Reflection is a phenomenon which occurs in opaque objects but reflection occurs in a droplet of water at the time of rainbow formation. Explain how?
Reflection of light from a mirror and reflection occurring in a droplet of water at the time of rainbow formation are two different physical phenomena.
The statement asserted in the question
"Reflection is a phenomenon which occurs in opaque objects" is incorrect. Light can not travel in opaque objects.
The scientific dictionary defines Opaque:
Adjective. Neither reflecting nor emitting light. Allowing little light to pass through, not translucent or transparent.
What perhaps in the mind of the student is:
Reflection takes place whenever ray of light travels from a transparent medium towards an opaque medium (shown in the figure below). The opaque medium does not let the light pass through. Instead reflects it back as per laws of reflection.
Why does reflection occur in a droplet of water at the time of rainbow formation. Both the droplet of water as well as the medium in which droplet is there (air), are transparent; absence of an opaque medium.
We must remember that reflection in the water droplet at the time of rainbow formation is total internal reflection. This occurs a when a ray of light, while traveling from a denser to rarer medium, reaches an interface and makes an angle of incidence exceeding the critical angle, as explained in the figure below.
Total internal refection in a water droplet and formation of rainbow.