Assertion (A): When a rainbow forms, the light at the water drop's inner surface is internally reflected. 

Reason (R): The critical angle is more than the angle formed by the refracted ray and the normal to the drop surface. 

The rainbow is formed when the light at the inner surface of the water drop gets internally reflected. This happens if the angle between the refracted surface and normal to the drop surface is greater than the critical angle. When a ray of white light is incident on a water drop, it suffers refraction at the outer surface and total internal reflection at the inner surface of the water drop. The light is split into seven colours inside the drop, and a rainbow is formed. The splitting of light takes place due to total internal reflection, which occurs when the incidence angle is greater than the critical angle. 

Hence, the correct option is (a) Both A and R are true, and R is the correct explanation of the assertion.