Refraction of Light
If the light rays are travelling from one medium to another they change their direction at the boundary between two mediums.
Refraction of light
When the light rays either bend or change their direction while passing from one medium to another it is called refraction of light. The refraction of light takes place when light travels from air into glass, from glass into air, from air into water or from water into air.
The example of optical instruments that work on the basis of refraction of light are camera, microscope etc.
Incident ray: The light rays passing from air into glass or water are called incident rays.
Refracted ray: When the light rays bend after passing into another medium, they are called refracted rays.
Normal: The point of incidence is called normal.
Angle of incidence: The angel between incident ray and normal is called angel of incidence.
Angle of refraction: The angel between refracted ray and normal is called angle of refraction.
The angle of refraction is either smaller or greater than angle of incidence.
Causes of refraction
Light travels in different speed in different mediums. For example light travels faster in air than in a glass. Therefore, it is due to the change of speed of light in different medium that the light rays are refracted.
Optically rarer medium
A transparent substance (medium) in which the speed of light is more is called optically rarer medium.
Optically denser medium
A transparent substance (medium) in which the speed of light is less is known as optically denser medium.
Glass is an optically denser medium than air and water.
Rules for refraction of light
Case 1: When light rays travel from optically rarer medium to denser medium then they bend towards normal. In this case angle of refraction is smaller than angel of incidence.
When light rays travel from air into glass or from air into water, it bends towards normal. This is because the speed of light rays decrease while travelling from air into glass or water.
Case 2: When light rays travel from optically denser medium to rarer medium then they bend away from the normal. In this case the angle of refraction is greater than angle of incidence.
When light rays travel from glass into air or from water into air they bend away from the normal. The speed of light rays increase while travelling from glass or water into air.
Case of light going from air into glass and again into air
In this case refraction of light takes place two times. One when it enters the glass slab from air and second time when it enters the air through glass slab.
When light rays travelling through air enters glass slab, they get refracted and bend towards the normal. Now the direction of refracted ray changes again when it comes out of the glass slab into air. Since the ray of light I know travelling from denser medium to rarer medium, it bends away from the normal.
In this case incident ray and the emergent ray are parallel to each other. The perpendicular distance between the original path of incident ray and the emergent ray coming out of the glass slab is called lateral displacement of the emergent ray of light and the angle which the emergent ray makes with the normal is called the angle of emergence.
Light falling perpendicularly on glass slab
When light falls perpendicularly or normally on the surface of a glass slab, it goes straight. There is no bending of ray of light on entering the glass slab or coming out of it. In this case angle of incidence and angle of refraction is zero.
The same happens if the ray of light falls perpendicularly on the surface of water.
Effects of refraction of light
- It is due to refraction of light that when we hold a stick obliquely and partially immersed in water it appears to be bend at the surface of water.
- An object appears to be raised when paced under water.
- Pool of water appears less deep than it actually is.
- If a lemon is kept in a glass of water it appears to be bigger when viewed from the sides of glass.
- It is due to refraction of light that stars appear to twinkle at night.
Laws of refraction of light
1) The incident ray, refracted ray and normal at the point of incidence, all lie in the same plane, i.e. the surface.
2) The ratio of sine of angle of incidence to the sine of angle of refraction is constant for a given pair of media.
Sine of angle of incidence/ sine of angle of refraction = Constant
Constant is called refractive index.
Or Sine i/ Sine r = constant
The refractive index of a medium helps to know the light-bending ability of that medium.
Refractive index and speed of light
Refractive index of medium 2 with respect to medium 1 is equal to the ratio of speed of light in medium 1 to the speed of light in medium 2.
Relative refractive index
When light travels from one medium to another other than vacuum and air, then the value of refractive index is called relative refractive index.
Refractive Index = Speed of light in vacuum/ Speed of light in medium
Or Refractive index = Speed of light in medium 1/ Speed of Light in medium 2
For example, light travelling from water into glass.
Absolute refractive index
When light travels from vacuum to another medium, it is called absolute refractive index.
The substance that has higher refractive index is optically denser than another substance having lower refractive index.
Also, the refractive index for light going from medium 1 to medium 2 is equal to the reciprocal of the refractive index of light going from medium to 2 to medium 1.
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