As day breaks, a reddish-white ball of light that we call the sun, rises. This red haze gives way to a golden-white sheen as the day advances and reappears when the sun sets in the evening. Does the sun really change colours, or does it only seem so? The explanation lies in-between.
Light from the sun consists of three kinds of radiation - ultra-violet rays, visible light and infra-red rays. Unlike visible light, ultra-violet rays and infra-red rays cannot be seen.
The visible light's rays are white and consist of seven colours - violet, indigo, blue, green, yellow, orange and red (known simply as VIBGYOR).
The earth's dust, gas and air particles scatter the sun's light in different directions. Violet light has a peculiar property due to which it gets scattered the most. Red light gets scattered the least.
The rising and setting sun
At sunrise the earth is rotating towards the sun. Similarly, during sunset, the earth is rotating away from the sun. During these times the sunlight has to travel at an angle and thus cover a longer distance through the atmosphere. It encounters more obstacles in its path. Most of the colours, like green and violet, get scattered before the light reaches the earth. Since red is scattered the least, it is this colour that we see as it reaches us.
During noontime, the sun is directly overhead and it appears white. This is because the sunlight has to cover a much lesser distance through the atmosphere and it faces less obstacles. Hence, it escapes a great deal of scattering.
In fact, if we were to calculate the distance the sun's rays have to travel through the denser part of the atmosphere (roughly around 150 kms) to any point on earth, we would find that this journey of the rays is almost 50 times more at sunrise and sunset than at noon.