A light beam is directed parallel to the axis of a hollow cylindrical tube. When the tube contains only air, the light takes 8.72 ns to travel the length of the tube, but when the tube is filled with a transparent jelly, the light takes 1.82 ns longer to travel its length. What is the refractive index of this jelly?

Explanation:

velocity of light in a medium of refractive index V = V₀ / μ

V₀  is velocity of light in air and μ is refractive index of light.

time to travel in tube with air =  length of tube / velocity of light

8.72 ns = L / V₀  L is length of tube .

time to travel in tube with jelly =  length of tube / velocity of light

8.72+ 1.82  = L / V  L is length of tube .

10.54 ns = L / V

dividing the equations

10.54 / 8.72 = V₀  / V

10.54 / 8.72 =   μ

1.21 =  μ

refractive index of jelly = 1.21 .