Light and electromagnetic waves
What this note covers
- Wave models and the wave equation
- The electromagnetic spectrum and solar radiation
- Refraction, total internal reflection and optical fibres
- Dispersion, rainbows and mirages
4 sections · 8 key terms & formulas · 5 common mistakes
A wave is a disturbance that carries energy from one place to another without carrying matter along with it. Physicists sort waves into two broad families. Mechanical waves, such as sound, water ripples and waves on a slinky, need a material medium: particles of the medium push or pull on their neighbours, and the disturbance passes along. Sound cannot cross the vacuum of space because there are no particles to pass it on. Electromagnetic (EM) waves are different.
They consist of linked, oscillating electric and magnetic fields that regenerate each other, so they need no medium at all. Every EM wave, from radio to gamma, travels through a vacuum at the same speed, c = 3.0 × 10⁸ m s⁻¹.
EM waves are transverse: the electric and magnetic fields oscillate at right angles to the direction the wave travels (and at right angles to each other). In a longitudinal wave, such as sound, the particles vibrate back and forth parallel to the direction of travel, producing compressions and rarefactions.
Four quantities describe any periodic wave:
- Amplitude (A): the maximum displacement from the rest (equilibrium) position. Larger amplitude means more energy carried; for light it corresponds to brightness.
- Wavelength (λ): the distance between two successive points in phase, for example crest to crest. Measured in metres.
- Period (T): the time for one complete oscillation, in seconds.
- Frequency (f): the number of complete oscillations per second, in hertz (Hz). f = 1/T.
Because a wave moves forward one wavelength in each period, its speed is v = λ/T, which gives the wave equation v = fλ. For EM waves in a vacuum this becomes c = fλ.
A key idea is what sets each quantity. The frequency is fixed by the source: it is the rate at which the source oscillates, and it does not change when the wave passes into a new medium. The speed is fixed by the medium. The wavelength then adjusts to fit, λ = v/f. So when light passes from air into glass, it slows down, its frequency stays the same, and its wavelength becomes shorter.
When reading a displacement–distance graph you can read amplitude and wavelength; from a displacement–time graph you read amplitude and period. Students often mix these up, so always check the horizontal axis label first.
f = 104.3 × 10⁶ Hz, c = 3.0 × 10⁸ m s⁻¹.
λ = c/f = (3.0 × 10⁸)/(1.043 × 10⁸) = 2.88 m (about 2.9 m).
T = 1/f = 1/(1.043 × 10⁸) = 9.59 × 10⁻⁹ s (about 9.6 ns).
In air: λ = 3.0 × 10⁸ / 4.6 × 10¹⁴ = 6.5 × 10⁻⁷ m (650 nm).
In water the frequency is unchanged: λ = 2.25 × 10⁸ / 4.6 × 10¹⁴ = 4.9 × 10⁻⁷ m (489 nm). The wavelength shortens because the medium slowed the wave.
Common questions
What does the VCE Year 11 Physics note "Light and electromagnetic waves" cover?
This note has 4 sections on Light and electromagnetic waves. It includes 8 key terms and formulas, plus 5 common mistakes. It covers 1, Unit 1 AOS 1: How are light and heat explained?. The full section list is shown under “What this note covers”.
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