sources of light (or artificial sources of light). Electric bulb, tube-light, kerosene oil lamp, candle and torch, are all man-made sources of light.

1.3 Luminous and Non-Luminous Objects

An object which gives out its own light is called a luminous object. The sun, other stars, burning candle, lighted electric bulb, lighted torch, glowing tube-light, television screen and firefly are all luminous objects. This is because they give out their own light. The luminous objects are, in fact, the sources of light. A luminous object can be seen because the light given out by it enters our eyes.

An object which does not give out its own light is called a non-luminous object. The non-luminous objects can be seen only when light coming from a luminous object falls on them. For example: a chair, a painting, a shoe, bag, etc. We see these when light from a luminous object (like the Sun, a torch or an electric light) falls on these and then travels towards our eye.

1.4 Transparent Materials

Those materials which allow all the light to pass through them are called transparent materials. The materials like glass, polythene, groundnut oil, air and water are transparent materials. They allow almost all the light to pass through

WIN POINT

Page 1

them. We can see clearly through the transparent materials. If we are able to see through an object clearly, it is allowing all the light to pass through it and hence it is a transparent object. The transparent objects allow all the light to pass through them because they do not absorb any light.

1.5 Translucent Materials

Those materials which allow only some of the light to pass through them are called translucent materials. The materials like ground-glass, butter paper, oiled paper, tissue paper, sheet of white cotton, muddy water and clouds are translucent materials. We cannot see clearly through a translucent material. The translucent objects absorb a good part of the light falling on them and allow only some of the light to pass through them. Looking through opaque, transparent or translucent material

1.6 Opaque Materials

Those materials which do not allow any light to pass through them are called opaque materials. Light cannot pass through opaque materials at all. The materials like Cardboard, book, wooden door, metal sheet, brick wall, and stone, etc., are all opaque materials. We cannot see anything through an opaque object. The opaque objects do not allow light to pass through them because they absorb or reflect all the light falling on them.

2. LIGHT TRAVELS IN A STRAIGHT LINE

The property of light travelling in straight lines is called rectilinear propagation of light. The formation of shadow by a source of light suggests that the light travels in straight lines. The fact that we cannot see around a corner also shows that light travels in a straight line.

Looking at a candle through a straight and a bent pipe

WIN POINT

Page 2

3. THE PINHOLE CAMERA

The pinhole camera is a simple device to see the images of various objects. A pinhole camera can also be used to take the photograph of an object if a photographic film is placed on the screen. The pinhole camera consists of a closed box having a small pinhole in the front and a translucent screen at the back. The translucent screen at the back side of the pinhole camera box is made of butter paper. The butter paper acts as a screen to receive the image of the object. The pinhole camera works on the principle that light travels in straight lines.

A sliding pinhole camera

3.1 Characteristics of the image in a pinhole camera:

(a) The image in a pinhole camera is inverted as compared to the object, because the light rays coming from the top and bottom of the object cross-over at the pinhole.

(b) The image in a pinhole camera is real (because it can be formed on a screen). (c) The image in a pinhole camera is of the same colour as the object. (d) The image in a pinhole camera can be smaller than the object, equal to the

object or bigger than the object (depending on the distance of object from the pinhole camera.)

3.2 A Natural Pinhole Camera

The pinhole camera effect can be observed in everyday life. On a sunny day, when we pass under a tree covered with a very large number of leaves, we often see bright circular patches of light on the ground. These bright circular patches of light are the pinhole images of the sun.

Pinhole images of the Sun under a tree

WIN POINT

Page 3

4. SHADOWS

When an object is placed in front of a source of light, it produces a shade (dark area) behind it. The ‘shade’ cast by an object is called its ‘shadow’. Shadows are formed when light is stopped by an object. A shadow is formed when an opaque object comes in the path of light and stops it. An object forms shadow on the opposite side to the source of light. The sunlight also forms shadows of the object which are on the ground or near the ground. For example, if we stand in the sun, our body casts a shadow on the ground. The shadow of an object can be seen only on a screen. The surfaces such as the wall of a room, a building and even ground act as screen for the shadows which we see in our everyday life.

We require three things to observe a shadow: a source of light an opaque object (to obstruct the path of light) a screen on which the shadow can be seen.

