Saturday, November 29, 2008

Why is the sky blue? Why is sunrise and sunset red?

Why is the sky blue? Or rather, why does sky appear blue?

Before understanding that, you need to know that visible light is formed by 7 coloured lights, namely, red, orange, yellow, green, blue, indigo and violet (the colours of rainbow). In that order, red has the longest wavelength and lowest frequency with violet on the reverse.

When light passes throught our atmosphere during the day, it collides with oxygen and nitrogen in the air. Light with the shortest wavelength (blue and violet) are scattered more by the collision and hence the sky will appear more blue and violet. However, our eye is more sensitive to blue light than violet, therefore we perceive sky as blue.

Why is sunrise and sunset red or reddish in colour?

During sunrise and sunset, the light from the sun has to travel a greater distance through the atmosphere to reach our eye than during the day. You probably look at my ugly drawing below to understand. Red line is the distance travelled by light through the atmosphere during sunrise while blue line is the distance travelled by light during the day

As such, by the time the light reaches our eyes, much of blue light has already been scattered (the blue light may have been scattered more than twice), making red light the dominant colour. Hence, sunrise will appear more reddish in colour and gradually more yellowish. The reverse for sunset.

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