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All 11 Flashcards — Electromagnetic waves and the EM spectrum
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Question
What is an electromagnetic (EM) wave?
Answer
A **transverse** wave of vibrating electric and magnetic fields (light is one example). It needs **no medium** and travels through a vacuum.
Question
How fast do EM waves travel in a vacuum?
Answer
They **all** travel at the same speed, **c = 3.00 × 10⁸ m s⁻¹** (the speed of light), whatever their region.
Question
Are EM waves transverse or longitudinal?
Answer
**Transverse** — the fields oscillate at right angles to the direction the wave travels.
Question
List the EM spectrum in order of increasing frequency.
Answer
**Radio → microwave → infrared → visible → ultraviolet → X-ray → gamma.** Wavelength falls, frequency and energy rise.
Question
Which end of the spectrum has the longest wavelength?
Answer
**Radio** — longest wavelength, lowest frequency, lowest energy. **Gamma** is the opposite end.
Question
Wave equation for an EM wave in a vacuum?
Answer
$c = f\lambda$ — speed of light = frequency × wavelength. Rearrange to $f = c/\lambda$ or $\lambda = c/f$. **Given** in the data booklet.
Question
One difference between a sound wave and an EM wave?
Answer
Sound **needs a medium** (it is mechanical); an EM wave **crosses a vacuum**. Also: sound is longitudinal, EM is transverse; EM is far faster.
Question
What oscillates in an EM wave vs a sound wave?
Answer
EM wave: **electric and magnetic fields**. Sound wave: the **particles of the medium** (e.g. air molecules).
Question
A wavelength of about one atom (≈ 10⁻¹⁰ m) is which region?
Answer
An **X-ray** (very short wavelength ⇒ very high frequency, about 10¹⁸ Hz).
Question
Most common EM-spectrum mistake?
Answer
Thinking different colours or regions travel at **different speeds** — in a vacuum they all travel at c.
Question
What is the slope of a graph of f against 1/λ for EM waves?
Answer
The **speed of light c** — because $c = f\lambda$ rearranges to $f = c(1/\lambda)$, a straight line through the origin.
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