Chapter 5: Light and electromagnetic waves

The chapter problem: the moorland observatory

An observatory stands on a moor far from any town. It studies the sky with radio waves, infrared radiation and visible light. Each kind of radiation carries different information about the universe.

These are the facts to use all the way through the chapter.

• The radio telescope receives a signal with a frequency of 150 MHz.
• An infrared camera detects radiation with a wavelength of 10 μm.
• A visible-light image uses light with a wavelength of 600 nm.
• The observatory walls are painted white.
• The visitor centre uses a lens to project a picture of the Moon. The picture is 2.0 cm tall, and its image on the wall is 0.30 m tall.
• Electromagnetic waves travel at 3.0 × 10⁸ m/s in a vacuum.

By the end of the chapter you will work out the wavelength of the radio signal, the frequencies of the infrared and visible radiation, and the magnification of the projected picture. You will place all three kinds of radiation in the electromagnetic spectrum. You will also explain why the white walls keep the building cooler than black walls would.

First contact: no working yet. Estimate. The radio signal has a frequency of 150 million hertz. Is its wavelength closer to 20 cm, 2 m or 20 m? Write your estimate down.

By the end of this chapter

You will be able to:

• describe the electromagnetic spectrum, and list its groups in order of wavelength and frequency;
• state the properties that all electromagnetic waves share;
• use the wave equation $v = f\lambda$ with electromagnetic waves;
• describe how electromagnetic radiation is absorbed, reflected, transmitted and refracted;
• describe the uses and the dangers of each group of electromagnetic waves;
• describe the action of convex and concave lenses, and draw ray diagrams;
• calculate the magnification of a lens;
• explain the colour of an object in terms of absorption and reflection;
• explain how the temperature of an object affects the radiation it emits.

Where it is used

Light and the rest of the electromagnetic spectrum carry almost everything we know about the world beyond the Earth. Radio telescopes map distant galaxies. Infrared cameras see warm objects in the dark. X-rays show a broken bone inside the body. Visible light gives us colour, and the colour of a star tells us how hot it is.

Your turn to connect it

Where in your own week do you use light or another kind of electromagnetic radiation? Write two examples. One should be a time you used light to see, and one should be a device that sends or receives a signal through the air.

Roadmap

1. 5.1 The electromagnetic spectrum
2. 5.2 Interactions of electromagnetic radiation with matter
3. 5.3 Lenses and ray diagrams
4. 5.4 Colour and frequency
5. 5.5 Black body radiation
6. Review