Properties of EM Waves

Section 1 of 7What all EM waves have in common

Transverse waves

• Transverse waves The vibrations are at right angles to the direction in which the wave transfers energy.

Travel through a vacuum

• Travel through a vacuum Unlike sound, they need no medium, which is how light from the Sun reaches the Earth across empty space.

Same speed in a vacuum

• Same speed in a vacuum Every EM wave travels at about 3 × 10⁸ m/s in a vacuum, whatever its frequency or wavelength.

Transfer energy

• Transfer energy They carry energy from a source to an absorber, without transferring matter.

Section 2 of 7The electromagnetic spectrum

Wave type	Wavelength	Frequency and energy
Radio waves	Longest	Lowest frequency, lowest energy
Microwaves	Long	Low
Infrared	Medium	Medium
Visible light	Short	Higher
Ultraviolet	Shorter	High
X-rays	Very short	Very high
Gamma rays	Shortest	Highest frequency, highest energy

Section 3 of 7Energy and danger

1Shorter wavelength

• Shorter wavelength Waves with a shorter wavelength have a higher frequency.

2More energy

• More energy Higher-frequency waves carry more energy per photon.

3More hazard

• More hazard Ultraviolet, X-rays and gamma rays can damage living cells, so the higher the energy, the more dangerous the radiation.

Section 4 of 7Examples of energy transfer

Source to absorber

Microwave oven

• Microwave oven Water molecules in food absorb microwaves, which transfers energy to them and heats the food from the inside.

Infrared

• Infrared All warm objects emit infrared radiation.
  • When it is absorbed by another object, that object's thermal energy store increases and it warms up.

Light from the Sun

• Light from the Sun Visible light lets living things see, and it is absorbed by plants for photosynthesis.

Ultraviolet

• Ultraviolet UV in sunlight allows our skin to make vitamin D, needed for healthy bones, teeth and muscles, but too much can burn skin.

Section 5 of 7Using the wave equation

1

Write the wave equation: wave speed (m/s) = frequency (Hz) × wavelength (m), or v = f × λ.

2

For an EM wave in a vacuum, use v = 3 × 10⁸ m/s.

3

Rearrange to find the unknown, for example λ = v ÷ f.

4

Worked example: a radio wave of frequency 1.5 × 10⁸ Hz has wavelength = 3 × 10⁸ ÷ 1.5 × 10⁸ = 2 m.

Section 6 of 7Key terms

Term	Meaning
Electromagnetic spectrum	The continuous family of EM waves, arranged from radio waves to gamma rays.
Transverse wave	A wave whose vibrations are perpendicular to the direction of energy transfer.
Source	The object that emits the waves.
Absorber	The object that takes in the wave and gains energy from it.
Vacuum	A region of space containing no matter.

Section 7 of 7Exam technique

1

Saying that different EM waves travel at different speeds in a vacuum — they all travel at the same speed.

2

Claiming EM waves need a medium to travel through; they do not, unlike sound waves.

3

Confusing wavelength with energyshorter wavelength means higher frequency and higher energy.

4

Saying that waves transfer matter; they transfer only energy.