HC Verma for Class 11 Physics Chapter 17:Light Waves

Chapter 17 Light Waves concentrates on the phenomenon of light. Light waves are of transverse, electromagnetic nature and are visible to the human eye. This wave nature of light rays was proved through a series of experiments on interference and diffraction. Similar to electromagnetic waves, it is observed that light can also travel through a vacuum. Polarisation clearly shows the transverse nature of light rays. Light is often referred to as 'visible light' to differentiate it from 'Ultraviolet light' and 'Infrared light'.

It is worth noting that light travelling through anything other than a perfect vacuum will scatter off the surfaces of existing (if any) particles. Thus, irrespective of how the speed of light is measured, light has the exact value of speed in a vacuum. When light travels from one medium to another, its velocity and wavelength change, but the frequency remains constant. Suppose light travels from air to a medium having refractive index n. In that case, the speed of light in the given medium is given by the formula: V=n/c, where c is the speed of light in air and n is the index of refraction.

Further, Young's double-slit experiment is also discussed in this chapter. He designed a simple experiment that demonstrated the phenomenon of interference. Monochromatic light, when passing through two narrow slits, is seen to illuminate a distant screen. In this process, a characteristic pattern of bright and dark fringes is formed.

Young has concluded that this interference pattern was caused by the superposition of overlapping light waves originating from the two slits. Regions of constructive interference, corresponding to bright fringes, are produced when the path difference from the two slits to the fringe is an integral number of wavelengths of the light. Destructive interference and dark fringes are produced when the path difference is a half-integral number of wavelengths. Thus, interference is the reason behind the formation of dark and bright fringes.

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