A shadow is obtained only on a screen

When a bird is on the ground, we can see its shadow which is formed by sunlight. In this case, the bird obstructs the sunlight to form shadow on the nearby ground. When the same bird is flying high up in the air, even then the bird obstructs the sunlight falling on it but its shadow is not seen on the ground because the ground is very, very far below the bird. Similarly, when as aeroplane is parked on the ground, we can see its shadow which is formed by sunlight. In this case the aeroplane obstructs the sunlight to form a shadow on the nearby ground. When the same aeroplane is flying high up in the air, even then the aeroplane obstructs the sunlight falling on it but its shadow is not seen on the ground because the ground is very, very far below the aeroplane. Shadows of animals hidden in your hand Sometimes the shadows of opaque objects are not seen clearly. For example, though our shadow and that of a ceiling fan formed by an electric bulb or a lighted candle can be seen clearly but the shadows formed by a fluorescent tube-light cannot be seen clearly.

WIN POINT

Page 4

The shadow of an object has the following characteristics:

The shadow of an object is erect. The shadow of an object is real. Irrespective of the colour of the object, the shadow is always black. The shadow can be smaller than the object, equal to the object or bigger

than the object. The two important differences between the pinhole images and the shadows are as follows:

(a) The pinhole image of an object is inverted whereas the shadow of an object is erect.

(b) The pinhole image of an object is of the same colour as the object but the shadow is always black.

5. REFLECTION OF LIGHT

The process of sending back the light rays which fall on the surface of an object, is called reflection of light. The objects having polished, shining surfaces reflect more light than objects having unpolished, dull surfaces. Silver metal is one of the best reflectors of light. A ray of light is the straight line along which light travels. A ‘bundle of light rays’ is called a ‘beam of light’.

5.1 Reflection of Light From Plane Surfaces: Plane Mirror

The incident ray, reflected ray and the normal to the reflecting surface lie in the same plane.

The ray of light which falls on the mirror surface is called the incident ray. The ray of light which is sent back by the mirror is called the reflected ray. The ‘normal’ is a line at right angle to the mirror surface at the point of

incidence. The angle of incidence is the angle made by the incident ray with the

normal at the point of incidence. The angle of reflection is the angle made by the reflected ray with the

normal at the point of incidence.

WIN POINT

Page 5

5.2 Laws of Reflection of Light

(1) First Law of Reflection. According to the first law of reflection of light: The incident ray, the reflected ray, and the normal (at the point of incidence), all lie in the same plane.

(2) Second Law of Reflection. According to the second law of reflection of light: The angle of reflection is always equal to the angle of incidence.

( i r ) A ray of light which is incident normally (or perpendicularly) on a mirror, is reflected back along the same path (because the angle of incidence as well as the angle of reflection for such a ray of light are zero).

5.3 Regular Reflection and Diffuse Reflection of Light

In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction. Regular reflection of light occurs from smooth surfaces like that of a plane mirror (or highly polished metal surfaces). A plane mirror produces regular reflection of light. Images are formed by regular reflection of light. In diffuse reflection, a parallel beam of incident light is reflected in different directions. The diffuse reflection is also known as irregular reflection or scattering. The diffuse reflection of light takes place from rough surfaces like that of paper, cardboard, chalk, table, chair, walls and unpolished metal objects. A sheet of paper produces diffuse reflection of light. No image is formed in diffuse reflection of light.

Parallel rays incident on Rays reflected from Regular reflection an irregular surface irregular surface

Objects and Images

Anything which gives out light rays (either its own or reflected by it) is called an object.

Image is an optical appearance produced when light rays coming from an object are reflected from a mirror (or refracted through a lens). An image is formed when the light rays coming from an object meet (or appear to meet) at a point, after reflection from a mirror (or refraction through a lens).

WIN POINT

Page 6

Real Images and Virtual Images

The image which can be obtained on a screen is called a real image. The image which cannot be obtained on a screen is called a virtual image.

A virtual image can be seen only by looking into a mirror (or a lens).

Formation of Image in a Plane Mirror Image of an Extended Object (or Finite Object) in a Plane Mirror 5.4 Lateral Inversion

When an object is placed in front of a plane mirror, then the right side of object appears to become the left side of image; and the left side of object appears to become the right side of image. This change of sides of an ‘object’ and its ‘mirror image’ is called lateral inversion. The phenomenon of lateral inversion is due to the reflection of light. Left hand appears on the right side in the image

Characteristics of an image formed by a plane mirror.

