Question

what causes refraction of light when it passes from one medium to another.?

Answer 1

speed of light causes refraction of light when it passed from on medium to the another

Answer 2

It is due to the difference in Optical Density of the two mediums that causes Refraction when light passes from one medium to the other. Optical Density is not the same as the Physical density.

To make things more clear for you:
Refraction in a medium, as we know, depends on the speed of light in that medium. Light travels slower in optically dense mediums, and faster in optically rarer mediums.

Speed of light = frequency X wavelength

In a ray of light travelling from one medium to another medium, Speed of light changes because wavelength changes, but frequency does not change. (Frequency is a unique property, which determines the type of electromagnetic radiation. It doesn't change because of a ray of light cannot change its property i.e. a UV ray remains a UV ray even after Refraction, a Visible Ray remains a Visible ray even after Refraction).

Hence, we can say that the Extent of Refraction depends on the change in Speed of light which is due to the change in the wavelength, and this change directly depends on the difference in optical density of the two mediums.