LS1151A Light microscopy electronic guide-general concepts-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

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

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

                                                Example

 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.