Physical Science / Unit 9: Waves, Sound and Light
9.4 The Electromagnetic Spectrum
Notes
Electromagnetic (EM) waves are transverse waves of oscillating electric and magnetic fields. They travel at the speed of light, $c = 3.0 \times 10^8 \text{ m/s}$, in a vacuum.
- Order from lowest to highest frequency: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays.
- Higher frequency means shorter wavelength and more energy. $c = f\lambda$.
- Uses: radio (broadcasting), microwaves (cooking, radar), infrared (remote controls, heat), UV (sterilizing, vitamin D), X-rays (imaging bones), gamma (cancer treatment).
- Visible light, from red (longest wavelength) to violet (shortest).
- Example: A radio wave with $f = 1.0 \times 10^8$ Hz has $\displaystyle \lambda = \frac{3.0 \times 10^8}{1.0 \times 10^8} = 3$ m.
Practice
Which EM wave has the highest frequency?
Gamma rays are at the high-frequency end.
Which has a longer wavelength?
Lower frequency waves have longer wavelengths; microwaves are lower than X-rays.
Radio waves have a frequency of $3.0 \times 10^6$ Hz. What is the wavelength? Use $c = 3.0 \times 10^8 \text{ m/s}$.
$\displaystyle \lambda = \frac{3.0 \times 10^8}{3.0 \times 10^6} = 100$ m.
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