High School

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Physics Ex-46

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In studying optics, it is convenient to classify objects into those thatemit light (luminous) and those that reflect light (non-luminous):

LUMINOUS (originate light) NON-LUMINOUS or ILLUMINATED (do not originate light)

Incandescent Opaque (light cannot pass through)Neon Transparent (light and images pass through)Fluorescent Translucent (light passes through, images do not)

BEAM OF LIGHT: A beam of light is a "bunch" of light rays.

CONVERGENCE: When rays of light come together to meet at a point, they are said to converge.

DIVERGENCE: When rays of light go away from a point, they are said to diverge.

MEDIUM: A medium is the material through which light travels. Optical mediums are classified according to density. The more "optically dense" a material is, the slower light travels in that medium.

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Physics Ex-46

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THE PRIMARY COLORS

Monochromatic light is light of one color.

Polychromatic light is light of many colors.

In everyday language we use names to represent monochromaticlight such as red, orange, yellow, green, blue and violet.

Furthermore, each of these colors has a variety of different shades.Collectively, all the different colors form the “light band” which ispart of the “electromagnetic spectrum”.

Physics Ex-46

Red, green and blue light (RGB) are called the primary colors.Adding these three primary colors together forms “white” light.Adding pairs of the primary colors form the secondary colors.

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Physics Ex-46

The color of an object is determined by the color (or colors)it reflects.

Suppose white light shines on a red object. The object willabsorb all the colors except red. Thus, it is red.

Study the illustration below.

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Additive color theory Subtractive color theory

Primary colors: RGB Primary colors: CMY

Red – Green – Blue Cyan – Magenta – Yellow

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Red = Magenta + Yellow Green = Cyan + Yellow Blue = Cyan + Magenta

Cyan = Green + Blue Magenta = Red + Blue Yellow = Red + Green

Black = Cyan + Magenta + Yellow White = Red + Green + Blue

Physics Ex-46

SHADOWS

Shadows are formed when light is obstructed. The light may betotally obstructed thereby forming a dark shadow known as anumbra, or it may be partially obstructed thereby forming a grayshadow known as a penumbra.

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Penumbra (grey)

Umbra (black)

Physics Ex-46

For example, in diagram-A below, a point source of light encounters anobstacle (a disc) causing an umbra (total darkness).

In diagram-B, a large source of light causes both an umbra and a penumbra.Note that when the source is a point, the shadow consists of an umbra(total darkness) with a well defined outline.

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From the side view,we have two rays

(one to the top of theobstruction, one to

the bottom of theobstruction) to definethe shadow (the light

source has no top andbottom portions).

Physics Ex-46Question-1

Define the following terms:

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a) Beam of light

A stream of photons emanating (coming from) a light source.

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b) Medium

The material through which light travels.

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c) Opaque

A material that blocks light (light cannot pass through).

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d) Transparent

A material through which light and images pass through.

e) Translucent

A material through which light can pass through butimages cannot pass through.

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Physics Ex-46Question-2

Define the meaning of the terms monochromatic andpolychromatic light.

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Monochromatic is light consisting of only a single color.

Polychromatic is light consisting of many colors.

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Physics Ex-46Question-3

Draw 5 rays of light that are parallel, converging and diverging.

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Physics Ex-46Question-4

What does RGB stand for?

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Red, Green and Blue.

Physics Ex-46Question-5

Fill in the primary and secondary colors in the chart below:

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P rim a ry co lo rs

S eco n d a ry co lo rs

( lig h t b lu e ) ( lig h t v io le t)

Red Green Blue

Yellow Cyan Magenta

Red Green Red Blue Green Blue

Physics Ex-46Question-6

Define an umbra and explain how it is formed.

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The umbra consists of total darkness and is caused by thetotal obstruction of the light rays.

Physics Ex-46Question-7

Define an penumbra and explain how it is formed.

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The penumbra consists of partial darkness and is causedby the partial obstruction of light rays.

Physics Ex-46Question-8

In your own words, define light intensity.

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Light intensity (also known as brilliance) refers to the totalnumber of photons emitted by a light source per second.

Physics Ex-46Question-9

Sketch the electromagnetic spectrum and indicate the positionof light.

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Physics Ex-46Question-10

In the given diagram, measure the length of the umbra andpenumbra of the shadow formed by the obstruction (line).

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Physics Ex-46Question-11

A beam of white light strikes a glass prism producing an arrayof different colors as illustrated below.

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What optical phenomena does this represent?

A) Specular reflection

B) Total internal reflection

C) Diffusion

D) Dispersion

By definition

Physics Ex-46

The diagram below illustrates light coming from the sun to theearth with the moon in between.

Question-12

Which of the following statements correctly describes the shadowof the moon on the earth?

I- Region Y is the penumbra

II- Regions X and Z are the umbra

III- Regions Y is the umbra

IV- Regions X and Z are the penumbra

A) I and II B) I and IV C) II and III D) III and IV

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Physics Ex-46

Blue filter

Question-13

The diagram below illustrates a white light source, a blue filterand a yellow object.

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What color will the object appear?

A) White

B) Black

C) Yellow

D) Blue

A yellow object illuminated by white light willabsorb all the colors except yellow. A blue

filter will filter all the colors except blue.Thus, when the blue light strikes the yellow

object, it will be absorbed and the objectwill appear black (since it reflects no light).

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All the colors Blue

Physics Ex-46Question-14

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A green filter is placed between a red light source and an object.

Red light sourceGreen filter

White object

What color is the object?

A) Red

B) Green

C) Yellow

D) White

E) Black

A green filter will block (absorb) all colorsexcept green. Since the light source givesoff red light, the green filter will absorb the

red light causing no light to fall on the object.

Physics Ex-46

Object-X Object-Y Object-Z

A) Blue Black Black

B) Blue Red Green

C) White Red Green

D) White Black Red

E) White Black Green

Question-15

Three different colored objects X, Y and Z, are illuminated bythree projectors. Each projector emits a different colored light.

The first projector causes object X to appear blue while theother objects appear black.

The second projector causes objects X and Y to appear redwhile object Z appears black.

The third projector causes objects X and Z to appear greenwhile object Y appears black.

What color do the objects appear if while light is used to illuminate the objects?

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X Y Z

1

2

3

White Red Green

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