Light changes speed depending on the medium

Light does not always travel at the same speed

In a vacuum, light travels at the well-known speed c = 3 × 10⁸ m/s. But as soon as it passes through a material medium (water, glass, air, etc.), it slows down. This slowing down depends on a quantity called the refractive index, denoted by n.

The refractive index

The refractive index n of a medium is defined by:

n = c / v

where v is the speed of light in that medium. As v ≤ c, we always have n ≥ 1. The index n has no units (it is a ratio).

Medium	Index n (approx.)
Vacuum	1
Air	1.00 (almost 1)
Water	1.33
Glass	1.50
Diamond	2.42

Why does light bend?

When a light ray passes from one medium to another (for example, from air to water) at an angle of incidence other than 0, its speed changes abruptly as it crosses the dioptre (the surface separating the two media). This change in speed causes the ray to change direction: this is refraction.

A practical example: a straw submerged in a glass of water appears to be broken at the surface. In reality, the straw is straight, but the light rays coming from it are deflected as they exit the water into the air, which tricks our eye.

A common pitfall to avoid

Do not confuse reflection (where the ray travels back through the same medium, as with a mirror) with refraction (where the ray changes both medium and direction). In many real-life situations, both phenomena occur simultaneously: some of the light is reflected, whilst some is refracted.