Q.Explain the formation of rainbow.

Sunlight entering a raindrop is refracted, then totally internally reflected off the back of the drop, then refracted again on exit; because refraction depends on wavelength (dispersion), different colours emerge at slightly different angles, producing the rainbow.

A rainbow is a natural phenomenon caused by the combined effect of refraction, total internal reflection, and dispersion of sunlight by spherical water droplets suspended in the air after rain.

Primary rainbow: Sunlight entering a water droplet is first refracted at the front (air-to-water) surface. Since the refractive index of water is slightly different for different wavelengths (violet bends more than red), the light disperses into its component colours. This dispersed light then undergoes one total internal reflection at the back inner surface of the drop, and is refracted again as it exits the front surface toward the observer. Because of the geometry of this double refraction plus one internal reflection, red light emerges at a slightly larger angle (about 42°) from the antisolar point than violet light (about 40°), so an observer with the sun behind them sees red on the outer edge of the arc and violet on the inner edge.

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