Wave speed, frequency and wavelength: Class 9 practice
For any wave, v = fλ. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Rearranging formulas
- Decimals
The idea behind wave speed, frequency and wavelength
For any wave, v = fλ. The period T = 1/f. In a given medium the speed is fixed, so raising frequency shortens wavelength.
Step-by-step method
- Write the known quantities with units.
- Use v = fλ (or T = 1/f).
- Rearrange for the unknown.
Worked example 1 (Basic)
A sound of frequency 1000 Hz travels at 330 m/s. Find its wavelength.
- λ = v/f = 330/1000
- = 0.33 m
- Answer: 0.33 m
Worked example 2 (Standard)
A wave has wavelength 0.34 m and speed 340 m/s. Find its frequency and period.
- f = 340/0.34 ≈ 1000 Hz
- T = 1/f = 0.001 s
- Answer: 1000 Hz, 0.001 s
Worked example 3 (Challenge)
An echo is heard 3 s after a shout. If sound travels at 330 m/s, how far is the reflecting wall?
- Sound travels there and back: 2d = 330 × 3
- d = 495 m
- Answer: 495 m
Common mistakes and misconceptions
- Using T for f
- Not converting cm to m
Practice questions
- (Basic) A sound of frequency 100 Hz travels at 340 m/s. Find its wavelength.
- (Basic) A sound of frequency 100 Hz travels at 1500 m/s. Find its wavelength.
- (Standard) A wave has wavelength 7.353 m and speed 5000 m/s. Find its frequency and period.
- (Standard) A wave has wavelength 7.5 m and speed 1500 m/s. Find its frequency and period.
- (Challenge) An echo is heard 5 s after a shout. If sound travels at 340 m/s, how far is the reflecting wall?
- (Challenge) An echo is heard 4 s after a shout. If sound travels at 330 m/s, how far is the reflecting wall?
Answer key with full working
- Q1: λ = v/f = 340/100 → = 3.4 m → Answer: 3.4 m
- Q2: λ = v/f = 1500/100 → = 15 m → Answer: 15 m
- Q3: f = 5000/7.353 ≈ 680 Hz → T = 1/f = 0.0014705882352941176 s → Answer: 680 Hz, 0.0014705882352941176 s
- Q4: f = 1500/7.5 ≈ 200 Hz → T = 1/f = 0.005 s → Answer: 200 Hz, 0.005 s
- Q5: Sound travels there and back: 2d = 340 × 5 → d = 850 m → Answer: 850 m
- Q6: Sound travels there and back: 2d = 330 × 4 → d = 660 m → Answer: 660 m
Syllabus note
This skill is listed under "Sound" for Class 9 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is wave speed, frequency and wavelength in?
"Sound", Class 9 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.