(a) The image formed in a plane mirror is virtual. It cannot be received on a screen. (b) The image formed in a plane mirror is erect. It is the same side up as the object. (c) The image in a plane mirror is of the same size as the object. (d) The image formed by a plane mirror is at the same distance behind the mirror as

the object is in front of the mirror. (e) The image formed in a plane mirror is laterally inverted.

(or sideways reversed)

WIN POINT

Page 7

Uses of Plane Mirrors

(a) Plane mirrors are used to see ourselves. The mirrors on our dressing table and in bathrooms are plane mirrors.

(b) Plane mirrors are fixed on the inside walls of certain shops (like jewellery shops, Saloons, etc.) to make them look bigger.

(c) Plane mirrors are fitted at blind turns of some busy roads so that drivers can see the vehicles coming from the other side and prevent accidents.

(d) Plane mirrors are used in making periscopes.

6. PERISCOPE

The periscope is an optical instrument (or device) which gives us a higher view than normal; it works on the reflection of light from two plane mirrors arranged parallel to one another. A periscope gives us a higher view than normal. For example, by using a periscope, we can see the objects on the other side of a high wall which cannot be seen by us directly. A periscope works on the reflection of light from two plane mirrors arranged parallel to one another. A periscope consists of a long tube having two plane mirrors M1 and M2 fitted at its two ends. The two plane mirrors are fitted in such a way that they are parallel to one another and their reflecting surfaces face each other. Each plane mirror, however, makes an angle of 45° with the side of the tube. There are two holes in the periscope tube: one hole is in front of the top mirror M1 and the other hole is in front of the bottom mirror M2. In a periscope, the top mirror reflects light and the bottom mirror reflects the ‘reflected light’. So, the working of a periscope demonstrates that reflected light can be reflected again.

Some of the Uses of Periscope are given below:

A periscope is used to see over the heads of a crowd. A periscope is used by soldiers sitting in a trench (or bunker) to observe the

enemy activities outside (over the ground). A periscope is used by a navy officer sitting in a submarine to see ships

over the surface of water in the sea (even though the submarine itself may be submerged under water).

WIN POINT

Page 8

7. KALEIDOSCOPE

The kaleidoscope is an optical instrument (or device) which works on principle of multiple image formation by plane mirrors when placed inclined at an angle. Note: When two plane mirrors are kept inclined at an angle ( ox ), they can form

multiple images of an object. Number of images formed = 360 1x

Mirror at the hair dresser shop Images in plane mirror at right angle to each other

The kaleidoscope contains inclined plane mirrors which produce multiple reflections of coloured glass pieces (or coloured plastic pieces) and create beautiful patterns. The kaleidoscope consists of three long and narrow strips of plane mirrors inclined at 60° to one another forming a hollow prism, and fitted into a cardboard tube. The coloured glass pieces act as objects and the inclined plane mirrors form multiple images of these glass pieces by repeated reflections, which look like beautiful patterns (or designs). An interesting feature of a kaleidoscope is that we can never see the same pattern again.

A Kaleidoscope

WIN POINT

Page 9

8. SPHERICAL MIRRORS: (REFLECTION OF LIGHT FROM CURVED SURFACES)

A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass.

Spherical mirrors: the shaded side is non-reflecting

(i) A concave mirror is that spherical mirror in which the reflection of light takes place at the concave surface (or bent-in surface).

(ii) A convex mirror is that spherical mirror in which the reflection of light takes place at the convex surface (or bulging-out surface).

Important definitions related to Spherical Mirrors

The centre of curvature of a spherical mirror is the centre of the hollow sphere of glass of which the mirror is a part. The centre of curvature of a concave mirror is in front of it but the centre of curvature of a convex mirror is behind it.

The radius of curvature of a spherical mirror is the radius of the hollow sphere of glass of which the mirror is a part.

The centre of a spherical mirror is called its pole. The straight line passing through the centre of curvature and pole of a

spherical mirror is called its principal axis. That portion of a mirror from which the reflection of light actually takes

place is called aperture of the mirror. The principal focus of a concave mirror is a point on its principal axis to

which all the light rays which are parallel and close to the axis, converge after reflection from the concave mirror. A concave mirror has a real focus. The focus of a concave mirror is in front of the mirror.

The focal length of a concave mirror is the distance between its pole and principal focus.

