Definition: Refraction (optics)

Definition: Refraction (optics) | Glossary entry

Refraction (optics)

Refraction is the phenomenon that causes a light ray to bend when it hits an optical lens surface (or other medium that can act to transmit light e.g. water).

Refraction occurs when light strikes the surface of the new medium. As the light enters a dense medium is slows the light wave down. The light is passing from an optical medium (like air) to another optical medium (like glass). As the density of the medium changes the light striking the surface is refracted. The refraction is most obvious when when the light strikes the new medium at an angle. This is because one side of the light wave hits before the other side of the wave. The result is to redirect the wave as it swings around on the slower side of the beam.

Refraction definition: Diagram showing how refraction works

• Diagram showing how refraction works •
Refraction definition: The light wave bends because light moves slower in glass than air. As one side of the wave hits the glass the other side of the wave is still moving fast – it swings the wave around.


When the light leaves the glass the wave speeds up on the side that leaves the glass first causing more deflection until the other side of the wave leaves the glass.

If light strikes the surface on a perfect perpendicular, it will pass straight through without bending – refraction still occurs (see diagram: • Simple lens details and principle rays • below).

Simple lens details and principle rays

• Simple lens details and principle rays •
Click image to view large


The path of the principle rays (in red in the diagram above) shows the refracted path of the light as it passes through the lens.

Refraction sometimes causes the colours in the light to split. This causes some colour aberrations which may be seen on the image. In a camera these are normally corrected by additional glass elements.

A detailed and technical explanation of refraction is beyond the scope of this short article. More information: Refraction From Wikipedia  External link - opens new tab/page.

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