A-Level Physics: Interference, Diffraction and Refraction

Interference, diffraction and refraction form the optics core of A-Level Physics, and their calculations share one discipline: keep the geometry straight and the units consistent. This quiz works through all three with realistic numbers.

The interference questions centre on Young's double slits: fringe spacing from wavelength, slit separation and screen distance (the classic w = λD/s calculation, 2.4 mm), the proportionality reasoning of what happens when the wavelength doubles, and the coherence conditions — same frequency, constant phase relationship — that make stable fringes possible at all.

Diffraction gets both qualitative and quantitative treatment: the single-slit pattern's structure (a central maximum twice the width of the secondary maxima, widening as the slit narrows), the grating equation applied to find a second-order angle of 41.3° from a 600-lines-per-millimetre grating, and the maximum-order calculation — dividing grating spacing by wavelength and taking the integer part — that students routinely overcomplicate.

The refraction questions apply Snell's law in both routine and boundary cases: the angle of refraction entering glass at 40° (23.7°), refractive index from the speed of light in the medium, the critical angle between two given media, and the attenuation mechanisms — absorption and scattering — that limit optical fibres.

Every value is supplied in the question, in keeping with the exam's data booklet, and each explanation walks the full calculation with the arithmetic shown, because in this topic the marks live in the intermediate steps.

Calculate double-slit fringe spacing and reason proportionally about changes
State the coherence conditions required for stable interference
Apply the diffraction grating equation and find the maximum observable order
Use Snell's law, refractive index and the critical angle with given values
Describe single-slit patterns and the attenuation mechanisms in optical fibres

Topic scope follows the A-level physics content — as organised, for example, in section 3.3.2 (Refraction, diffraction and interference) of the AQA A-level Physics (7408) specification: interference and coherence, single-slit and grating diffraction, refraction at plane surfaces, total internal reflection and fibre optics.

Sample question

Young's double slits: light of wavelength 600 nm falls on two slits 0.50 mm apart, and fringes appear on a screen 2.0 m away; the fringe spacing w is marked between two neighbouring bright fringes.

In a Young's double-slit experiment, light of wavelength 600 nm passes through two slits separated by 0.50 mm. The screen is placed 2.0 m from the slits. What is the fringe spacing on the screen?

1.2 mm
2.4 mm
3.0 mm
4.8 mm

See the answer

2.4 mm

The fringe spacing is given by $w = \frac{\lambda D}{s}$. Substituting the values: $w = \frac{600 \times 10^{-9} \text{ m} \times 2.0 \text{ m}}{0.50 \times 10^{-3} \text{ m}} = 2.4 \times 10^{-3} \text{ m} = 2.4 \text{ mm}$.

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