CHAPTER 16. OPTICAL PHENOMENA; PROPERTIES OF MATTER 16.2
Why is the sky blue? ESCHE
The sun emits light at many different wavelengths, including all of thevisible wave-
lengths. Light which is made up of all the visiblewavelengths appears white. So what
causes the sky to look blue?
The atmosphere consists of molecules of different gases as well as tiny dust grains.
Light from the sun scatters off the molecules in the air (called Rayleigh scattering). The
chance that the light will scatter off the gas molecules is higher for shorter wavelengths.
The short wavelength blue light is therefore scattered more than the other colours.
At noon, when the light from the sun is coming straight down (see thepicture), the
scattered blue light reaches your eyes from all directions and so the skyappears blue.
The other wavelengths do not get scattered muchand therefore miss yourline of sight
and are not seen. At sunrise or sunset, the direction of the light coming from the sun is
now straight towards your eyes (see the picture). Therefore the scattered blue light can’t
be seen because it is scattered out of your lineof sight. The redder colours (oranges
and reds) can now be seen because they are notscattered as much and still fall in your
line of sight.
At noon, sun is overhead
sun emits radiation
of all wavelengthsblue light scattered
by large angle
into the eye
red light scattered little
and misses the eyeAt sunrise/sunset, sun isat horizon
blue light scattered
by large angle
away from the eyered light scattered
into the eyeExercise 16 - 1
- Explain how visible light is reflected from metals.