Topic 5: Waves and Particle Nature of Light — Edexcel A-Level Physics
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Topic 5: Waves and Particle Nature of Light explained
The wave equation links the three quantities that describe wave travel: speed v equals frequency f multiplied by wavelength λ, so v = fλ.
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The logic is that in one period the wave advances by one wavelength, and f oscillations occur each second, so the distance covered per second is f × λ. Keep units consistent: frequency in hertz (s⁻¹), wavelength in metres, giving speed in m s⁻¹. The equation applies to all progressive waves, including sound and electromagnetic waves. Rearrange it as f = v/λ or λ = v/f depending on the unknown. For example, a wave of frequency 250 Hz and wavelength 1.36 m travels at v = 250 × 1.36 = 340 m s⁻¹, a typical speed of sound in air. Remember that when a wave passes from one medium to another its speed and wavelength change but its frequency stays the same, so use the wave equation with the values appropriate to each medium.
Your focus
- Apply v = fλ to calculate speed, frequency or wavelength.
- Rearrange the wave equation for any one variable.
- Explain why frequency is unchanged when a wave changes medium.
Topic 5: Waves and Particle Nature of Light exam tips
Marking Points
- States and applies v = fλ with speed in m s⁻¹, frequency in Hz and wavelength in m.
- Rearranges the equation correctly to f = v/λ or λ = v/f for the required unknown.
- Substitutes consistent SI units before calculating.
- Recognises that frequency is unchanged when a wave crosses into a new medium while speed and wavelength change.
- Calculates correctly, for example 250 Hz × 1.36 m = 340 m s⁻¹.
Examiner Tips
- 💡Write down v = fλ, then rearrange for the unknown before inserting numbers to avoid arithmetic slips.
- 💡Convert any wavelength given in centimetres or nanometres to metres before calculating.
- 💡Sanity-check the answer: sound in air is about 340 m s⁻¹ and light in a vacuum about 3 × 10⁸ m s⁻¹.
Common Mistakes
- Mixing units, such as wavelength in centimetres with speed in m s⁻¹; the correction is to convert all quantities to SI units first.
- Assuming frequency changes when a wave enters a new medium; the correction is that frequency stays constant while speed and wavelength change.
- Using the equation without rearranging correctly for the unknown; the correction is to isolate the required quantity before substituting.