# HC Verma for Class 11 Physics Chapter 20:Dispersion and Spectra

Chapter 20 Dispersion and Spectra focus on different phenomena involved in light, such as refractive index, focal length, etc. When white light falls on a screen after passing through a prism, the seven colours, i.e., VIBGYOR, are mainly seen. This band of colours observed on the screen is called a spectrum. This is because the prism splits white light into different colours. This splitting of white light into its constituent colours is called dispersion.

Refractive Index (Index of Refraction) is the ratio between the speed of light in a vacuum and the speed of light in the second medium of greater density. Few other important concepts of this chapter include angular dispersion and dispersion power of prism.

Angular dispersion can be defined as the difference in the angles of deviation of two extreme colours in the spectrum of white light. The dispersion angle contains all the colours in it that are obtained by the dispersion of white light.

The prism's dispersive power indicates the difference in refraction between the highest and lowest wavelengths of light entering the prism. It can be expressed in terms of the angle between the two extreme wavelengths.

The distance noted between the lens and the image sensor when the subject is in focus is termed the lens's focal length. It is usually stated in millimetres. It is worth noting that a diverging lens's focal length and power is usually negative in value. For instance, if the distance to F is 5.00 cm, then the focal length is f=–5.00 cm, and the power of the lens is P=–20 D.