CBSE Class 7 Answered

Propagation of light from one place to another place  is a wave phenomenon that doesn't need a medium through which to propagate.

In the mid 1800s, Scottish physicist James Clerk Maxwell established that light is a form of electromagnetic energy that travels in waves. But unlike other waves like sound waves which require a medium to propagate, light does not require a medium to propagate. No requirement of a medium for light propagation is explained by the nature of electromagnetic vibrations. When a charged particle vibrates, it produces an electrical vibration or oscillating electric field that automatically induces a oscillating magnetic one. This oscillating magnetic one again produces oscillating electric field so on, thus light propagates through electric and magnetic field produced by light itself.-- physicists often visualize these vibrations occurring in perpendicular planes. The paired oscillations propagate outward from the source; no medium, except for the electromagnetic field that permeates the universe, is required to conduct them.

When an electromagnetic source generates light, the light travels outward as a series of concentric spheres spaced in accordance with the vibration of the source. Light always takes the shortest path between a source and destination. A line drawn from the source to the destination, perpendicular to the wave-fronts, is called a ray. Far from the source, spherical wave fronts degenerate into a series of parallel lines moving in the direction of the ray. Their spacing defines the wavelength of the light, and the number of such lines that pass a given point in a given unit of time defines the frequency.