Question

Secondary rainbow refraction after a reflection of sunlight

Answer 1

After a rain, there are still some tiny water droplets remained in the air. If there is sunshine a white sunbeam will be reflected and refracted by these tiny droplets. Both the primary and secondary rainbows are phenomena that formed by the reflection and refraction of sunlight in tiny water droplets.

Answer 2

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In theory, all rainbows are double rainbows, but since the secondary bow is always fainter than the primary, it may be too weak to spot in practice. Secondary rainbows are caused by a double reflection of sunlight inside the water droplets.