Note: A plane mirror neither converges parallel rays of light nor diverges them. The focal length of a plane mirror can be considered to be ‘infinite’ or ‘infinity’.

WIN POINT

Page 10

Relation between Radius of Curvature and Focal Length of a Spherical Mirror

The focal length of a spherical mirror (a concave mirror or a convex mirror) is equal to half of its radius of curvature. If f is the focal length

of a spherical mirror and R is its radius of curvature, then: 2Rf

8.1 Rules for obtaining Images formed by Spherical Mirrors:

The image is formed at that point where at least two reflected rays intersect (or appear to intersect).

Rule 1. A ray parallel to the principal axis, after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror. This is illustrated in the following figures:

Rule 2. A ray passing through the principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis. This is illustrated in the following figures:

Rule 3. A ray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror, after reflection, is reflected back along the same path. This is illustrated in the following figures. The light rays comeback along the same path because the incident rays fall on the mirror along the normal to the reflecting surface.

WIN POINT

Page 11

Rule 4. A ray incident obliquely to the principal axis, towards a point P (pole of the mirror), on the concave mirror or a convex mirror, is reflected obliquely. The incident and reflected rays follow the laws of reflection at the point of incidence (point P), making equal angles with the principal axis.

8.2 Formation of different types of Images by a Concave Mirror:

The type of image formed by a concave mirror depends on the position of object in front of the mirror. The object can be placed at different positions (or different distances) from a concave mirror to get different types of images. For example, the object can be placed:

(a) between the pole (P) and focus (F), (b) at the focus (F), (c) between focus (F) and centre of curvature (C), (d) at the centre of curvature (C), (e) beyond the centre of curvature (C), and (f) at far-off distance called infinity.

(a) (b) (c)

(d) (e) (f)

WIN POINT

Page 12

8.3 Summary of the Images Formed by a Concave Mirror

Position of object Position of image Size of image Nature of image 1. Between pole P and F 2. At focus (F) 3. Between F and C 4. At C 5. Beyond C 6. At infinity

Behind the mirror At infinity Beyond C

At C Between F and C

At focus (F)

Enlarged Highly enlarged

Enlarged Equal to object

Diminished Highly diminished

Virtual and erect Real and inverted Real and inverted Real and inverted Real and inverted Real and inverted

8.4 Uses of Concave Mirrors:

Concave mirrors are used as shaving mirrors to see a large image of the face. Concave mirrors are used by dentists to see the large

images of the teeth of patients. Concave mirrors are used as reflectors in torches,

vehicle head-lights and search lights to get powerful beams of light.

Concave mirrors are used as doctor’s head-mirrors to focus light coming from a lamp on to the body parts of a patient (such as eye, ear, nose, throat, etc.) to be examined by the doctor.

Concave dishes are used in TV dish antennas to receive TV signals from the distant communications satellites.

Large concave mirrors are used in the field of solar energy to focus sun’s rays for heating solar furnaces.

8.5 Formation of image by a Convex Mirror

Whatever be the position of an object in front of a convex mirror, the image formed by a convex mirror is:

Behind the mirror Virtual and erect Diminished (smaller than the object)

When the distance of the object is changed from the convex mirror, then only the position and size of the image changes. There are two main positions of an object in case of a convex mirror:

(a) anywhere between the pole (P) and infinity (b) at far-off distance called infinity.

WIN POINT

Page 13

(a) (b)

8.6 Summary of the Images Formed by a Convex Mirror

Position of object Position of image Size of image Nature of image 1. Anywhere between

pole P and infinity 2. At infinity

Behind the mirror between P and F

Behind the mirror at focus (F)

Diminished

Highly diminished

Virtual and erect

Virtual and erect

8.7 Uses of Convex Mirrors

Convex mirrors are used as rear-view mirrors in vehicles (like cars, trucks and buses) to see the traffic at the rear side (or back side).

Big convex mirrors are used as ‘shop security mirrors’.

Convex mirror as side view mirror 8.8 Identification between a Plane Mirror, a Concave Mirror and a Convex Mirror without touching them:

We can distinguish between these mirrors just by looking into them, that is, by bringing our face close to each mirror, turn by turn. All of them will produce an image of our face but of different types. Following points briefly explain the image in different mirrors: A plane mirror will produce an image of the same size and our face will look normal. A concave mirror will produce a magnified image and our face will look much bigger. A convex mirror will produce a diminished image and our face will look much smaller.

