5.6 - Electromagnetic Waves

EM waves transfer energy from a source to an absorber

• Electromagnetic (EM) waves are transverse waves that transfer energy from a source to an absorber.
• For example, an electric heater (source) emits infrared waves that are absorbed by your body (absorber), warming you.
• EM waves are vibrations of electric and magnetic fields, not particles, so they can travel through a vacuum (such as empty space).

The speed of EM waves

• All EM waves travel at the same speed through a vacuum (and through air): 3.0 × 10⁸ m/s (the speed of light).
• EM waves travel at different speeds in different materials.

EM waves form a continuous spectrum of seven groups

• EM waves cover a wide range of wavelengths and frequencies.
• They're grouped into seven bands, with no sharp boundaries between them, forming a single continuous spectrum.

In order from longest wavelength (lowest frequency) to shortest wavelength (highest frequency):

radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays

As wavelength decreases, frequency increases across the spectrum (since $v=f\lambda$).

Visible light

• Our eyes detect only visible light, which is a narrow band of the spectrum.
• Within visible light, the colour depends on the wavelength. From longest wavelength to shortest:

red, orange, yellow, green, blue, indigo, violet

Interactions at a boundary

• Different wavelengths interact differently at a boundary between materials.
• A wave may be absorbed, transmitted, reflected or refracted, depending on both the material and the wavelength.

Typical sources and detectors for each band

• Radio waves - Source: a radio transmitter. Detector: a radio or TV aerial.
• Microwaves - Source: a microwave transmitter (or a microwave oven). Detector: an aerial or satellite dish.
• Infrared - Source: any warm or hot object (e.g. a heater or the Sun). Detector: a thermistor, or a thermal imaging camera (CCD).
• Visible light - Source: a lamp, the Sun, or an LED. Detector: the retina of the eye, an LDR, or photographic film.
• Ultraviolet - Source: the Sun, or a UV lamp. Detector: a fluorescent material (which absorbs UV and emits visible light).
• X-rays - Source: an X-ray machine. Detector: photographic film or a digital detector plate.
• Gamma rays - Source: radioactive decay in atomic nuclei. Detector: a Geiger-Muller tube and counter.