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MCAT Content / Geometrical Optics / Refraction Refractive Index N Snells Law N1 Sin 01 N2 Sin 02

Refraction, refractive index n; Snell’s law: n1 sin θ1 = n2 sin θ2

Topic: Geometrical Optics

Refraction: when light ray changes its direction in a new medium depending on the incident angle and the changes in speed. n1sinθ1 = n2sinθ2

A ray of light changes direction when it passes from one medium to another. The angle of incidence (towards the surface, the incidence ray) and angle of refraction (of the refracted ray) are measured relative to a perpendicular to the surface at the point where the light ray crosses it. This perpendicular line is called the normal line – if light enters a medium with a higher density, it bends towards the normal line.

In the image below, the angle of incidence and the angle of refraction are respectively θ1 and θ2.

The change in the direction of the light ray depends on how the speed of light changes, which is related to the indices of refraction of the mediums. In mediums that have a greater index of refraction, the speed of light is less. Here n1 and n2 are the indices of refraction for medium 1 and 2.

n = c/v,

where v is the speed of light in the material, c is the speed of light in vacuum

For a ray at a given incident angle, a large change in speed causes a large change in direction, and thus a large change in refraction angle. This is expressed in Snell’s Law:

n1sinθ1 = n2sinθ2


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Key Points

• The change of a light ray’s direction (loosely called bending) when it passes through variations in matter is called refraction.

• The index of refraction is n=c/v, where v is the speed of light in the material, c is the speed of light in vacuum, and n is the index of refraction.

• Snell’s law of refraction: n1sinθ1=n2sinθ2.

Key Terms

Incidence ray: light ray towards the surface between 2 mediums

Refraction ray: light ray outgoing from surface between 2 mediums

Normal line: perpendicular line where two surfaces meet

Index of refraction: for a material, the ratio of the speed of light in vacuum to that in the material

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