LIGHT

Write down everything you

know about light!

INTRODUCTION

• We step outside on a sunny day and our surroundings are awash with every colour of the spectrum. How does the energy known as light produce such spectacular effects?

• The answer is that white light, radiated from the sun, is perhaps not really white at all! White light is every colour radiating together, and it is only when this light is split that we see the dazzling array of colour that we know

What are the colours that make up white light?

White light =

ROY G BIV

HEADSTART BOOKLET

• Successful completion of this booklet will allow you to:– Explain the refraction of light in various situations– Compare the effects of concave and convex lenses

on light and explain how they form images– Describe how white light can be split into its

constituent colours– Explain the interaction of white light with coloured

objects and describe how coloured filters are used

HEADSTART BOOKLET

• Bring your booklet to every class• It will need to be submitted at the end of

Headstart for assessment• Must choose ONE homework activity from the

end to complete before the book is submitted for assessment

• Your booklet is NOT for taking notes – it will be used for an assessment at the end of the unit. Your exercise book must be used for notes

What do you notice about the pencil in the water? Can you explain what is happening?

• Put a coin in a polystyrene cup and move back until the coin is just out of sight. Stand in this position while a friend pours water into the cup.

• What happens?• Draw a diagram to explain what has happened.

REFRACTION OF LIGHT• As light moves from one medium to another,

its speed changes

• This causes the light wave to change direction, or bend (except when light hits a surface at right angles)

Air

Water

Ray of light: the incident ray

Where light would go if travelling in a straight line

Where light really goes: the refracted ray

REFRACTION VS. REFLECTION

• Refraction and reflection animations

REFRACTION OF LIGHT• To investigate the bending of light as it moves from one

medium to another, we use a reference line called the normal

• The normal is an imaginary line that is at right angles to the boundary between the two materials

Air

WaterBoundary

Normal

ANGLE OF REFRACTION• Angle between the normal and the path of the

refracted light

Glass

Air Normal

Refracted ray

Incident ray

Angle of refraction

ANGLE OF INCIDENCE

• Angle between the normal and the ray of light approaching a surface

Glass

Air NormalIncident ray

Angle of incidence

CRITICAL ANGLE• The angle at which there is total reflection of light

as it hits a surface – called total internal reflection• i.e. There is NO refracted ray of light

Glass

Air

Normal

Incident ray Reflected ray

Critical angle

PRAC

• Complete the ‘Demonstrating Refraction’ Prac

REFRACTION OF LIGHT• The direction that light bends as it moves from

one medium to another depends on the density of each material

• How did the light bend (in relation to the normal) as it entered the glass block?

• How did the light bend (in relation to the normal) as it left the glass block?

REFRACTION OF LIGHT

• When light moves from a less dense medium to a denser medium, the light slows down

• This causes the light to bend towards the normal

Glass

Air Normal

REFRACTION OF LIGHT

• When light moves from a denser medium to a less dense medium, the light gets faster

• This causes the light to bend away from the normal

Air

Glass Normal

REFRACTION AND FISHING

• Hunters who use spears to catch fish know about refraction. Light bends as it leaves the water so the fish always appears closer to the surface than it really is.

REFRACTION AND FISHING

• Where would the hunter aim to spear a fish?• Why?

REFRACTION AND FISHING• Aim in front of the fish• Light from the fish comes out of the water and bends

away from the normal (speed increases)• Our brain knows that light travels in straight lines. For

this reason, we think that the fish is located in a straight line from our eyes to the water

TOTAL INTERNAL REFLECTION• Why do you think diamonds sparkle?

TOTAL INTERNAL REFLECTION

• Light is refracted into the diamond, then totally internally reflected twice within the diamond, producing the sparkle

TOTAL INTERNAL REFLECTION• What does total internal reflection mean? Use

the following diagram to explain.

LENSES

• Look through a concave and then a convex lens at your classmates and at your book.

• What do your friends look like through each of the lenses?

