State the cause of refraction.

Hint:To solve the question we have to know about the refraction. What refraction is. We know that, while passing from one medium to another if light slows down the second medium is said to be optically denser than the first medium. And if the light speeds up the second medium is said to be optically rarer than the first medium. Thus we can say, water and glass are optically denser than air.

Complete answer:
We know that the change in direction of the path of light when it passes from one transparent medium to another transparent medium is called refraction. Refraction of light is essentially a surface phenomenon. We know that, when a ray of light passes from one medium to another medium its direction changes because of the change in speed of light in going from one medium to another.

In passing from one medium to another if light slows down it bends towards the normal. And if it speeds up it bends away from the normal. We know that,If the speed of light remains the same in passing from one medium to another the light rays do not change. It passes undivided.

Note: From above information we can say, glass is optically denser than water. We can say that the reflected ray from the boundary surface will not be shown although it is always there. So, we know that, when a ray of light gets refracted from rarer to denser medium, the speed of light decreases.