P3.104SD0 · Section P3: Waves

The electromagnetic spectrum

Key learnings

EM spectrum — order and the common speed
The EM spectrum is a continuous family of transverse waves, all travelling at the same speed in a vacuum, $c = 3 \times 10^{8}$ m/s. In order of increasing frequency (decreasing wavelength): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma — so radio has the longest $\lambda$ /lowest $f$ and gamma the shortest. Visible, long to short $\lambda$ : red, orange, yellow, green, blue, indigo, violet. Since $v = f\lambda$ with $v$ fixed, higher frequency means shorter wavelength.
A use of each EM band
Learn one use per band. Radio — broadcasting. Microwaves — cooking; satellite/mobile signals. Infrared — heaters, remote controls, thermal imaging. Visible — vision, photography, optical fibres. Ultraviolet — fluorescent lamps, security marking. X-rays — imaging bones, security scanners. Gamma — sterilising equipment/food, treating cancer. Trap: if a question asks for a shared property or a similarity/difference, giving a use earns nothing.
Detrimental effects and protection
The harm depends on the band. Microwaves — internal tissue heating. Infrared — skin burns and heating. Ultraviolet — damages skin cells (sunburn, skin cancer) and eyes. X-rays and gamma — ionising: they damage or mutate cells and can cause cancer. Protect: sunscreen, covered skin and UV sunglasses (UV); lead shielding/aprons and limited exposure (X-rays/gamma). Pattern: higher-frequency bands damage cells; lower-frequency (microwave, IR) mainly heat.

Common mistakes

Drawn from real examiner reports.

All EM waves travel at the same speed

In a vacuum every EM wave travels at $3 \times 10^{8}$ m/s — gamma rays are not faster than radio waves; the bands differ in frequency and wavelength, not speed. The related idea that "all EM waves are harmful" is also wrong: radio waves and visible light are not dangerous in normal use.

June 2023 Paper 1P Q6(a)(i); June 2024 Paper 1P Q3(b)(i)

State the harm from X-rays specifically

For a detrimental effect, "harms cells" or "is bad for you" is too vague. X-rays and gamma rays damage or mutate cells and can cause cancer — give the mechanism. The strongest answers name both the harm and, where asked, a matching protective measure (lead shielding, limited exposure, distance).

June 2023 Paper 1P Q6(a)(ii)

Infrared heats — it does not cause cancer

Match the effect to the band. Infrared and microwaves heat and burn tissue; they are NOT creditworthy for "causes cancer / cell mutation" — that harm belongs to the ionising, high-frequency bands (ultraviolet, X-rays, gamma). Also, "burns" on its own was judged too vague for infrared: say it heats and burns the skin or tissue, not a single word.

June 2024 Paper 1P Q3(a)

Comparing waves — say HOW they differ

"They have different frequencies/wavelengths" earns nothing — state HOW they differ, e.g. "infrared has a lower frequency and longer wavelength than visible light". Weaker candidates reversed the two: from $v = f\lambda$ with $v$ fixed, the higher-frequency wave has the shorter wavelength — do not swap them.

June 2024 Paper 1P Q3(b)(ii)

Don't answer a property with a use

When a question asks for a similarity, difference or property of two waves, giving a use scores nothing. A property is something like "both are transverse" or "both travel at the same speed in a vacuum"; a use is what we do with the radiation (cooking, imaging). Read the command word and answer the category it asks for.

June 2024 Paper 1P Q3(b)(i)

Frequency is complete waves per second

Frequency must be defined as "the number of complete waves passing a point per second" (unit hertz, Hz), not "how often a wave happens". Do not confuse it with wavelength (measured in metres): for a fixed wave speed the two are inversely related, so a higher frequency goes with a shorter wavelength.

Exam tips

Fix the order, then read the command word
Learn the order (radio, microwave, infrared, visible, ultraviolet, X-ray, gamma) — frequency rising, wavelength falling, left to right. Then read whether the question wants a use, a detrimental effect or a property; the wrong category is a frequent lost mark.
Match the effect to the radiation
When stating harms, pair them correctly: microwaves and infrared heat and burn; ultraviolet, X-rays and gamma damage/mutate cells and cause cancer. Giving a heating effect for gamma, or a cancer effect for infrared, loses the mark.
Substitute before rearranging in v = fλ
For a wave-speed calculation, write $v = f\lambda$ and substitute the data before rearranging — a correct substitution earns a mark even if the algebra then slips. Use $v = 3 \times 10^{8}$ m/s in free space, wavelength in metres, frequency in Hz; state the unit.

Revision notes