CIE A Level Physics复习笔记7.2.1 Electromagnetic Spectrum

• 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

• 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

• 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