CIE A Level Physics复习笔记7.2.1 Electromagnetic Spectrum
Properties of Electromagnetic Waves
- Visible light is just one part of a much bigger spectrum: The Electromagnetic Spectrum
- All electromagnetic waves have the following properties in common:
- They are all transverse waves
- They can all travel in a vacuum
- They all travel at the same speed in a vacuum (free space) — the speed of light 3 × 108 ms-1
- The speed of light in air is approximately the same
Oscillating electric and magnetic fields in an electromagnetic wave
- These transverse waves consist of electric and magnetic fields oscillating at right angles to each other and to the direction in which the wave is travelling (in 3D space)
- Since they are transverse, all waves in this spectrum can be reflected, refracted, diffracted, polarised and produce interference patterns
Uses of electromagnetic waves
- Electromagnetic waves have a large number of uses. The main ones are summarised in the table below
Exam Tip
You will be expected to recall the common properties of all electromagnetic waves in an exam question, however the speed of light will be given on the data sheet.
From Radio Waves to Gamma Rays
- The electromagnetic spectrum is arranged in a specific order based on their wavelengths or frequencies
- This order is shown in the diagram below from longest wavelength (lowest frequency) to shortest wavelength (highest frequency)
Energy, wavelength and frequency for each part of the electromagnetic spectrum
- The higher the frequency, the higher the energy of the radiation
- Radiation with higher energy is highly ionising and is harmful to cells and tissues causing cancer (e.g. UV, X-rays, Gamma rays)
- The approximate wavelengths in a vacuum of each radiation is listed in the table below:
EM spectrum wavelengths and frequencies
- To alternatively find the range of frequencies, convert the wavelengths using the wave equation: c = fλ where c is the speed of light: 3.0 × 108 m s-1
Worked Example
A is a source emitting microwaves and B is a source emitting X-rays.The table suggests the frequencies for A and B. Which row is correct?
Exam Tip
You will be expected to memorise the range of wavelengths for each type of radiation, however you don’t need to learn the frequency ranges by heart. Since all EM waves travel at the speed of light, you can convert between frequency and wavelength using the wave equation in an exam question.
Visible Light
- Visible light is defined as the range of wavelengths (400 - 700 nm) which are visible to humans
- Visible light is the only part of the spectrum detectable by the human eye
- However, this is only 0.0035% of the whole electromagnetic spectrum
- In the natural world, many animals, such as birds, bees and certain fish, are able to perceive beyond visible light and can see infra-red and UV wavelengths of light
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