Sunday, January 11, 2015

Refraction of Light and Snell's Law

The phenomena of the light going into the other medium after striking a boundary separating the two media is called refraction.

When a light travels from one medium to another medium, its speed and wave length will change. But the frequency of the light is not going to change even when the medium is changed. It is just because frequency is the characteristic property of the source and it is independent of the medium. Frequency can be changed only when the source is changed.

The direction of the propagation of the light also changes when the light changes its medium. The phenomenon of transmission of light from one medium to another with the change in speed is called refraction.

Refraction at a plain surface

The reflected light will change its path as per the change of the medium.

When a light rays moving from a rarer medium to denser medium it moves towards from the normal. It means angle of incidence is more than angle of refraction.

When a light rail is moving from denser medium to rarer medium, it moves away from the normal. In this case angle of refraction is more than angle of incidence.

In the following diagram the path followed by the light on the angle of deviation is shown.

Laws of refraction

The incident ray, reflected ray and the normal lies in the same plane.

During refraction the ratio of sin angle of incidence to the sin angle of the fraction remains constant for a given pair of media and for a given wavelength. This la is called Snell’s law.

This ratio is also called refractive index. This refractive index is defined for a pair of media. When no second medium is given by default, it can be understood that the second medium is vacuum.

The ratio is the refractive index of the second medium to where the light is going with respect to refractive index of the first medium from where the light is coming.
The angle of incidence shall be taken in the first medium from where the light is coming and angle of refraction shall be taken in the second medium to where the light is going.

Being frequency is constant, the ratio of refractive indices can be written in the inversely proportional to the velocity of the light in the corresponding media. It is simply because if the refractive index is more velocity of the light is less in that medium and vice versa.

If the second medium is taken as vacuum than the refractive index is called a absolute refractive index of the medium.

If the light is taking equal time in travelling different distances in different medium, then we can prove that the product of refractive index in the distance is constant. It is proved in the following diagram.

If the light travels same distance into different media into different times then we can prove that the time taken to travel is directly proportional to refractive index.

Refraction doesn't takes place if angle of incidence and angle of refraction are equal to 0.
Refraction don’t takes place when the two mediums are having same refractive index that means there shall be a change in the media for the reflection to takes place.

Optical path is defined as the distance traveled by the light in vacuum in the same time in which the travels in a medium.

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