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?

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.