Explain how primary rainbow is formed …

Rainbow

• Rainbow is a phenomenon due to combined effect of dispersion, refraction and reflection of sunlight by spherical water droplets of water.
• Rainbow appears when the sun is shining in on one part of the sky (say near western horizon) while it is raining in the opposite part of the sky (say eastern horizon).
• Sunlight is first refracted as it enters a raindrop, which causes the different wavelengths (colours) of white light to separate.
• Longer wavelength of light (red) are bent the least while the shorter wavelength (violet) are bent the most.
• The rays strike the inner surface of the water drop and get internally reflected if the angle between the refracted ray and normal to the drop surface is greater than the critical angle (48º, in this case).
• The reflected light is refracted again as it comes out of the drop as shown in the figure.