Hi there,

I'm seeing this and similar rainbow reflections in one of dance studios around 4-5pm and at home when the sunbeams pass through the faceted clear crystal that I hang sometimes on my window. In the studio it's usually shimmering and I have to find the right angle to see it. At home, it's just there emiting colorful drops like a discoball.

I was researching how it actually happens and what I found was interesting to me.
So, sunlight is composed of various color waves that together make it white (we know that). When the sun beam changes the mediums (air to glass), light waves slow down. Different color waves have different frequencies and therefore pass with different speeds. The waves with higher frequencies move faster and refract the least and the waves with lower frequencies move slower and refract the most. This causes the "white" beam to be broken down into different light colors when they pass through glass. When it passes through trism is becomes even more evident. In case of the dance studio air, window glass and mirror serve as different mediums and create this kind of beautiful rainbow. Science!