Chapter 7

Chapter 7 – The Electromagnetic Spectrum. Here are the questions and answers. If you spot anything that looks wrong let me know, because I want to know!   🙂

1. a) What is meant by wavelength?
The distance from a certain point on one wave to the same point of the next wave.
E.g. From on wave crest to another.

b) Which is longer – a nanometer  or an Ångström?
A nanometer is longer than an Ångström. A nanometer is one thousand-millionth of a metre, and there ten Ångström in a nanometer.

c) What is the difference between wavelength and frequency?
Wavelength is the length of a wave, whilst frequency is the amount of waves that pass a certain point in one second. This is measured in Hz. E.g If 3 waves passed me in one second, then that wave is 3Hz.

2. a) Why does the detector of an infra-red telescope have to be kept very cold?
The detector has to be kept cold, so it will not interfere with the infra-red emissions it detects from the Universe.

b) Why is UKIRT, the United Kingdom Infra-red Telescope, set up on the top of Mauna Kea in Hawaii?
Because the water vapour in the air blocks the infra-red emissions from the Universe, the telescope had to be built at a high altitude, and the top of Mauna Kea is 4205 metres high.

c. Why do we have to use rockets and space-probes for X-ray astronomy?
We have to use rockets and space-probes for X-ray astronomy because the atmosphere completely blocks all traces of them. So to escape that problem, we must send equipment above the atmosphere, using rockets and space-probes.

3. Arrange the following wavelengths in to order of length, beginning with the shortest:
Infra-red, visible light, ultra-violet, radio waves, microwaves, gamma-rays, X-rays.

Gamma-rays, X-rays, Ultra-violet, Visible light, Infra-red, Microwaves and Radio waves.

4. Describe the main principle of a radio telescope such as the Lovell Telescope at Jodrell Bank.

The main principle of a radio telescope, such as the Lovell Telescope, is to collect radio waves and present them onto a graph. The term radio telescope, however, is a little misleading as you do not receive any optical type pictures, until all the data has been collected. Then a computer may have the chance to generate an image.Jodrell Bank, Cheshire, UK

5. Explain the principle of the Doppler effect. If a source of light shows a red shift, is it approaching or receding from the observer?
If a police car is moving towards you, the closer it gets, the higher the pitch of the noise. Once it has passed, the noise starts to lower again. This is because when it is moving towards you, the wave lengths are being squashed together causing them to shorten. That increase the frequency and makes a higher pitched noise. When the noise is moving away, then the wave lengths are being stretched. This lengthens the waves and slows the frequency. this lowers the pitch. It works the same for light, only if it is approaching, then it will look more blue than it usually is (blue shift) and if it is receding then it will look more red than it usually is (red shift).

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