#### • Class 11 Physics Demo

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# Angle of Deviation Prism

Angle of deviation prism is an important tool to calculate the angle of deviation. Two important principles are involved in understanding the concept of angle of deviation prism, which are as follows:-

• Reflection and
• Refraction.
The second concept is more important to understand the angle of deviation prism.  This concept is actually occurred in the prism when the ray of light is passed into the prism.  In terms of refraction we can easily describe the angle of deviation prism.  Basically the total amount of refraction in ray of light through a prism when a ray of light is passed through the prism is usually expressed in terms of the main subject that is angle of deviation. If we will describe in a detailed way then its definition is as follows:-

“It is defined as the angle between the incident ray of a light which is entering in the first face or side of the prism and the refracted ray that is coming out from the second face or side of the prism.”

To understand the angle of deviation in prism it is important to understand why the refraction occurs in the prism. The simple answer is because of difference in velocity of light inside and outside of the prism. By inside and outside, we mean that the rarer medium and denser medium respectively. In the rarer medium the velocity of light is always higher than the denser medium.  In the case of prism air is the rarer medium and prism itself is denser medium. So when a ray of light is entering inside the prism, then the speed of the light decreases and hence it is refracted that is bending with some angle. So this is basic phenomenon to understand the concepts of angle of deviation in prism.

It is also important to note that the angle of deviation in prism varies with the wavelength. This is because the refraction indices are different for different wavelengths. So in the visible light spectrum, the colors which have longer wavelength deviates more. And the colors which have shorter wavelength deviates less.  So it is clear that there are various concepts which need to be understood before understanding the angle of deviation in prism. Two more important points which we shall now as follows:-

• Speed of light in vacuum or air = 3 x 10 ^8 m/s.
• Speed of light in prism = 2 x 10 ^8 m/s (approximately)
This difference of speed of light in vacuum or prism is actually related to the phenomenon refraction which in turns related to the angle of deviation prism. If this difference is not there then there would not be any refraction and finally there will not be any term like angle of deviation.

At last it is important to note that the angle of deviation can be calculated or it can be measured with the help of spectrometer. And also it is important to note that with the help of formulas and laws of refraction one can easily find the angle of refraction also.