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What is refraction?

Refraction is the change in direction of a wave as it crosses a boundary between media, caused by a change in its speed.

Refraction explained

Light slowing down bends towards the normal; speeding up bends away. If the angle of incidence is zero, the ray passes straight through with no bending.
Beyond the critical angle no refraction occurs and total internal reflection takes over — the principle behind optical fibres and endoscopes.

Key formula

n1sin⁡θ1=n2sin⁡θ2n_{1}\sin\theta_{1} = n_{2}\sin\theta_{2}

Worked example

Light entering glass (n=1.5n = 1.5) at 40∘40^{\circ} refracts to about 25∘25^{\circ} from the normal.

Examiner tip

Angles are always measured from the normal, never from the surface. Mark the normal with a dashed line on every ray diagram.

Refraction: common questions

What is the critical angle?
The angle of incidence in the denser medium at which the refracted ray travels along the boundary; beyond it, total internal reflection occurs.
Why does a straw look bent in water?
Light from the submerged part refracts as it leaves the water, so your eye traces it back along a shifted straight line.

Related terms

Wave speed

Wave speed is how fast a wave transfers energy through a medium, equal to frequency multiplied by wavelength.

Frequency

Frequency is the number of complete waves or oscillations passing a point each second, measured in hertz.

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