Light waves change speed as they pass through the between different substances (or mediums) such as glass, air, and water. These changes in speed cause a change of direction. This is known as .

The phenomenon you're referring to is known as "Refraction". Here's a step-by-step explanation:

1. Light travels at different speeds in different mediums. In a vacuum (like space), light travels at its maximum speed. But when it passes through any medium (like glass, water, or air), it slows down.

2. The change in speed depends on the medium's "refractive index" - a measure of how much the medium can bend light.

3. When light enters a medium with a different refractive index, it changes speed, which causes it to change direction. This bending of light is what we call "refraction".

4. The amount of bending (or angle of refraction) depends on the angle at which the light hits the new medium (known as the angle of incidence) and the refractive indices of the two mediums.

5. This is why a straw in a glass of water looks bent or broken. The light from the straw is refracted, or bent, as it moves from the water to your eyes through the air.