9. REFRACTION

The change in direction of light when it passes from one medium to another obliquely is called refraction of light. The bending of light when it goes from one medium to another obliquely is called refraction of light. The refraction (or bending) of light takes place at the boundary between the two media.

WIN POINT

Page 14

Refraction and reflection of light

Cause of Refraction: The refraction of light is due to the change in the speed of light on going from one medium to another. Thus, when light goes from one medium to another, its speed changes and this change in speed of light causes the refraction of light. Greater the difference in the speeds of light in the two media, greater will be the amount of refraction (or bending) of light.

10. SPHERICAL LENSES: (REFRACTION of LIGHT by SPHERICAL LENSES)

The working of a lens is based on the refraction of light rays when they pass through it. A lens is a piece of transparent glass bound by two spherical surfaces. There are two types of lenses: Convex lens and Concave lens.

(a) A convex lens is thick at the centre but thinner at the edges. (b) A concave lens is thin in the middle but thicker at the edges.

Converging action of a convex lens Diverging action of a concave lens (CONVERGING LENS) (DIVERGING LENS)

WIN POINT

Page 15

Important definitions related to Spherical Lenses

The centre point of a lens is known as its optical centre. The principal axis of a lens is a line passing through the optical centre of

the lens and perpendicular to both the faces of the lens. The principal focus of a convex lens is a point on its principal axis to

which light rays parallel to the principal axis converge after passing through the lens. A lens has two foci. The two foci of a lens are at equal distances from the optical centre, one on either side of lens.

The focal length of a lens is the distance between optical centre and principal focus of the lens.

10.1 Rules for obtaining Images formed by Spherical Lenses:

Rule 1. A ray of light from the object, parallel to the principal axis, after refraction from a convex lens, passes through the principal focus on the other side of the lens. In case of a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens.

Rule 2. A ray of light passing through a principal focus, after refraction from a convex lens, will emerge parallel to the principal axis. A ray of light appearing to meet at the principal focus of a concave lens, after refraction, will emerge parallel to principal axis.

Rule 3. A ray of light passing through the optical centre of a lens will emerge without any deviation.

WIN POINT

Page 16

10.2 Image formation in Convex lenses using Ray Diagrams

A convex lens is also known as a converging lens because it converges (brings to a point), a parallel beam of light rays passing through it. The type of image formed by a convex lens depends on the position of object in front of the lens. The object can be placed at different positions (or different distances) from a convex lens to get different types of images. For example, the object can be placed:

(a) between the optical centre (C) and focus (b) at the focus (c) between f and 2f (d) at 2f (e) beyond 2f (f) at infinity

(a) (b)

(c) (d)

(e) (f)

WIN POINT

Page 17

10.3 Summary of the images formed by a Convex Lens

Position of object Position of image Size of image Nature of image 1. Between f and lens 2. At f (at focus) 3. Between f and 2f 4. At 2f 5. Beyond 2f 6. At infinity

On the same side as object At infinity Beyond 2f

At 2f Between f and 2f At f (at focus)

Enlarged Highly enlarged

Enlarged Same size as object

Diminished Highly diminished

Virtual and erect Real and inverted Real and inverted Real and inverted Real and inverted Real and inverted

10.4 Uses of Convex Lenses

Convex lenses are used in spectacles to correct the defect of vision called hypermetropia (or long sightedness).

Convex lens is used for making a simple camera. Convex lens is used as a magnifying lens by palmists,

watchmakers, etc. Convex lenses are used in making microscopes, telescopes

and slide projectors (or film projectors).

10.5 Image formation in Concave lenses using Ray Diagrams

Whatever be the position of an object in front of a concave lens, the image formed by a concave lens is:

Virtual and erect Diminished (smaller than the object)

When the distance of the object is changed from the concave lens, then only the position and size of the image changes. There are two main positions of an object in case of a concave lens:

(a) Anywhere between the optical centre and infinity. (b) at infinity.

(a) (b)

10.6 Summary of the Images Formed by a Concave Lens

Position of object Position of image Size of image Nature of image 1. Anywhere between

centre (C) and infinity 2. At infinity

Behind optical centre (C) and focus (F) At focus (F)

Diminished Highly diminished

Virtual and erect

Virtual and erect

WIN POINT

Page 18

10.7 Uses of Concave Lenses

Concave lenses are used in spectacles to correct the defect of vision called myopia (or shortsightedness).

