Sunsets appear red primarily due to the scattering of light as it passes through the Earth's atmosphere. When the sun is low on the horizon, its light travels through a greater thickness of atmosphere, encountering particles like dust and water vapor. This scattering process disperses shorter wavelengths of light, such as blue and green, while allowing longer wavelengths, like red and yellow, to dominate our view. Additionally, the angle of sunlight affects the amount of scattering, intensifying the reddish hues at sunset. This phenomenon also explains why the sky appears blue during the day, as shorter wavelengths are scattered more effectively.