Question

1. Explain why the refractive index of any material with respect to air is always greater 1.
2. In the figure below a light ray travels from air into the semi-circular plastic block. Give a reason why the ray does not deviate at the semi-circular boundary of the plastic block.
   
3. Complete the ray diagram of the above scenario when the light ray comes out of the plastic block from the top flat end.

Answer 1

1. The refractive index of a medium with respect to air is given by `"speed of light in the air"/"speed of light in the medium"`. Since the speed of light in the medium is always less than the speed of light in air, hence the above ratio is always greater than 1.
2. The ray of light is undergoing normal incidence at the air-plastic block interface. And for normal incidence, there is no deviation.
3. 
   
   (Credit arrows, refracted ray moving away from normal)