Concave lens is used as eye-lens in Galilean telescope. Concave lenses are used in combination with convex lens to make high

quality lens systems for optical instruments. Concave lens is used in wide-angle spyhole in doors.

11. POWER OF A LENS

The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it. The power of a lens depends on its focal length. The power of a lens is defined as the reciprocal of its focal length in metres.

Power of a lens = 1( )focal lengthof thelens in metres

1Pf

where P = Power of the lens and f = focal length of the lens (in metres)

A lens of short focal length has more power whereas a lens of long focal length has less power. The unit of the power of a lens is dioptre. One dioptre is the power of a lens whose focal length is 1 metre. The power of a convex lens is positive. The power of a concave lens is negative. If a number of lenses are placed in close contact, then the power of the combination of lenses is equal to the algebraic sum of the powers of individual lenses.

P = p1 + p2 + p3 + ……….

12. DISPERSION OF LIGHT

The band of seven colours formed on a white screen, when a beam of white light is passed through a glass prism, is called spectrum of white light. The seven colours of the spectrum can be denoted by the word VIBGYOR where V stands for Violet, I for Indigo, B for Blue, G for Green, Y for Yellow, O for Orange and R for Red. The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light. White light is a mixture of seven colours. Dispersion of white light by the glass prism

WIN POINT

Page 19

The dispersion of white light occurs because colours of white light travel at different speeds through the glass prism. When white light consisting of seven colours falls on a glass prism, each colour in it is refracted (or deviated) by a different angle, with the result that seven colours are spread out to form a spectrum. The red colour is deviated the least. The violet colour is deviated the maximum.

12.1 Re-Combination of Spectrum Colours to Give White Light

The seven coloured lights of the spectrum can be recombined to give back white light.

Recombination of the spectrum of white light A top with seven colours

12.2 The Rainbow The rainbow is an arch of seven colours visible in the sky which is produced by the dispersion of sun’s light by raindrops in the atmosphere. The rainbow is actually a natural spectrum of sunlight in the sky. The rainbow is formed in the sky when the sun is shining and it is raining at the same time. A rainbow is produced by the dispersion of white sunlight by raindrops (or water drops) in the atmosphere. Each raindrop acts as a tiny glass prism splitting the sunlight into a spectrum. The raindrops in the atmosphere act like many small prisms. As white sunlight enters and leaves these raindrops (or water drops), the various coloured rays present in white light are refracted by different amounts due to which an arch of seven colours called rainbow is formed in the sky.

*****

WIN POINT

Page 20

CLASS-7th (Science) Topic: LIGHT WORKSHEET-1

1. Name two natural sources of light and two man-made sources of light. 2. What name is given to the material (or object):

(a) Which allows all the light to pass through it ? (b) Which does not allow light to pass through it at all ? (c) Which allows light to pass through it only partially ?

3. State three differences between Real Image and Virtual Image. 4. What is meant by reflection of light ? What type of objects reflect more light ? 5. What is periscope ? How two plane mirrors in a periscope are arranged:

(a) with respect to one another ? (b) with respect to the ends of the tube ?

6. Draw a ray diagram to explain reflection of light in a plane mirror. 7. What type of image is formed:

(a) in a plane mirror ? (b) on a cinema screen ? (c) in a lake ? (d) in a pinhole camera ?

8. Write the characteristics of image formed by a plane mirror. Also mention the uses of a plane mirror. 9. Define shadows. What are the conditions necessary for shadow formation? Write two differences between the pinhole images and the shadows. 10. What is meant by :

(a) Luminous objects and (b) Non-luminous objects ?

Name two luminous objects and two non-luminous objects. 11. Rearrange the boxes given below to make a sentence that helps us understand opaque objects. 12. Classify the given objects or materials as opaque, transparent or translucent and luminous or non-luminous:

WIN POINT

Page 21

Air, water, a piece of rock, a sheet of aluminium, a mirror, a wooden board, a sheet of polythene, a CD, smoke, a sheet of plane glass, fog, a wall,

a piece of red hot iron, an umbrella, a lighted fluorescent tube, a sheet of carbon paper, the flame of a gas burner, a sheet of cardboard, a lighted torch, a sheet of cellophane, a

wire mesh, kerosene stove, sun, firefly, moon. 13. Can you think of creating a shape that would give a circular shadow if held in one way and a rectangular shadow if held in another way ? 14. In a completely dark room, if you hold up a mirror in front of you, will you see a reflection of yourself in the mirror ?

