LS1151A Light microscopy electronic guide-general concepts-refraction

1/3 What is refraction

2/3 Refraction in a biconvex lens

3/3 Refractive index

  • Created by: mat
  • Created on: 26-11-11 19:21

What is refraction?

Refraction is bending of a light ray when it passes from one medium (e.g. air) to another medium of a different density (e.g. glass).

Light slows down when it passes into a denser medium and speeds up when it travels into a < dense medium.

If a light ray enters at an angle (except perpendicular), the direction of the ray changes.

          2 factors therefore contribute to the degree of refraction:

1) comparative densities of the two media – the greater the difference in density, the greater the degree of bending;

2) the angle at which the light ray strikes the second medium – the greater the angle, the greater the degree of refraction

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Refraction in a biconvex lens

When light hits the curved surface of an object of greater density, the direction of refraction depends on the angle of curvature of the object.

The > the curvature, the > the degree of bending.

A biconvex object like a microscope/ human eye lens, refracts light rays that enter it like a series of prisms and then converges the rays onto a single focal point (F) :

1 parallel light rays strike lens

2 focal length (f) is the distance from centre of lens to focal point

3 the focal point (F) is where the image forms

Conversely, a concave surface diverges light rays (spreads them apart).

Therefore concave lenses are used for correcting refractive errors of the eye e.g. myopia

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Refractive index

The refractive index (RI) of a substance is a measure of the speed of light in that substance.

 It is expressed as a ratio speed of light in vacuum : speed of light in medium.

simple mathematical description of RI: n = light velocity of vacuum / light velocity of medium


 RI of glass is ~1.5, meaning that light travels ~ 1.5x faster in  vacuum than in glass.

This is because when light travels from air (RI = 1.000277) into glass (RI = ~ 1.5), it is slowed and refracted.

When light leaves glass and returns to air, it accelerates and is refracted again.

Cover glass slips used in microscopy have an RI of ~1.518.

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