General Wave Properties: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 11 · Read time: ~8 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
What a wave is
A wave transfers energy from one place to another without transferring matter. The particles of the medium only oscillate about fixed positions.
You can see wave motion in a vibrating rope, a slinky spring and water waves in a ripple tank. A wavefront is a line joining points on a wave that are in the same position of their oscillation, for example along a crest.
Describing a wave
Amplitude (A): the maximum displacement of a particle from its rest position. Unit: m.
Wavelength (λ): the distance between two consecutive points in phase, for example crest to crest. Unit: m.
Frequency (f): the number of complete waves produced per second. Unit: Hz.
Period (T): the time for one complete oscillation. T = 1/f.
Wave speed (v): the distance travelled by a wavefront per second. v = fλ.
Trick: Check which graph you are reading. A **displacement-distance** graph lets you read off wavelength. A **displacement-time** graph lets you read off period, and then f = 1/T. Mixing them up is one of the most common ways to lose easy marks.
Trap: Amplitude is measured from the rest position to a crest, not from crest to trough. A crest-to-trough measurement is twice the amplitude.
Transverse and longitudinal waves
Transverse waves: particles oscillate perpendicular to the direction of energy transfer, forming crests and troughs. Examples: water waves, waves on a rope, all electromagnetic waves.
Longitudinal waves: particles oscillate parallel to the direction of energy transfer, forming compressions and rarefactions. Examples: sound waves, a slinky pushed back and forth.
When a wave enters a new medium, its frequency stays the same (set by the source), while its speed and wavelength change together so that v = fλ still holds.
Worked Example
A wave generator in a ripple tank produces 15 complete waves in 3.0 s. The distance between 6 consecutive crests is measured as 10 cm.
(a) Calculate the frequency and the period of the waves. [2]
f = 15 ÷ 3.0 = 5.0 Hz
T = 1 / f = 1 / 5.0 = 0.20 s
(b) Calculate the wavelength. [1]
6 consecutive crests span 5 wavelengths.
λ = 10 ÷ 5 = 2.0 cm = 0.020 m
(c) Calculate the speed of the waves. [2]
v = fλ
= 5.0 × 0.020
= 0.10 m/s
Why this question is a good test of the topic: the trap is in part (b). Six crests contain only five gaps, so dividing by 6 gives the wrong wavelength and carries the error into part (c).
Frequently Asked Questions
Do waves transfer matter?
No. Waves transfer energy. The particles of the medium only oscillate about their rest positions.
What is the difference between transverse and longitudinal waves?
In transverse waves, particles oscillate perpendicular to the direction of energy transfer. In longitudinal waves, they oscillate parallel to it, forming compressions and rarefactions.
What is a wavefront?
A line joining points on a wave that are in the same phase, for example all the points along one crest.
Does frequency change when a wave enters a new medium?
No. Frequency is set by the source. Speed and wavelength change instead.
How do I find wavelength from a graph?
Use a displacement-distance graph and measure from one crest to the next. A displacement-time graph gives the period instead.
Wave graphs still confusing?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author.