$(a)$ What happens to a ray of light when it travels from one medium to another having equal refractive indices?
$(b)$ State the cause of refraction of light.

(N/A) When a ray of light travels from one medium to another having equal refractive indices,no refraction or bending occurs. The light continues to travel in a straight line because there is no change in the optical density between the two media.
$(b)$ The cause of refraction of light is the change in the speed of light as it travels from one medium to another. When light enters a medium with a different optical density,its velocity changes,causing it to deviate from its original path.