The Atmosphere

relate the blue sky to the preferential scattering of light in its atmosphere
demonstrate an understanding of the benefits of the Earth’s atmosphere to humankind
demonstrate an understanding that the Earth’s atmosphere is transparent to visible light, microwaves and some radio waves
interpret data on the effect of the Earth’s atmosphere on infra-red, ultra-violet and X-rays
describe where infra-red, ultra-violet and X-ray observatories are sited and explain the reasons why

The Earth's atmosphere is about 100km thick. The part of this that we can live in, the troposphere, is only about 14km thick. Compare that to the radius of the Earth which is about 6,500km. If the Earth were the size of a bowling ball then its liveable atmosphere would be the thickness of the varnish on its surface. Yet the atmosphere is of vital importance to us. We could not survive without it. Without carbon dioxide there could be no plant life on Earth. Without the oxygen produced by plants there could be no animal life. Plants were around (in one form or another) around a billion years before any kind of animal life came into being. Without ozone to absorb harmful ultra violet rays from the sun we could not survive.

Obviously visible light can pass though the atmosphere. It is changed though when it does.

Light from the Sun is white. It contains all the colours of the visible spectrum ROYGBIV. When this light passes through gas molecules the colours with a longer wavelength (ROYG) more or less pass straight through. Colours with a shorter wavelength (BIV) are absorbed by gas molecules and then reemitted but in any direction. This process is called Rayleigh scattering.

Unless we look directly at the Sun we do not see the light which has NOT been scattered. We only see the scattered blue light.

pic   Ian Britton - Freefoto.com

Sunrise and sunset is often red because light from the Sun has to travel through much more atmosphere to get to us when the Sun is low in the sky. All the blue that was in this light has been scattered and only the red light remains. This also explains, as we shall see later, why our moon appears red during a lunar eclipse.

Draw a diagram to explain why sunlight has to travel through more atmosphere to get to us at sunrise and sunset.

Other Wavelengths

Visible light is just a very small part of the electromagnetic spectrum. Stars, and other cosmic objects, may emit many other types of electromagnetic wave and much is to be learnt from detecting and studying these also.

	Microwaves can pass through the atmosphere quite easily. This is one of the reasons that satellite signals are in the form of microwaves.
	Some radio waves can also get through. Radio telescopes are very important for observing large scale objects in the cosmos such as galaxies.

What about infra red, ultra violet and X rays? These are also very important to astronomers.

Study the graph below carefully. It shows what wavelengths are absorbed by the atmosphere and what gets through. It also shows what gas in the atmosphere absorbs what wavelengths.

You should see that water vapour is mostly responsible for absorbing infra red. Oxygen and ozone are responsible for absorbing most of the high frequency waves, i.e. ultra violet and x rays.

So where would be the best place to put a telescope that worked in the infra red, ultra violet or x ray region?

Ideally in space above the atmosphere. As well as optical instruments the Hubble space telescope also contains ultra violet and infra red instruments and it is just one of NASA's "Great observatories". These include the Chandra X ray observatory and the Spitzer infra red telescope.   all pics NASA
	Hubble	Chandra	Spitzer

Of course putting a telescope in space is incredibly expensive and not very easy to fix when things don't work.

Many large Earth based telescopes are found on top of mountains, e.g. in Chile. Why?