Waves & The Electromagnetic Spectrum Notes

Edexcel IGCSE Physics: Revision notes

Key facts

  • Waves transfer energy and information without transferring matter.
  • Transverse waves vibrate at 90° to the direction of travel; longitudinal waves vibrate parallel to it.
  • Wave equation: v=fλv = f\lambda; and f=1Tf = \dfrac{1}{T}.
  • The Doppler effect: a source moving towards you raises the observed frequency.
  • The EM spectrum, by increasing frequency: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
  • All EM waves travel at the same speed in free space.

Types of wave

A wave transfers energy without moving matter. Transverse waves vibrate at right angles to the direction of travel; longitudinal waves vibrate along it.

Amplitude is the maximum displacement from rest; wavelength is the distance between identical points on neighbouring waves; frequency is waves per second (Hz); period is the time for one wave.

A wavefront joins points in phase, such as all the crests.

24681012−1.5−1−0.50.511.5xycrest (amplitude 1)troughwave
A transverse wave: amplitude is 1 and wavelength is 2π.

Transverse

Vibrations:
At 90° to direction of energy transfer
Examples:
Light, all EM waves, water waves

Longitudinal

Vibrations:
Parallel to direction of energy transfer
Examples:
Sound: compressions and rarefactions

Which is a longitudinal wave?

The wave equation

Wave speed equals frequency times wavelength, and frequency is one divided by the period.

vv is wave speed (m/s), ff is frequency (Hz) and λ\lambda is wavelength (m); TT is the period (s).

All waves, including EM waves, can be reflected and refracted.

  • Wave equationv=f×λv = f \times \lambda
  • Frequency and periodf=1Tf = \dfrac{1}{T}

Worked example

A wave has frequency 50 Hz and wavelength 4 m. What is its speed?

A wave has a period of 0.02 s. What is its frequency?

The Doppler effect

When a source moves relative to an observer, the observed frequency and wavelength change: higher frequency if approaching, lower if receding.

The Doppler effect applies to all waves, including sound (a passing ambulance) and light (red-shift from receding galaxies).

A source moving towards you squashes the waves; moving away stretches them.

Source approaching

Observed wavelength:
Decreases
Observed frequency:
Increases
Sound:
Higher pitch

Source receding

Observed wavelength:
Increases
Observed frequency:
Decreases
Sound:
Lower pitch

A galaxy's light is red-shifted. The galaxy is

The EM spectrum

The EM spectrum is a continuous family of transverse waves, all travelling at the same speed in free space.

From radio to gamma, wavelength decreases and frequency increases. In free space all EM waves travel at the same speed, 3.0×1083.0 \times 10^8 m/s.

Within visible light, red has the longest wavelength and violet the shortest.

  1. 1

    Radio waves

    longest wavelength, lowest frequency

  2. 2

    Microwaves

  3. 3

    Infrared

  4. 4

    Visible light

    red to violet

  5. 5

    Ultraviolet

  6. 6

    X-rays

  7. 7

    Gamma rays

    shortest wavelength, highest frequency

Wavelength decreases and frequency increases left to right

Which has the highest frequency?

Uses of EM waves

Each type of EM wave has its own uses, from broadcasting to sterilising equipment.

The use depends on the wave's properties, so learn one use for each type.

Radio waves broadcast. Microwaves cook food and carry satellite signals. Infrared heats and gives night vision.

Use

Radio:
Broadcasting and communications
Microwaves:
Cooking and satellite transmissions
Infrared:
Heaters and night vision
Visible light:
Optical fibres and photography

Wave

Radio:
Radio waves
Microwaves:
Microwaves
Infrared:
Infrared
Visible light:
Visible light

Use

Ultraviolet:
Fluorescent lamps
X-rays:
Imaging internal structure, medical
Gamma rays:
Sterilising food and medical equipment

Wave

Ultraviolet:
Ultraviolet
X-rays:
X-rays
Gamma rays:
Gamma rays

Which wave is used to sterilise medical equipment?

Dangers of EM waves

Each type of high-energy radiation harms the body differently, so limit exposure and use shielding.

Microwaves heat body tissue internally. Infrared burns the skin. Ultraviolet damages surface cells and can cause blindness. Gamma rays ionise cells deep inside the body, causing cancer and mutation.

Protect yourself by limiting time, using shielding such as lead, and wearing sunscreen or protective clothing for UV.

Danger

Microwaves:
Internal heating of tissue
Infrared:
Skin burns
Ultraviolet:
Surface cell damage, blindness
Gamma rays:
Cancer, mutation

Protection

Microwaves:
Limit exposure
Infrared:
Limit exposure
Ultraviolet:
Sunscreen, eye protection
Gamma rays:
Lead shielding, limit time

Which radiation can cause cancer and mutation?

Try an exam question

Radio waves travel at 3.0×1083.0 \times 10^8 m/s. A radio station broadcasts at a frequency of 100 MHz (1.0×1081.0 \times 10^8 Hz). Calculate the wavelength of these waves.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.