Short on time? This page is the recap. For explanations, Figures 1 and 2 and the worked examples, use the full study guide.
Recap
- An EM wave is an electric field and a magnetic field oscillating together. E and B are perpendicular to each other and in step.
- Both fields are perpendicular to the direction of travel, so EM waves are transverse and can be polarized. The polarization direction is the direction of E.
- EM waves are usually modelled as plane waves: flat wavefronts perpendicular to the direction of travel.
- EM waves need no medium. In a vacuum every EM wave travels at c, whatever its frequency. In air the speed is very close to c.
- The spectrum runs from kilometre wavelengths (radio) to picometre wavelengths (gamma). You need the order, not the exact ranges.
- Visible EM waves are called light; the word is sometimes used for the whole spectrum.
Key relationships
| Relationship | Symbols and units | Use it to |
|---|---|---|
| c = fλ | c = 3.00 × 10⁸ m/s, f in Hz, λ in m | move between frequency and wavelength |
| T = 1/f | T in s | find the period of the oscillating fields |
| d = ct | d in m, t in s | find travel times and distances (halve for an echo) |
| Order (decreasing wavelength, increasing frequency) |
|---|
| radio → microwave → infrared → visible → ultraviolet → X-ray → gamma |
| visible: red → orange → yellow → green → blue → violet |
Assumptions behind the numbers
- Speed in air is taken as c.
- Unit prefixes: nm = 10⁻⁹ m, MHz = 10⁶ Hz, GHz = 10⁹ Hz.
Mistakes to avoid
- Making high-frequency waves faster. All EM waves share c in a vacuum.
- Putting B parallel to E, or along the direction of travel. All three directions are mutually perpendicular.
- Using B for the polarization direction. Use E.
- Reversing the spectrum or swapping infrared and microwave, or ultraviolet and X-ray.
- Forgetting unit prefixes (MHz, GHz, nm) in c = fλ.
- Not halving the distance for an echo.
Quick self-check
- A wave travels along +x and its E field oscillates along z. Along which axis does B oscillate? (y)
- What is the frequency of an EM wave with a wavelength of 0.20 m? (1.5 × 10⁹ Hz)
- How long does light take to travel 6.0 × 10³ m? (2.0 × 10⁻⁵ s)
- Which has the higher frequency, green light or red light? (Green)
Next: practice questions.