HOTS

15. How many images will be formed when two plane mirrors are inclined at an angle:

(a) 30° ? (b) 60° ? 16. A ray of light makes an angle of 40° with the plane mirror. Find the angle of incidence and angle of reflection. 17. A tall device which contains plane mirrors can be used to see over the heads of crowd (as in a football match).

(a) What is the name of this device ? (b) How many plane mirrors does it use ? (c) On what principle does this device work ?

18. A man is sitting on a chair in a dark room near one of the walls. A big mirror is hung on the wall facing the man. When a torch light is put on the man from a distance, then we can see X of the man on the wall behind him. At the same time Y of the man can be observed in the mirror on the wall.

(a) What is X ? (b) What is Y ? (c) State one way in which X differs from Y.

******

WIN POINT

Page 22

CLASS-7th (Science) Topic: LIGHT WORKSHEET- 2

1. If an object is placed at a distance of 5 cm from a plane mirror, how far would it be from its image ? 2. Write the word LEFT as you would see it in a plane mirror. 3. What type of mirror is the:

(a) back-side surface of a shining steel spoon? (b) inside-surface of a shining steel spoon?

4. Define the following terms: (a) Refraction (b) Lens (c) Rectilinear propagation

5. What is meant by dispersion of light ? Name a natural phenomenon which is caused by the dispersion of sunlight in the sky ? 6. What happens when a beam of sunlight is passed through a glass prism ? What is spectrum ? 7. List three points of difference between concave lens and convex lens. 8. What kind of lens can form:

(a) an inverted image smaller than the object (b) an erect image larger than the object.

9. Draw diagram for the following: (a) Concave Mirror (b) Convex Mirror (c) Concave Lens (d) Convex Lens

10. Fill in the blanks: (a) An image that cannot be obtained on a screen is called ________. (b) Image formed by a convex ______ is always virtual and smaller in size. (c) An image formed by a ____mirror is always of the same size as that of object. (d) An image which can be obtained on a screen is called a _____ image. (e) An image formed by a concave ________ cannot be obtained on a screen. (f) The inner surface of a steel spoon acts as a ________mirror. (g) The outer surface of a flat steel plate acts as a ________mirror. (h) The outer shining surface of a round bottom steel bowl acts as a _____ mirror. (i) The inner surface of the reflector of a torch acts as a_______ mirror.

11. Mark T if the statement is true and F if it is false: (a) We can obtain an enlarged and erect image by a convex mirror. (T/F) (b) A concave lens always form a virtual image. (T/F)

WIN POINT

Page 23

(c) We can obtain a real, enlarged and inverted image by concave mirror. (T/F) (d) A real image cannot be obtained on a screen. (T/F) (e) A concave mirror always form a real image. (T/F)

12. Match the items given in Column I with one or more items of Column II. Column I Column II (a) A plane mirror (i) Used as a magnifying glass. (b) A convex mirror (ii) Can form image of objects spread over a large area. (c) A convex lens (iii) Used by dentists to see enlarged image of teeth. (d) A concave mirror (iv) The image is always inverted and magnified. (e) A concave lens (v) The image is erect and of the same size as the object. (vi) The image is erect and smaller in size than the object.

13. State the characteristics of the image formed by a plane mirror. 14. Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings. 15. What is a virtual image ? Give one situation where a virtual image is formed. 16. Give two uses each of a concave and a convex mirror. 17. Which type of mirror can form a real image ? 18. Which type of lens forms always a virtual image ?

Choose the correct option in following questions (Qtn no. 19 to 30) 19. A virtual image larger than the object can be produced by a:

(a) concave lens (b) concave mirror (c) convex mirror (d) plane mirror

20. David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be

(a) 3 m (b) 5 m (c) 6 m (d) 8 m 21. The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be:

(a) 1 m/s (b) 2 m/s (c) 4 m/s (d) 8 m/s 22. Boojho and Paheli were given one mirror each by their teacher. Boojho found his image to be erect and of the same size whereas Paheli found her image erect and smaller in size. This means that the mirrors of Boojho and Paheli are, respectively: (a) plane mirror and concave mirror. (b) concave mirror and convex mirror. (c) plane mirror and convex mirror. (d) convex mirror and plane mirror.

