January 8, 2015

10th Physical Sciences Important Questions and Answers

Mirage is an optical illusion where it appears that water has collected on the road at a distant place but when we get there, we do not find any water. This phenomenon is because of total internal reflection.

Q: Explain the formation of a mirage? (4 Marks)

A: 1) During a hot summer day, air just above the road is very hot and the air at higher altitudes is cool.

2) It means that the temperature decreases with height. As a result the density of air increases with height.

3) We know that the refractive index of air increases with density. Thus the refractive index of air increases with height.

4) So the cooler air at the top has greater refractive index than the hotter air just above

5) Light travells faster through the thinner hot air than through the denser cool air above it.

6) When light from a tree passes through a medium just above the road, whose refractive index decreases towards the ground, it suffers, refraction and takes a curved path because of total internal reflection.

7) This appears to the observer as if the ray is reflected from the ground. Hence we feel the illusion of water being present on the road.

8) The inverted virtual image of the tree is seen by the observer. This is a mirage.