Waves & The Electromagnetic Spectrum

Wavefronts and rays

• A wavefront is a line joining all the points of a wave that are currently in step (for example, every point at the top of its peak)
• Wavefronts are usually drawn from above, like contour lines on a map:
  • Closely spaced wavefronts indicate a short wavelength
  • Widely spaced wavefronts indicate a long wavelength
• A ray is an arrow drawn perpendicular to the wavefronts, showing the direction of travel of the wave

Amplitude

• The amplitude (A) of a wave is the distance from the undisturbed (rest) position to either a peak or a trough, not the full peak-to-trough height
• Amplitude is measured in metres (m)
• The amplitude of a wave is a measure of how much energy it carries; a wave with twice the amplitude transfers roughly four times the energy in the same time

Wavelength

• The wavelength (λ) of a wave is the distance from one point on the wave to the same point on the next wave:
  • in a transverse wave, peak-to-peak (or trough-to-trough)
  • in a longitudinal wave, centre of one compression to centre of the next
• Wavelength is measured in metres (m) and uses the Greek letter lambda (λ)

Frequency

• The frequency (f) of a wave is the number of complete waves that pass a fixed point each second
• Frequency is measured in hertz (Hz); 1 Hz = one wave per second
• A wave with a higher frequency carries more energy per second (for the same amplitude and medium)

Period

• The period (T) of a wave is the time taken for one complete wave to pass a fixed point
• Period is measured in seconds (s)
• Frequency and period are reciprocals of one another:

f = 1 / T and equivalently T = 1 / f