Electromagnetic Waves

The Electromagnetic Spectrum

Electromagnetic waves are transverse waves. They transfer energy from the source of the waves to an absorber. 

For example, infrared waves are emitted from the sun. The sun is a source of electromagnetic radiation. The infrared waves travel to the Earth and heat up the air. The air is the absorber. 

Electromagnetic waves are transferred by vibrations of electric and magnetic fields. This means they do not require a medium to travel through. Hence, electromagnetic waves can travel through a vacuum such as space. 

There are lots of types of electromagnetic waves that form a continuous spectrum of electromagnetic radiation. Different types of electromagnetic waves have different frequencies, wavelengths and properties. But all electromagnetic waves travel at the same speed – the speed of light. The full spectrum of electromagnetic waves is shown below. 

Humans can only see a very small portion of the electromagnetic spectrum. The waves in this portion are called visible light. 

Refraction of Electromagnetic Waves

When a wave crosses a boundary between two materials it may be refracted. How much the wave is refracted depends on how much the wave speeds up or slows down, which depends on the difference in density between the two materials. 

Previously, we have represented refraction using ray diagrams as shown below. 

Alternatively, refraction can be shown on wave front diagrams, such as the one shown below. 

 On these diagrams, each line represents a wave front. Wave fronts are closer together where the wave speed is decreased which causes the wavelength to be shorter. 

Required Practical 

Investigating Absorption and Emission of Infrared Radiation

A Leslie cube is a hollow metal cube with the following different surfaces on each of its vertical faces: 

• Matt black paint
• Matt white paint
• Shiny metal
• Dull metal

You can investigate infrared (IR) emission and absorption with a Leslie cube using the following process. 

Doing the experiment

1. Set up the Leslie cube on a heat-proof mat with an infrared detector placed 5 \text{ cm} from the surface. 
2.  Fill the Leslie cube with boiling water and place the lid on. Leave for 1 \text{ minute} to allow the surfaces to warm up.
3. Use the infrared detector to measure the amount of radiation emitted from the surface.
4. Repeat the infrared measurement for each of the surfaces. Make sure the distance between the surface and the detector is the same for each measurement.

Analysing the Results

Plot a bar chart of the results. The surface with the greatest infrared emission is the best emitter of infrared radiation. This surface will also be the best absorber of infrared radiation.