Refraction is the bending of light when it passes from one medium to another, like from air to water or glass to air.
Why does it bend?
Because the speed of light changes when it enters a different medium.
In air, light travels fastest.
In denser materials like water or glass, it slows down.
This change in speed causes the light to change direction — that's refraction.
In a vacuum, light travels at about 300,000 km/s (kilometers per second) or 3 × 10⁸ m/s.
But in other materials, it slows down:
In water: ~225,000 km/s
In glass: ~200,000 km/s
Formula:
n = c/v
n = refractive index
c = speed of light in vacuum (3 × 10⁸ m/s)
v = speed of light in the medium
Higher n → Slower light → More bending.
Light is an electromagnetic wave. When it passes through a material, it interacts with the atoms, which temporarily absorb and re-emit the light. This delay causes the effective speed to decrease.
But — the light still travels at the same speed between atoms, just gets delayed due to interactions.