Deviation of Light Rays
Rules for Deviation of Light Rays during Refraction
Light ray passing from rarer (air) to denser (water) medium bends towards the normal at the surface of separation of two media. This makes the angle of incidence i (angle between the incident ray and the normal at the point of incidence) larger than that of the angle of refraction r (angle between the normal and the refracted ray).
Light ray passing from denser (water) to rarer (air) medium bends away from the normal. This makes the angle of incidence i (angle between the incident ray and the normal at the point of incidence) smaller than that of the angle of refraction (angle between the normal and the refracted ray).
Light ray that strikes the surface of separation of the two media normally, does not deviate from its path.
Terms Related to Refraction of Light
Incident ray: The ray which strikes the surface of separation of two optical media
Refracted ray: The ray which travels in the second medium after deviation
Normal at the point of incidence: A perpendicular drawn at the point where the incident ray strikes the surface of separation
Angle of incidence: The angle which the incident ray makes with the normal at the point of incidence
Angle of refraction: The angle which the refracted ray makes with the normal at the point of incidence
Effects of Refraction
Fish in aquarium appear bigger than their original size due to refraction.
When a pencil is dipped partially in a beaker of water, it appears to be bent and short.
A coin kept at the bottom of a bowl appears to be raised.
Laws of Refraction
The incident ray, the normal and the refracted ray, all lie in a plane.
For a given pair of optical media, the ratio of the sine of the angle of incidence to the sine of angle of refraction is a constant, i.e., sin i / sin r = constant (Snell's Law).
The constant is known as the refractive index of the second medium with respect to the first medium.
Refractive Index
The extent to which a light ray bends depends in a medium depends on the refractive index of the medium. The ratio of velocity of light in vacuum to that in a medium, which is the absolute refractive index (m) of the medium, is the measure of the ability of light to bend in the given medium. Measuring speed of light is difficult. Hence, Snell’s law helps to determine the refractive index. According to Snell’s law, the refractive index is given by
n = sin i/sinr
Refraction of light is due to the fact that the speed of light is different in different media. Example The speed of light in air is 3´108 m/s, and in water it is 2.25´108 m/s. The extent of refraction can be found if we know an optical property of the medium, called its refractive index. The refractive index or absolute refractive index n of a medium is defined as the ratio of the speed of light in vacuum to its speed in the medium. It is a pure number and has no unit. Now, if c is the speed of light in vacuum and v the speed of light in a medium, then the refractive index of the medium is given by
n = c/v Note that the speed of light in the medium, v, is inversely proportional to its refractive index N. Example The refractive index of water nw = 1.33 and that of glass ng = 1.52. Since the refractive index of glass, ng, is more than that of water, nw, the speed of light in glass, vg, is less than the speed of light in water, vw.
If the refractive index of a medium is greater than that of another, then the first medium is said to be optically denser than the second.
Example glass is optically denser than water. It is important to note that the optical density of a medium is different from its mass density. The optical density of a medium is related to its refractive index and depends on the speed of light in it, whereas mass density is the mass per unit volume. When light travels from a denser medium, like water, to a rarer medium, say air, its speed increases and it bends away from the normal. In such a case, the angle of incidence, i, will be less than the angle of refraction, r. On the other hand, if light travels from a rarer medium, like air, to a denser medium, say water, its speed decreases and it bends towards the normal. The angle of incidence, i, is greater than the angle of refraction, r. The relationship between the angle of incidence, i, and the angle of refraction, r, is explained by the laws of refraction.
Refraction of Light through A Rectangular Glass Slab
When a light ray, incident at an angle, passes through a glass slab, the emergent ray is shifted laterally. The lateral shift depends on the thickness and refractive index of the glass slab.
When a light ray bends from denser to rarer medium, it bends away from the normal. If the angle of incidence gradually increases, the angle of refraction too increases. At a particular angle of incidence in the denser medium, the refracted ray emerges along the surface. That particular angle is the critical angle. If the angle of incidence is greater than the critical angle, the ray undergoes total internal reflection. It is due to this phenomenon that we observe rainbows and mirages in deserts.
