1. What is stroboscope? A hand stroboscope is used to freeze the wave.
slits
It has a disc with 6 to 12 uniformly spaced slits.
The disc is rotated by hand about an axis through its centre.

2. The waves are seen only when a slit passes in front of the eye.
screen
stroboscope
observer
wave pattern
The waves are seen only when a slit passes in front of the eye.

3. How many slits pass in front of the eye per second?
3 revolutions per second
How many slits pass in front of the eye per second?
24 slits
The strobe frequency of the hand stroboscope is 24 Hz.
Strobe frequency =
number of slits on the disc 
number of revolutions per second

4. ‘Freezing’ wave patterns
Viewing without stroboscope
0 s s
0.025 s
Frequency of water wave = 20 Hz s
0.05 s s
0.075 s
The wave moves a distance of one wavelength every 0.05 s. s
0.1 s
observer

5. Viewing with strobe frequency at 20 Hz
The time interval between successive sightings is 0.05 s. s
0.05 s s
The wave moves one wavelength each time we see it.
0.075 s s
0.1 s
The wave pattern is ‘frozen’.
observer
observation

6. Viewing with strobe frequency at 10 Hz
The time interval between successive sightings is 0.1 s. s
0.05 s s
The wave moves two wavelengths each time we see it.
0.075 s s
0.1 s
The wave pattern is ‘frozen’.
observer
observation

7. The wave appears to be ‘frozen’.
Time interval between two successive sightings
An integral multiple of the period (T, 2T, 3T…) =
The wave appears to be ‘frozen’.
The corresponding strobe frequencies
(f is wave frequency)
Strobe frequency is slightly higher than
The wave appears to move backwards slowly.
Strobe frequency is slightly less than
The wave appears to move forward slowly.

8. Viewing with strobe frequency at 40 Hz
The time interval between successive sightings is s. s
0.025 s s
The wave moves half a wavelength each time we see it.
0.05 s s
0.075 s
The wave pattern is ‘frozen’ but its wavelength appears to be halved. s
0.1 s
This is called double viewing.
observer
observation
Simulation: Stroboscope

9. How can we determine the speed of a wave?
Frequency, wavelength and speed
How can we determine the speed of a wave?
1. Find the frequency and wavelength of the wave.
2. Calculate the wave speed using the wave equation v = f.

10. How can we find the frequency of the wave?
The wave frequency is the highest possible strobe frequency that can freeze the wave without observing a reduced wavelength.

11. How can we find the wavelength of the wave?
Method 1
1. Place a transparent ruler in the ripple tank.
2. Freeze the wave.
3. Read the wavelength directly.

12. Method 2
1. Freeze the wave.
2. Measure the apparent wavelength on the screen.
3. Calculate the actual wavelength using the following formula:

13. That’s the end of Section 11.2
Check Point
Key Ideas
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Section 11.3
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