A good diffraction answer names what differs between the two diagrams, links it to gap width compared with wavelength, and states what stays the same. Stating only “it spreads more” earns little.
This lesson is part of wave and image interpretation. The underlying ideas are in reflection, refraction and diffraction.
What does the original diagram show?
Picture two ripple tank photographs with the same water depth and the same vibrator frequency. In diagram P, the gap is 1 cm wide and the wavelength is 1 cm. In diagram Q, the gap is 5 cm wide and the wavelength is still 1 cm.
In P, the waves leave the gap as semicircles that spread widely. In Q, the waves leave as nearly straight fronts with slight bending only at the edges.
Worked example: building the explanation
The question is: “Compare the diffraction in P and Q and explain the difference.”
- Observation: the waves spread much more after the gap in P than in Q.
- Reason: in P the gap width equals the wavelength, so diffraction is strong. In Q the gap is five times the wavelength, so diffraction is weak.
- What stays the same: the wavelength, frequency and speed are unchanged after each gap, since the water depth is the same.
This has three parts, so it suits a three-mark question. Each sentence carries a point a marker can tick.
A second comparison: same gap, different wavelength
Suppose one gap of width 2 cm is used twice. With wavelength 4 cm the waves spread widely. With wavelength 1 cm they spread little.
The explanation reverses the focus: a longer wavelength compared with the gap gives more diffraction. Notice that the ratio of gap to wavelength, not the gap alone, decides the result.
The mistake that costs marks
A common wrong statement is “the wavelength becomes bigger after the gap because the wave spreads”. Spreading does not stretch the wave.
| Claim | Why it is wrong |
|---|---|
| Wavelength increases after the gap | Wavelength stays the same in the same medium |
| Speed decreases in the gap | Speed depends on the medium, which has not changed |
| A narrow gap makes waves weaker, so they spread | Amplitude may fall, but spreading comes from the gap-to-wavelength ratio |
Check yourself
In diagram R, a 3 cm gap is used with waves of wavelength 6 cm. In diagram S, the gap is 3 cm and the wavelength is 1 cm. Which shows more diffraction, and why?
Answer
Diagram R shows more diffraction.
The gap in R (3 cm) is smaller than the wavelength (6 cm), so the waves spread widely. In S the gap is three times the wavelength, so the waves pass almost straight.
The wavelength, frequency and speed stay the same after the gap in both diagrams.
What to study next
Try the integrated practice set to mix all four skills. The mistake log and paper-error review helps you track wording slips.
To practise written answers with a teacher, see online one-to-one Physics tuition.