Let's perform an experiment,

Take a glass jar, fill 3/4 of it with water and dip a straw or stick as shown in the figure.

What do you observe?

Yes, at the separation of the two mediums (here, Air - Water), the straw seems to be bend.

Such optical phenomenon is called "REFRACTION".

But wait! 

Will there be a deviation in the direction of a ray of light incident normally?

There was no refraction. Right?

What causes a deviation in the path of the light ray when it passes from one medium to another?

Yes, It's the speed of light through each medium.

The speed of light is different for different mediums. The speed of light through a medium depends on the optical density of the medium.

Remember! This optical density is not related to the material density.