Refraction of Light
Refraction is the bending of light rays when it passes from one optical medium to another.
Light travels at different speed in different optical medium. It travels fastest in vacuum / air.
Optical Density is a measure of how much the speed of light is slowed down when it passes through a medium.
When a light ray incidents perpendicular to the boundary of two mediums, it will not be refracted. It simply passes through the medium.
When a light ray incidents at an acute angle to the boundary,
Note that light is both reflected and refracted at the boundary of two optical mediums.
You can play with the applet below to have a visual understanding on the concept of Refraction of Light.
Two Laws of refraction
First Law: The incident ray, the normal and the refracted ray all lie in the same plane.
Second Law: For two particular mediums, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.
i.e, sin i⁄sin r = constant
Refractive index of a medium is given as
It is also defined as the ratio of the speed of light in vacuum or air (c) to the speed of light in medium (v). i.e n = c⁄v
(1) Refractive index is
(2) A medium with a higher refractive index is optically denser, and light travels slower in it.
The diagram shows the path of a light ray travelling through three different transparent medium P, Q and R. Which medium have the lowest and highest refractive indices respectively?
When light ray passes from P to Q, it bends away from Normal => P is denser than Q;
When light ray passes from Q to R, it bends towards the Normal => R is denser than Q;
When light ray passes from R to P, it bends towards the Normal => P is denser than R.
Optical density: P > R > Q
Speed: P < R < Q as light travels slower in optically denser medium
Refractive Index: P > R > Q as n = c / v
When doing calculation, be very careful with the formula of refractive index! The numerator is always the sine of angle in air or vacuum. Also, angle of incidence and refraction are angles between the normal and light rays, not between boundary and light rays.
Effect of Refraction
The effect of refraction can cause human have a misperception of depth. Also, the image is distorted and it is not always directly above the object although some textbooks stating otherwise. Refer to this applet to have a better understanding on this.