Physics / Unit 8: Light and Optics
8.1 The Electromagnetic Spectrum
Notes
Light is an electromagnetic (EM) wave: a transverse wave of changing electric and magnetic fields that needs no medium.
- All EM waves travel at the speed of light in a vacuum: $c = 3.0 \times 10^8 \text{ m/s}$. Use $c = f\lambda$.
- In order of increasing frequency (decreasing wavelength): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma ray.
- Higher frequency means more energy. UV, X-rays and gamma rays can damage cells.
- Visible light runs from red (longest wavelength, about 700 nm) to violet (shortest, about 400 nm).
- Example: green light with $\lambda = 5.0 \times 10^{-7} \text{ m}$ has $\displaystyle f = \frac{3.0 \times 10^8}{5.0 \times 10^{-7}} = 6.0 \times 10^{14} \text{ Hz}$.
Practice
Which type of EM wave has the highest frequency?
Gamma rays are at the high-frequency, high-energy end.
Sunlight travels $1.5 \times 10^{11} \text{ m}$ to Earth. How long does the trip take?
$\displaystyle t = \frac{d}{c} = \frac{1.5 \times 10^{11}}{3.0 \times 10^8} = 500 \text{ s}$ (about 8 minutes).
Compared with microwaves, X-rays have:
X-rays are higher on the spectrum; all EM waves share the same speed in a vacuum.
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