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In a homogeneous medium ( having the same density throughout) , light travels in a straight line path. But when a ray of light travelling from one transparent medium is incident obliquely on the surface of another transparent medium, a part of light is reflected back in the same medium and the rest of light is transmitted into the other medium, in a direction different from its initial path (i.e. it gets refracted).

In the question above, air and rain drop act as two transparent media with different densities, hence a light ray travelling from air to water gets reflected as well as refracted.