When light travels from air into glass it slows down because glass is more (optically) dense than air.

This change in speed can cause the light to bend at the boundary between the two transparent materials.

The change in direction of a beam of light as it travels from one material to another is called refraction.

The change in direction of a beam of light as it travels from one material to another is called refraction.

The normal is a construction line drawn at right angles to the surface of the glass block.

Angle of incidence i = angle between the incident ray and the normal.

Angle of refraction r = angle between the refracted ray and the normal.

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Angles of incidence and refraction are always measured between the normal and the corresponding ray of light.

Direction of refraction

Glass is denser than air, so when light passes from air into glass it slows down.

If the ray meets the boundary at an angle to the normal, it bends towards the normal.

Light speeds up as it passes from glass into air because air is less dense than glass.

If the ray meets the boundary at an angle to the normal, it bends away from the normal.

The greater the change of speed of light at a boundary, the greater the refraction.

Light is bent more by glass than by water because glass is denser than water and so slows it down more.