SSC CGL Physics · Sound
Sound Waves and the Doppler Effect
The wave equation and what changes when sound moves from air into water, the Doppler effect, and the range of human hearing.
Why this matters
Seven questions in the bank: one from the 2024 shifts and six from the 2025 shifts. Four use the one equation v = f x wavelength, so learn it with the rule that frequency never changes between media.
Concept 1 of 2: The wave equation
Definition
The rules:
- v = f x wavelength.
- Frequency stays the same when sound passes from one medium to another.
- Sound is faster in water (about 1,500 m/s) than in air (about 340 m/s), so going from air into water its wavelength grows.
- Time = distance / speed.
Wave equation
- Speed of the wave
- Frequency
- Wavelength
Worked example
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 1 · Sound · Sound Waves and the Doppler Effect
Faster medium, longer wavelength
Concept 2 of 2: The Doppler effect and hearing
Definition
Doppler effect:
- The heard frequency changes with relative motion of source and listener: higher when they close in, lower when they separate.
- Source and listener moving at the same speed in the same direction: no change.
Hearing: humans hear about 20 Hz to 20,000 Hz. Below is infrasound; above is ultrasound.
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 2 · Sound · Sound Waves and the Doppler Effect
No relative motion, no change
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (1)
- The wave equation
Wave equation
Reference tables (1)
Watch out for (2)
- Faster medium, longer wavelength→ The wave equation
- No relative motion, no change→ The Doppler effect and hearing
Test yourself on a real paper
Sit a past SSC CGL paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.