Question

Explain with the help of a labeled ray diagram why a pencil partly immersed in water appears to be bent at the water surface. State whether the bending of pencil will increase or decrease if water is replaced by another liquid which is optically more dense than water. Give reason for your answer.

Answer 1

Hint: When light travels from one medium to another medium it bends, this phenomenon is known as refraction. Refraction occurs because the speed of light changes in different mediums, in denser medium refraction is more and vice versa.

Complete step by step answer:

Here a pencil AO is partially immersed in water. when light travels from one medium to another it bends. When the light goes from rarer to denser medium it refracts towards the normal and when it goes from denser to rarer medium it refracts away from the medium. Water is denser than air. So light rays OC and OD are coming from water and it gets refracted away from the normal.

We can see rays CX and CY. If we extend these rays backward to CI and DI. At point I a virtual image is formed of point O. Point I is nearer to the water surface hence it seems that the pencil is bent at the water surface. Snell’s law gives the relation between refractive index and angle of incidence and angle of refraction.
${\mu _1}\sin i = {\mu _2}\sin r$
Here ${\mu _1}$ is the refractive index of medium one from which light is coming, ${\mu _2}$ is the refractive index of medium two to which light is going, $\sin i$ is the angle of incident and $\sin r$ is the angle of refraction.

Hence in the denser medium the refraction of light is more. Therefore, the bending of the pencil will increase if water is replaced by another liquid that is optically more dense than water.

Note:If we see the pencil right from above and not at an angle we will not be able to see refraction because then light will come from the water perpendicularly since the angle of the incident will be zero so the angle of refraction will be zero hence no refraction occurs.