• What does your book look like through each of the lenses?

• Suggest some uses for each type of lens.

CONVEX LENSES• Also called converging lenses because they

focus parallel light rays to a point• Your eyes contain these type of lenses

• Parallel rays come to a focus after passing through a convex lens

PRAC

• Complete ‘Investigating Convex Lenses’ Prac

CONVEX LENSES• Were the images upright or inverted?

• If the object is not past the focal length, the image is upright

CONVEX LENSES• Were the images upright or inverted?

• If the object is past the focal length, the image is inverted

CONVEX LENSES• Was the image bigger or smaller?

• Images may be bigger or smaller

• However, images are mostly bigger than the real object

CONVEX LENSES• Was the image bigger or smaller?

Where would the object have to be for the image to be smaller than the object?

CONVEX LENSES

• Why did the plastic lens and glass lens have different focal lengths?

CONVEX LENSES

• What is the relationship between focal length and thickness?

CONCAVE LENSES• A concave lens can be easily recognised by its thicker

rim• Also called diverging lenses because they spread out

parallel rays of light• Produce images that are always upright, smaller in

size and virtual in nature

REAL AND VIRTUAL IMAGES

• Virtual images are seen on a mirror• Real images can be projected on a screen

CONCAVE LENSES

• Use a concave lens to try to obtain an image of your laboratory windows.

• What do you find?

PRAC

• Complete ‘Investigating Concave Lenses’ Prac

Re-Cap

• What have you learnt about lenses and light so far?

VISIBLE LIGHT & THE COLOUR SPECTRUM

• Visible light is only one section of a wide variety of waves known as the electromagnetic spectrum

• The electromagnetic spectrum also includes: UV rays, infrared rays, gamma rays

VISIBLE LIGHT & THE COLOUR SPECTRUM

• The electric field travels in waves – the energy alternates in positive and negative directions

• Positive direction

• Negative direction

VISIBLE LIGHT & THE COLOUR SPECTRUM

• All components of the electromagnetic spectrum consist of a changing electric field at right angles with a changing magnetic field.

A field is an area in space where an introduced electrically charged/magnetic field will experience a force.

VISIBLE LIGHT & THE COLOUR SPECTRUM

• For example: a magnetic field is an area where if you introduce a magnet into it, the magnet will experience a force– A compass has a magnet in it that experiences a

force which pushes the needle to point North

PRAC

• Complete the ‘Slinky’ Activity

COLOUR SPECTRUM• The light that you can see is the visible spectrum• This includes red, orange, yellow, green, blue, indigo

& violet• This can be abbreviated to ROY G BIV• Light wavelengths are extremely small, being less

than 1,000 of a millimeter

COLOUR SPECTRUM• Each colour of light has a different wavelength

• Special cells called cones in the retinas of your eyes respond to different wavelengths and determine the colour you see• Colour your graph to match that on the screen

COLOUR SPECTRUM• The white light that comes from the sun and

light bulbs is a mixture of all the colours of the visible spectrum

• It consists of mixed waves of different wavelengths and frequencies all travelling at the same speed

• The frequency of a light wave refers the number of waves that pass a specific point in one second

• Visible light is released from atoms when ‘excited’ electrons drop to a lower energy level

COLOUR SPECTRUM

• ROY G BIV – They Might Be Giants Video

• List the colours of the visible spectrum• Apart from their colour, what is different

about the different colours in the visible spectrum?

• Which colour of visible light has the greatest wavelength?

• What colour of visible light has the highest frequency?

• Which colours is the human eye most sensitive to?

