Reflection bounces a wave off a surface, refraction bends it as its speed changes, and diffraction spreads it around an edge or through a gap. In every case the frequency stays the same.
This lesson is part of SPM Physics waves. It uses the formula from relating speed, frequency and wavelength.
What changes in each effect?
Answer by checking four quantities: speed, wavelength, frequency and direction.
| Effect | Speed | Wavelength | Frequency | Direction |
|---|---|---|---|---|
| Reflection | Same | Same | Same | Changes (angle in = angle out) |
| Refraction | Changes | Changes | Same | Changes at an angle |
| Diffraction | Same | Same | Same | Spreads out |
Worked example: a ripple tank going shallow
An original ripple tank has deep water where waves of frequency 5.0 Hz have wavelength 6.0 cm. The speed there is 5.0 × 6.0 = 30 cm s⁻¹.
The waves enter a shallow region, and the speed falls to 20 cm s⁻¹. The frequency is still 5.0 Hz, because the vibrating source has not changed.
The new wavelength is λ = v ÷ f = 20 ÷ 5.0 = 4.0 cm. If the waves meet the boundary at an angle, they bend toward the normal because they slow down.
Why does bending happen?
The part of the wavefront that enters the slower region first slows first. The rest of the wavefront catches up, so the line of the wavefront turns.
A wave travelling straight into the boundary slows down without turning. Speed changes without direction changing.
When does diffraction happen most?
Diffraction is greatest when the gap is about the same size as the wavelength. A narrower gap, or a longer wavelength, gives more spreading.
For example, ripples of wavelength 2.0 cm passing through a 2.0 cm gap spread into almost circular waves. Through a 20 cm gap they continue mostly straight, with slight curving at the edges.
The mistake that costs marks
The slip is to say frequency changes in refraction, because wavelength and speed do. The source sets the frequency, so it cannot change.
| Statement | Wrong | Right |
|---|---|---|
| Frequency in shallow water | Decreases | Stays 5.0 Hz |
| Wavelength in shallow water | Stays 6.0 cm | Becomes 4.0 cm |
A second slip is to say diffraction changes the wavelength. The wave spreads, but each crest still has the same spacing.
Check yourself
Straight water waves of wavelength 3.0 cm and speed 15 cm s⁻¹ pass through a gap 12 cm wide. State how diffraction could be increased, and find the frequency.
Answer
Frequency = v ÷ λ = 15 ÷ 3.0 = 5.0 Hz.
Diffraction increases if the gap is narrowed toward the wavelength, for example to 3.0 cm, or if the wavelength is made longer by lowering the frequency.
What to study next
When two diffracted waves overlap, they produce interference. Continue with reading interference patterns, and come back to this page when you practise in the waves practice set.
To practise explanation answers with a teacher, see online one-to-one Physics tuition.