The electromagnetic spectrum: order, uses and dangers

Memory aid: Raging Martians Invaded Venus Using X-ray Guns.

Moving from radio waves to gamma rays:

• Wavelength decreases.
• Frequency increases.
• Energy increases, so the waves become more dangerous to living cells.

The wave equation links them: $v = f\lambda$. Because the speed is fixed in a vacuum, a higher frequency means a shorter wavelength. See the wave equation guide.

A radio station broadcasts at 100 MHz ($1 \times 10^8$ Hz). Find the wavelength.

Check calculations like this with our wave speed calculator.

• Microwaves and infrared can heat body tissue, causing burns.
• Ultraviolet can cause sunburn, premature skin ageing and increase the risk of skin cancer.
• X-rays and gamma rays are ionising: they can cause mutations in DNA and increase the risk of cancer.

The risk depends on the type of radiation and the dose. Radiographers stand behind lead screens and wear dosimeters to limit their exposure.

• Radio waves travel long distances and can diffract around hills and buildings.
• Microwaves pass through the atmosphere to satellites; certain frequencies are absorbed by water molecules in food.
• X-rays pass through soft tissue but are absorbed by bone, giving a shadow image.
• Gamma rays have enough energy to kill cells, including bacteria and cancer cells.

Higher-tier questions ask you to explain the link between a property and a use, so learn the reason, not just the use.

Visible light is a small part of the spectrum. White light is a mix of colours: red, orange, yellow, green, blue, indigo and violet. Red light has the longest wavelength and violet the shortest. A prism separates white light into these colours because each colour refracts by a different amount.

• Saying EM waves need a medium. They can travel through a vacuum (unlike sound).
• Saying different EM waves travel at different speeds in a vacuum. They all travel at the same speed.
• Mixing up the order of X-rays and ultraviolet.
• Calling all EM radiation dangerous. Only high doses or ionising types are a significant risk.

1. Which EM wave has the longest wavelength?
2. Name two ionising types of EM radiation.
3. A wave has a frequency of $6 \times 10^{14}$ Hz. Find its wavelength.
4. Why are X-rays used to image bones?

Answers: 1. Radio waves. 2. Any two of ultraviolet, X-rays and gamma rays. 3. $\lambda = 3 \times 10^8 \div 6 \times 10^{14} = 5 \times 10^{-7}$ m (500 nm, visible light). 4. They pass through soft tissue but are absorbed by bone.