Refraction of light waves

Light travels at different speeds through different materials. It travels at 3x108 m/sec in a vacuum, at a

 slightly lower speed through air, and at 2x108 m/sec through glass. In a diamond, light travels at about 40 per cent of its speed in a vacuum.

When light goes from one medium to another, its speed changes. Because of this, the light rays bend. The process of light bending when passing from one medium to another is known as refraction. A transparent substance in which light travels is known as a medium. Air, glass, water, ice and diamond are examples of such media.

Different media are said to have different optical densities. A medium in which the speed of light is more when compared with another medium is known as an optically rarer medium. Air is an optically rarer medium as compared to water or glass.

A medium in which the speed of light is less when compared with another medium is known as an optically denser medium. Glass is optically denser than air and water. When a ray of light goes from a rarer medium to a denser medium, it bends towards the normal (at the point of incidence). When a ray of light goes from a denser

medium to a rarer medium, it bends away from the normal (at the point of incidence).

Consider the pane of thick window glass as shown in the picture (Figure A). When light falls normally to the surface of a glass slab (from point A through the glass to point B), it will go in a straight path. In this case, light encounters the glass perpendicularly (at a 90 degree angle) and hence there is no bending of the ray of light.

However, when it goes from A to C, the emerging light is displaced, but parallel to the incident light. A ray of light (Aa) travelling in air is directed on the glass block. On entering the glass block, it is refracted and bends towards the normal.

Again, a change of direction takes place when the ray of light (ac), travelling in glass emerges into the air at point c. Since the ray of light (ac) goes from a denser medium (glass) into the rarer medium (air), it bends away from the normal and goes in the direction cC. The incident ray (the ray which hits the medium)/Aa and the emergent ray/cC are parallel to each other.

How a road mirage is formed

You may have seen a pool of water lying on the road some distance ahead of you on a sunny day. Were you ever able to reach it? No, you could always see it at some distance.

This incident can also be explained through the concept of refraction. The water you see on the road is a mirage formed by light rays coming from the low section of the sky in front of you.

The air is very hot just above the road surface and cooler above. Light travels faster through the thinner hot air than through the denser cool air above it. As the rays of light descend, instead of coming to us from the sky in straight lines, they continuously bend towards the horizontal.

Light from the sky picks up speed in air near the ground because that air is warmer and less dense than the air above. When the light grazes the surface and bends upward, the observer sees a mirage. A mirage is not, as many people mistakenly believe, a 'trick of the mind'.

Refraction occurs when the average speed of light changes in going from one transparent medium to another. Consider a pair of wheels connected to an axle. As the wheels roll gently down from a smooth sidewalk onto a grass lawn, the wheels meet the grass at some angle; they will be deflected from their straight-line course.

On meeting the lawn, the wheels roll more slowly due to interaction with the grass and the left wheel slows down first. This is because it meets the grass first while the right wheel is still on the sidewalk. The faster-moving right wheel tends to pivot above the slower-moving left wheel because during the same time interval, the right wheel travels farther than the left one. This action bends the direction of the rolling wheels toward the "normal".

A light wave bends in a similar way. Thus, the phenomenon(occurrence that appears) of a mirage can be drawn when light rays are refracted as they pass from air layers that are less dense to more dense air.

The refraction of light is a very complicated subject and there's a lot more to write about it. We'll bring you more details on refraction in our next The World of Science page.

Compiled by Janani Amarasekara