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
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
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.