In Class 10 Physics (The Human Eye and the Colourful World), one of the most beautiful and visual topics is the Scattering of Light. This single physics phenomenon is the exact reason why the sky is blue, why clouds are white, and why sunsets are a gorgeous fiery red.
If you were standing on the Moon, the sky would look pitch black even during the daytime! This is because the Moon has no atmosphere (no gas particles). With no particles to scatter the sunlight, the sky remains completely dark.
Light usually travels in a perfectly straight line. However, the Earth's atmosphere is filled with microscopic obstacles like dust, smoke, water droplets, and gas molecules (Nitrogen and Oxygen).
Definition: When a beam of light strikes these tiny atmospheric particles, the particles absorb the light and instantly re-emit it, throwing (scattering) the light in all random directions. This process is called the Scattering of Light.
A physicist named Lord Rayleigh discovered a mathematical rule for scattering. He proved that the amount of scattering depends on the wavelength (color) of the light.
When pure white sunlight enters the Earth's atmosphere, it hits the gas molecules. Because blue light has a very short wavelength, the gas molecules scatter the blue light aggressively in all directions across the sky. When you look up, your eyes detect this scattered blue light coming from everywhere, making the sky appear blue.
During a sunset, the sun is very low on the horizon. Sunlight has to travel through a much thicker layer of the atmosphere to reach your eyes. Because the journey is so long, almost all the blue light gets scattered away and lost in space. Only the longest wavelengths (Red and Orange) are strong enough to survive the long journey and reach your eyes, making the sun look red.
Clouds are made of large water droplets. Unlike tiny gas molecules that only scatter blue light, these large water droplets are big enough to scatter *all* the colors of sunlight equally. When all colors (VIBGYOR) mix together, our eyes perceive it as pure white.
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