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LIGHT

Light is a small part of the electromagnetic spectrum and travels by waves just likeradio waves. But it is easier to represent light by rays.

If one throws a ball perpendicularly to a wall, the ball will bounce back along the same path. But

if the ball is thrown at an angle to a wall, the ball will be reflected back at an angle.

REFLECTION

Light does the same thing.

When drawing diagrams of reflection of light, imagine a ball being thrown at a walland bouncing back. This way you understand better what is happening.

The above diagram shows that:

ANGLE OF INCIDENCE = ANGLE OF REFLECTION

The angle of incidence is the angle fejn qed jaqa id-dawl.

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When drawing diagrams of reflection of light always draw the NORMAL. The normal is alwaysperpendicular to the surface and is drawn as a dotted line to show that it is not one of the rays oflight. One measures angles always with the normal.

The formation and properties of the image by a plane mirror:

Consider an object A. It may be a candle giving out light. But it can be any object reflecting thelight falling on it.After reflection by the mirror, the light from A enters the eye and the eye thinks that the light iscoming form a point A behind the mirror. That is how the eye sees the image of the object. Therays behind the mirror are shown dotted because there is no light behind the mirror light onlyappears to come from there.

The image in a plane mirror is as far behind the mirror as the object is infront.

REFRACTION

Let us imagine two men carrying a pole between them. We are looking at them from above theirheads.First they are travelling on concrete where they can walk swiftly and then they enter an areawhere there is mud and hence they cannot walk as fast as before.

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The speed of light in glass or in water is smaller than its speed in air. So if light is travelling in airand then enters water, that is light is changing the medium, then light will also change directionexactly as the two men did when they entered the area of mud..

This change in direction because there is a change in the medium is called REFRACTION.

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Speed of light in air / speed of light in glass = refractive index of glass

Note that when light travels from air to glass, the refracted ray bends towards the normal

and when light travel form glass to air the refracted ray bends away from the normal.

When one looks at a fish in the sea, the fish appears to be closer to the water surface than itreally is. This occurs because of the refraction of light as shown in the figure below.

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Consider light passing from glass to air as shown in the diagram below.

Note that all angles are measured with the normal.As the incident angle in the glass increases, the light is refracted out into the air away from thenormal.(brown and blue rays)But as the angle of incidence in glass is increased further, light will just emerge along thesurface of the glass. (red ray)Hence for angles of incidence smaller than the red angle, all light is refracted out into the air butfor angles greater than this angle all light is reflected internally in the glass. This special angle iscalled the CRITICAL ANGLE for glass.

For example if one is sick at the hospital and the body temperature keeps below a certain value,the patient has a chance that he will recover. But if his body temperature goes well above thatvalue he may die. Hence that value of temperature is CRITICAL for the patient.

Light is being totally internally reflected through a semi-circular glass block.

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Total internal reflection is used in OPTICAL FIBRES as shown in the diagram below.

Today optical fibres are used instead of copper wires to carry signals in communications.

Below are examples of refraction and total internal reflection of light through a glass block andthrough prisms.

Note that one always draws the normal to help him see how the ray of light will bend away ortowards the normal.

Remember:WHEN LIGHT TRAVELS FROM AIR TO GLASS, THE REFRACTED RAY BENDS TOWARDSTHE NORMALWHEN LIGHT TRAVEL FORM GLASS TO AIR THE REFRACTED RAY BENDS AWAY FROM

THE NORMAL.

The prism on the RHS shows total internal reflection.

But light of different colours are refracted at different angles as shown below.

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When white light is refracted through a prism, the light opens up in different colours becauseeach light of a different colour is refracted at a different angle.The order of colours, Red, orange, yellow, green, blue, indigo, violet, can be remembered by thesentence Richard of York gave battle in vain.

This spreading of white light into different colours is called DISPERSION.

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Now we have some questions form the SEC Physics exam

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