Question

what is the cause of dispersion of light through a glass prism?

Answer 1

because of the structure of the prism

Answer 2

Cause of dispersion of white light:

In vacuum, light rays of different colours travel with the same speed. But, in refracting media like glass, water, etc., lights of different colours travel with different speeds.
The speed of violet colour is the least while the speed of red colour is the greatest in glass. So, the refractive index of glass is largest for violet colour and least for red colour.
On passing through the prism, the violet colour is deviated most while the red colour is deviated least. Other colours are deviated by intermediate angles.
So, different component colours of white light get dispersed on passing through a glass prism.