EM Waves & Polarization
A short lesson with diagrams, a worked example, a problem to try, and a quick check. Then quiz yourself on 5 practice questions.
Light is an electromagnetic wave: electric and magnetic fields oscillating perpendicular to each other and to the direction of travel. All EM waves travel at c = 3.0 × 10⁸ m/s in a vacuum; they differ only in frequency and wavelength. Because they’re transverse, they can be polarized.
1. The EM spectrum
No medium needed: EM waves cross empty space (sunlight reaches us through a vacuum).
E, B and the direction of travel are all perpendicular. If the wave moves along x and E oscillates along y, B oscillates along z.
2. Polarization
Unpolarized light vibrates in every direction. A filter passes only one direction, and half the intensity.
A second filter at 90° to the first passes nothing. At the same angle, it passes everything that reached it.
Only transverse waves can be polarized. Sound, being longitudinal, can’t.
3. Example, then try it
Unpolarized light of intensity 80 W/m² hits a vertical polarizing filter. What comes out? What gets through a second vertical filter? A horizontal one instead?
First filter: half gets through: 40 W/m², vertically polarized.
Second vertical filter: 40 W/m² (already lined up). Horizontal filter: 0.
An AM radio station broadcasts at 1.0 MHz, and a dental X-ray has a frequency of 3.0 × 10¹⁸ Hz. Find each wavelength. Which travels faster through space?
Radio: λ = c/f = (3.0 × 10⁸) ÷ (1.0 × 10⁶) = 300 m.
X-ray: λ = (3.0 × 10⁸) ÷ (3.0 × 10¹⁸) = 1.0 × 10⁻¹⁰ m, about the size of an atom.
Speed: the same, c. Higher frequency means shorter wavelength, not more speed.
Answer: 300 m and 1.0 × 10⁻¹⁰ m; both travel at c.
4. Common mistakes
Which of these waves can be polarized?
Polarization picks out one direction of sideways vibration. Light is transverse; sound in air is longitudinal, so it can’t be polarized.
Ready to practice?
5 questions on EM Waves & Polarization, each with a hint and an explanation for every wrong answer.