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The sky appears blue because of a phenomenon called Rayleigh scattering. When sunlight enters Earth's atmosphere, it collides with gas molecules (primarily nitrogen and oxygen). Sunlight is composed of all colours of the visible spectrum, each with a different wavelength. Blue light has a shorter wavelength (~450 nm) compared to red light (~700 nm), and shorter wavelengths scatter far more strongly — the intensity of scattering is proportional to the inverse fourth power of wavelength. Because of this, blue light is scattered in every direction across the sky, whereas the longer-wavelength red and orange light passes through much more directly. When you look at any part of the sky away from the Sun, you are seeing this scattered blue light. At sunrise and sunset, sunlight must travel through a much greater thickness of atmosphere before reaching your eyes. By then, the blue light has been scattered away almost entirely, leaving the longer-wavelength reds and oranges to dominate, which is why sunsets appear red and orange.
answered by Omniscientia Team · 152 words · 18 Mar 2026