Why does a straw appear to be bent in glass of water?

When you look at a straw in a glass of water, it appears to be bent at the point where it enters the water. This is an optical illusion caused by refraction, the bending of light as it passes from one medium to another.

When light passes from air to water, it bends, or refracts, towards the normal, the imaginary line perpendicular to the surface of the water. This is because the speed of light is slower in water than it is in air. Because the light from the straw is refracted towards the normal when it enters the water, the image of the straw appears to shift upward.

The degree of refraction depends on the angle at which the light hits the surface of the water. Light that hits the water at a steeper angle is refracted more than light that hits the water at a shallower angle. This means that the top of the straw, which is closer to the surface of the water, appears to be bent more than the bottom of the straw.

As a result of refraction, straws appear to be bent in a glass of water. This is an optical illusion caused by the different speeds at which light travels in air and water.