The Electromagnetic Spectrum: MDCAT Physics notes
The Electromagnetic Spectrum for MDCAT: order of radio to gamma waves, c = fλ, properties and uses, UV and ozone, darkroom red light, AM and FM.
Electromagnetic waves
Electromagnetic waves are transverse waves of oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel. They need no medium and all travel in vacuum at the same speed
$$c = f\lambda = 3\times10^8\ \text{m s}^{-1}$$
Because $c$ is fixed, a higher frequency means a shorter wavelength. Photon energy $E = hf$ rises with frequency.
Example: a wave of frequency 6 kHz has $\lambda = 3\times10^8/6\times10^3 = 5\times10^4$ m = 50 km.
The spectrum
In order of increasing frequency (decreasing wavelength):
| Region | Approx. wavelength | Source / use |
|---|---|---|
| Radio waves | Longer than about 1 m | Radio and TV broadcasting |
| Microwaves | About 1 mm to 1 m | Radar, mobile phones, microwave ovens |
| Infrared | About 700 nm to 1 mm | Heat radiation, remote controls, night vision |
| Visible light | About 400–700 nm | Vision; red is longest, violet shortest |
| Ultraviolet | About 10–400 nm | Sterilisation, vitamin D; causes sunburn and skin cancer |
| X-rays | About 0.01–10 nm | Medical imaging, crystal structure |
| Gamma rays | Shorter than about 0.01 nm | From nuclei; cancer therapy, sterilising equipment |
Gamma rays have the highest frequency, highest photon energy and greatest penetrating power. Radio waves have the lowest frequency and longest wavelength. The regions overlap at their boundaries.
Visible light and the darkroom
Energy radiated in the visible range is called light. Red light has the lowest frequency and longest wavelength of visible light, so its photons carry too little energy to affect photographic film. That is why red safe-lights are used in darkrooms.
Ultraviolet and the ozone layer
The ozone layer in the upper atmosphere absorbs most of the Sun's harmful UV. Where it is depleted, direct exposure to sunlight increases the risk of skin cancer as well as sunburn and eye damage.
Modulation
To carry sound or data, a high-frequency carrier wave is modulated.
| Type | What is varied | What stays constant |
|---|---|---|
| Amplitude modulation (AM) | Amplitude of carrier | Frequency of carrier |
| Frequency modulation (FM) | Frequency of carrier | Amplitude of carrier |
FM is less affected by electrical noise, which mostly disturbs amplitude.
Key formulas
- $c = f\lambda$
- $E = hf = hc/\lambda$
Common MDCAT traps
- X-rays are highly penetrating, but gamma rays rank higher in frequency and penetration.
- Microwaves are electromagnetic, not sound or longitudinal waves.
- Red light is used in darkrooms because of low frequency and long wavelength.
- AM keeps frequency constant; FM keeps amplitude constant.
- Convert kHz and MHz to Hz before using $\lambda = c/f$.
Quick revision
- All EM waves travel at $3\times10^8$ m s$^{-1}$ in vacuum.
- High frequency means short wavelength.
- Order: radio, microwave, IR, visible, UV, X-ray, gamma.
- Ozone depletion raises UV exposure and skin cancer risk.