Here's why:
* Refraction is the bending of light as it passes from one medium to another. This bending occurs because the speed of light changes as it moves from a medium with one refractive index to another with a different refractive index.
* Refractive index is a measure of how much light slows down when it enters a medium.
* When light enters a denser medium (higher refractive index), it slows down and bends towards the normal (an imaginary line perpendicular to the surface). When it enters a less dense medium (lower refractive index), it speeds up and bends away from the normal.
Examples of refraction:
* A straw in a glass of water appears bent because the light from the straw is refracted as it passes from water (higher refractive index) to air (lower refractive index).
* Rainbows are formed by the refraction and reflection of sunlight through water droplets in the atmosphere.
So, observing the change in direction of light (refraction) as it enters a different medium is a direct indication of its change in speed.