TRANSMISSION AND ABSORPTION OF LIGHT

• When light hits a surface it can be reflected, refracted, transmitted or absorbed

• If a material only absorbs light, no light will pass through it

• If a material transmits light, the light is able to pass through it

OPAQUE MATERIALS

• If a material reflects or absorbs light, it is opaque

• These materials do not transmit any light at all• This means that no light passes through the

object and we cannot see what is on the other side

• E.g. Wood, metal, mirror

TRANSPARENT MATERIALS

• Transparent materials transmit light• This means that they let light pass through,

allowing us to see what is on the other side• E.g. Glass

TRANSLUCENT MATERIALS

• Translucent materials scatters the light that passes through it

• The objects behind it are not clearly visible• May be fuzzy or unclear• E.g. Frosted glass, some plastics

COLOUR SPECTRUM• Why do we see colours?

COLOUR SPECTRUM• Pigments on the surface of an object give it

colour• An object is seen as red if it reflects red light

towards our eyes and absorbs orange, yellow, green, blue, indigo and violet light

• In reality, some of the other colours of light may also be reflected, but this will just affect the shade of red we see

COLOUR SPECTRUM• Why does a sheet of paper look white?

• White light is made up of all the colours of the visible spectrum, therefore white objects reflect all light colours

• Black objects absorb all colours, reflecting none

COLOUR SPECTRUM• White light can also be

made using just three colours of the spectrum:• Red, blue and green

• For this reason, these are called the primary colours

• Red + green = yellow• Red + blue = magenta• Green + blue = cyan

COLOUR SPECTRUM

• Why is the sky blue?• Video – Why is the sky blue?

RETINAL FATIGUE• What happens when our eyes tire?• What is retinal fatigue & how does it work?

• Cone cells in the retina of our eye are responsible for detecting colour

• Video – Is that a colour photo?

RETINAL FATIGUECone cells in the retina of the eye

detect colour

Red light from an object hits

cone cells

Red-sensitive cone cells give off

a chemical

This chemical stimulates nerve receptors to send messages to the

brain, telling you that you’re seeing something red

After a few seconds the chemicals deplete, so that part of your retina becomes less sensitive to red light

If you look at an object of another colour, you will see it minus the red

(e.g. If you look at something white you will see cyan, which is all light minus red)

Called inverse colours

RETINAL FATIGUE: stare at the dot in the centre of the box

RAINBOWS

• A rainbow is a common and spectacular optical phenomenon, but how is it formed?

• It is a complex process involving refraction, total internal reflection and the dispersion of sunlight within water droplets

RAINBOWS • Prism animation

Light is refracted as it enters a

droplet of water

It gets dispersed in to the colours of the spectrum

Light is refracted again as it leaves the water

droplet, each colour at a different angle

Total internal reflection occurs

at the back surface of the

droplet

Red light is always refracted at 42O, meaning it will always be at the top of the rainbow, while violet is refracted at 400, so it will always be at the bottom

RAINBOWS

• We see a rainbow only occasionally because conditions need to be right for total internal reflection to occur

• This will be when the Sun is at our back, usually low in the sky

• On some occasions, light is reflected twice within the droplets of water to produce a second rainbow, like a mirror image above the first

RAINBOWS

• No two people ever see exactly the same rainbow, because the rainbow that you see is formed by a particular set of droplets that direct rays of the particular colour towards your eyes

• The droplets that form the orange part of your rainbow may produce the blue section of the rainbow seen by a friend standing next to you

RAINBOWS

• Why does the white light separate into the spectrum of colours when forming a rainbow?

• Why is red always at the top of the rainbow?• Explain why the rainbow you see is one of a kind.• Why does a rainbow only form when the sun is

at a particular angle? What would happen at the other times?

• Have you ever seen a double rainbow? How does it happen? • Video – Double Rainbow

RAINBOWS

• CHALLENGE QUESTION: Gina and Clare decide to wear their new clothes to the nightclub. Gina looks stunning in her red top, blue pants and black shoes and Clare is resplendent in her white top, yellow skirt and black shoes. The lighting inside the nightclub is nearly all red and blue. What would Gina and Clare’s clothes now look like?

COLOUR SPECTRUM

• Complete ‘Filter Wheel’ Prac

COLOUR SPECTRUM

• Video – Filter Animations

SUMMARY

• Complete summary sheet