WIN POINT

Page 24

23. Which of the following can be used to form a real image ? (a) Concave mirror only. (b) Plane mirror only. (c) Convex mirror only. (d) Both concave and convex mirrors.

24. If an object is placed at a distance of 0.5 m in front of a plane mirror, the distance between the object and the image formed by the mirror will be: (a) 2 m (b) 1 m (c) 0.5 m (d) 0.25 m 25. You are provided with a concave mirror, a convex mirror, a concave lens and a convex lens. To obtain an enlarged image of an object you can use either (a) concave mirror or convex mirror. (b) concave mirror or convex lens. (c) concave mirror or concave lens. (d) concave lens or convex lens. 26. A rainbow can be seen in the sky: (a) when the sun is in front of you. (b) when the sun is behind you. (c) when the sun is overhead. (d) only at the time of sun rise. 27. An erect and enlarged image can be formed by: (a) only a convex mirror. (b) only a concave mirror. (c) only a plane mirror. (d) both convex and concave mirrors. 28. You are provided with a convex mirror, a concave mirror, a convex lens and a concave lens. You can get an inverted image from:

(a) both concave lens and convex lens. (b) both concave mirror and convex mirror. (c) both concave mirror and convex lens. (d) both convex mirror and concave lens.

29. An image formed by a lens is erect. Such an image could be formed by a (a) convex lens provided the image is smaller than object. (b) concave lens provided the image is smaller than object. (c) concave lens provided the image is larger than object. (d) concave lens provided the image is of the same size.

30. If f is the focal length of the lens, then the power of a lens is equal to:

(a) ଵ

() (b)

ଵ ()

(c) ଵ ()

(d) ଵ

ଵ ()

*****

WIN POINT

Page 25

CLASS-7th (Science) Topic: LIGHT WORKSHEET- 3

1. The image formed by a lens is always virtual, erect and smaller in size for an object kept at different positions in front of it. Identify the nature of the lens. 2. State whether the following statements are True or False.

(a) A concave lens can be used to produce an enlarged and erect image. (b) A convex lens always produces a real image. (c) The sides of an object and its image formed by a concave mirror are always

interchanged. (d) An object can be seen only if it emits light.

3. What type of mirror is used as a side mirror in a scooter ? Why is this type of mirror chosen ? 4. The given figures show the path of light through lenses of two different types, represented by rectangular boxes A and B. What is the nature of lenses A and B ?

5. Boojho made light from a laser torch to fall on a prism. Will he be able to observe a band of seven colours ? Explain with a reason. 6. State the correct sequence (1-7) of colours in the spectrum formed by the prisms A and B as shown in figure.

7. The side mirror of a scooter got broken. The mechanic replaced it with a plane mirror. Mention any inconvenience that the driver of the scooter will face while using it ? 8. The concave reflecting surface of a torch got rusted. What effect would this have on the beam of light from the torch ?

WIN POINT

Page 26

9. An erect and enlarged image of an object is formed on a screen. Explain how this could be possible. 10. Two different type of lenses are placed on a sheet of newspaper. How will you identify them without touching ? 11. A shopkeeper wanted to fix a mirror which will give a maximum view of his shop. What type of mirror should he use? Give reason. 12. The distance between an object and a convex lens is changing. It is noticed that the size of the image formed on a screen is decreasing. Is the object moving in a direction towards the lens or away from it ?

HOTS

13. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal ? Why ? 14. Find the focal length of a lens of power 2.0 D. What type of lens is this ? 15. An object is placed at the given distances from a convex lens of focal length 10 cm:

(a) 8 cm (b) 15 cm (c) 25 cm Which position of the object will produce:

(i) A diminished real image ? (ii) A magnified real image ? (iii) A magnified virtual image ?

16. A person is standing in front of a big plane mirror. The distance between the mirror and his image is 5 m. If the person moves 2 m towards the plane mirror, what would be the distance between the person and his image ? 17. Suppose we wish to obtain the real image of a distant tree. Explain two possible ways in which we can do it. 18. It was observed that when the distance between an object and a lens decreases, the size of the image increases. What is the nature of this lens ? If you keep on decreasing the distance between the object and the lens, will you still able to obtain the image on the screen ? Explain. 19. You are given three mirrors of different types (a Plane Mirror, a Concave Mirror and a Convex Mirror). How will you identify each one of them ?

*****

WIN POINT

Page 27