Lesson B · 3.10–3.13
Order, shared properties, the specification uses, and the hazards with a protection sentence.
What I want on the page in 30 seconds
Learn the order as a chant: radio, micro, infrared, visible, UV, X-ray, gamma. Visible sits in the middle as ROYGBIV. Then pair each band with one use and, where the spec asks, one hazard. Do not invent uses that are not on the list if the question says “state a use from the specification.”
3.10–3.11
Visible light is just one slice of a continuous electromagnetic spectrum that runs from radio waves to gamma rays (3.10). All EM waves are transverse. In a vacuum (free space) they all travel at
$$c = 3.0 \times 10^8\,\text{m/s}$$That is why $v=f\lambda$ for a radio wave uses $3.0\times 10^8$, not $340$. Frequency and wavelength are inversely related: as you go from radio to gamma, $\lambda$ falls and $f$ rises (3.11).
Increasing frequency and energy → ← increasing wavelength
Visible colours in order of increasing frequency: red, orange, yellow, green, blue, indigo, violet. Red has the longest visible wavelength; violet the shortest.
Trap
“All waves travel at $3\times 10^8\,\text{m/s}$” is false. That is EM waves in a vacuum. Sound is ~$340\,\text{m/s}$ in air and needs a medium. Water ripples are slower still.
3.12–3.13
Learn the specification list. If the paper says “explain a use,” add a because: microwaves are absorbed by water in food, so the food’s internal energy rises.
| Band | Specification use (3.12) | Hazard and protection (3.13) |
|---|---|---|
| Radio | Broadcasting and communications | Not on the 3.13 hazard list |
| Microwaves | Cooking; satellite transmissions | Internal heating of body tissue. Metal screens / do not stand in a high-power beam |
| Infrared | Heaters; night-vision equipment | Skin burns. Limit time; insulating handles; do not stare into IR lamps |
| Visible | Optical fibres; photography | Very bright sources can damage eyes (not the 3.13 list, but sensible) |
| Ultraviolet | Fluorescent lamps | Damage to surface cells and blindness. Sunscreen, clothing, UV-filter goggles |
| X-rays | Internal structure of objects and materials, including medical images | Ionising — keep dose low; lead shields; stand behind a screen |
| Gamma | Sterilising food and medical equipment | Cancer and mutation. Time, distance, shielding (lead / concrete) |
The spec names hazards for microwaves, infrared, ultraviolet and gamma. X-rays are ionising in the same family as gamma; I still want a protection sentence if they appear.
3-mark hazard answer
Ultraviolet can damage surface cells / cause blindness (or skin cancer) [1]. Wear sunscreen / cover skin / use UV-filter glasses [1]. Reduce time in strong sunlight [1].
Watch
List the seven EM bands in order of increasing frequency. State the speed they all have in a vacuum.
Give one specification use of gamma rays and one detrimental effect of excessive exposure.