10. LIGHT – REFLECTION AND REFRACTION
Light seems to travel along straight-line paths in a transparent medium.
Light does not travel in the same direction in all media. It appears that when travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes. This phenomenon is called refraction of light. E.g.
- The bottom of a tank or a pond containing water appears to be raised.
- When a thick glass slab is placed over a printed matter, the letters appear raised.
- A pencil partly immersed in water in a glass tumbler appears to be displaced at the interface of air & water.
- A lemon kept in water in a glass tumbler appears to be bigger than its actual size, when viewed from the sides.
Refraction is varied in different media such as kerosene, turpentine, transparent plastic slab etc.
Experiments to demonstrate the refraction of Light
Place a coin at the bottom of a bucket filled with water.
With our eye to a side above water, try to pick up the coin in one go. We do not succeed.
Reason: Reflected light coming from the submerged coin in water (denser medium), on entering air (rarer medium), bend away from the normal due to refraction of light and image size becomes larger than its actual size. Thus, coin appears to be closer than its actual distance.
Place a large shallow bowl on a table and put a coin in it.
Move away slowly from the bowl. Stop when the coin just disappears from our sight.
Ask a friend to pour water into the bowl without disturbing the coin. The coin becomes visible again. This is because the coin appears slightly raised above its actual position due to refraction of light.
Draw a thick straight line on a white paper. Place a glass slab over the line such that one of its edges makes an angle with the line.
Look at the portion of the line under the slab from the sides. The line under the glass slab appears to be bent at the edges. It is due to the refraction of light.
Place the glass slab normal to the line. The part of the line under the glass slab does not appear bent. It appears in a straight line. Because a ray of light perpendicular to the plain of a refracting medium does not change its angle due to refraction.
Look at the line from the top of the glass slab. Part of the line appears to be raised. This is due to refraction of light.