Diffraction — AQA A-Level Physics

Diffraction is the spreading of waves when they pass through a gap or around an obstacle. In AQA A-Level Physics, you study single slit diffraction and diffraction gratings. You need to understand how the pattern depends on slit width and wavelength, and how to use the grating equation to make accurate measurements of wavelength.

This topic is essential for understanding wave behaviour and is a key part of the Waves and Optics section. It links to superposition, interference, and the wave-particle duality. Diffraction gratings are used in spectrometry to analyse light from stars and in everyday devices like CD readers, so the principles have wide practical applications.

• →Diffraction is the spreading of waves when they pass through an aperture or around an obstacle; it is most noticeable when the aperture size is comparable to the wavelength.
• →In single slit diffraction, the central maximum is twice as wide as the secondary maxima, and its width is inversely proportional to the slit width.
• →The diffraction grating equation d sin(theta) = n(lambda) relates the grating spacing d, diffraction angle theta, order n, and wavelength lambda.
• →For a diffraction grating, maxima are sharp and bright because many slits interfere constructively at specific angles, while minima are dark due to destructive interference.
• →The number of orders visible depends on the wavelength and grating spacing; the maximum order is given by n_max = d / lambda (when sin(theta) = 1).

• Monochromatic light gives a central maximum that is much brighter and wider than the secondary maxima, with minima between the maxima.
• The intensity of the secondary maxima decreases as the order increases, so the pattern fades away from the centre.
• White light gives a white central maximum because all wavelengths overlap there.
• The secondary maxima from white light are coloured, with the shorter wavelengths closer to the centre and the longer wavelengths spread further out.
• The pattern is only clearly observed when the slit width is comparable to the wavelength of the light used.
• For a fixed slit width, the central maximum becomes wider as the wavelength increases.
• For a fixed wavelength, the central maximum becomes narrower as the slit width increases.
• The central maximum is widest when the slit width is small compared with the wavelength.
• The central maximum is narrowest when the slit width is large compared with the wavelength.
• The relationship is qualitative: no calculation is required, but the direction of change must be stated correctly.
• State that at normal incidence the path difference between adjacent slits is d sin θ.
• Apply the grating equation d sin θ = nλ to calculate wavelength, slit spacing or order.
• Identify the zero-order maximum at θ = 0° and explain that higher orders occur symmetrically about it.
• Explain that maxima are sharp because many slits contribute, with destructive interference between them.
• Describe the use of a spectrometer or laser to measure angles of diffraction and determine wavelength.
• Explain that white light gives a continuous spectrum in each non-zero order, with violet diffracted least and red most.
• State that the path difference between light from two adjacent slits is d sinθ, where d is the slit spacing and θ is the angle from the straight-through direction.
• Explain that constructive interference occurs when the path difference equals a whole number of wavelengths, so d sinθ = nλ.
• Identify n as the order of the maximum, with n = 0 for the central maximum and n = 1, 2, 3 … for successive maxima.
• Use the equation to calculate d, θ, n or λ, converting angles to radians where required and using sinθ correctly.
• Recognise that the derivation applies to any pair of adjacent slits and that all such pairs reinforce at the same angle.
• Explain how the equation predicts that decreasing d or increasing λ increases the diffraction angle for a given order.
• Explain that a diffraction grating produces sharp maxima at angles determined by d sinθ = nλ, allowing wavelength measurement.
• Describe how a grating separates light into its component wavelengths, producing a spectrum that can be analysed.
• Give a specific application such as spectroscopy in astronomy to identify elements in stars from absorption or emission lines.
• Explain the use of gratings in spectrophotometers or similar instruments to select or analyse particular wavelengths.
• Compare gratings with prisms, noting that gratings give more widely separated and more precisely measurable wavelengths.
• Use the equation to explain why a grating can resolve two close wavelengths when the order or number of slits is sufficient.

• 💡Sketch the pattern with a wide central maximum and progressively fainter secondary maxima, labelling the central maximum clearly.
• 💡For white light, state that the central maximum is white and that the secondary maxima are coloured, with blue nearer the centre than red.
• 💡Use the phrase 'monochromatic light' correctly to mean light of a single wavelength or colour, and contrast it with white light in your answer.
• 💡Use comparative language such as 'wider', 'narrower', 'increases' and 'decreases' to make the direction of change clear.
• 💡Link the effect to the ratio of wavelength to slit width, for example by saying that a smaller slit width relative to the wavelength gives a wider central maximum.
• 💡When comparing red and blue light through the same slit, state that red light gives a wider central maximum than blue light because red has a longer wavelength.
• 💡Write the grating equation as d sin θ = nλ and define each symbol before substituting values.
• 💡Check that the angle you use is measured from the normal to the grating, not from the grating surface.
• 💡When calculating d from lines per metre, use d = 1 ÷ N and keep units consistent, converting to metres where needed.
• 💡Sketch two adjacent slits and mark the path difference d sinθ before writing the equation; this makes the derivation clear and earns method credit.
• 💡When calculating, keep d and λ in the same unit, usually metres, and give the angle to an appropriate number of significant figures.
• 💡If asked to derive the equation, state the condition for constructive interference explicitly rather than just quoting d sinθ = nλ.
• 💡Link each application explicitly to the grating equation, for example by stating that different λ values give different θ values.
• 💡Use precise terminology such as monochromatic, spectral line and order when describing applications.
• 💡When comparing gratings with prisms, refer to the sharpness and angular separation of maxima rather than vague statements about accuracy.
• 💡Always state the diffraction grating equation and define each symbol when answering calculation questions. This secures method marks even if arithmetic errors occur.
• 💡When describing diffraction patterns, use comparative language such as 'wider', 'narrower', 'brighter', 'dimmer' and refer to the central maximum and secondary maxima explicitly.
• 💡For 'explain' questions, link the pattern to path difference and superposition. For example, minima occur when waves from different parts of the slit arrive out of phase and cancel.

• Thinking that the central maximum is narrower than the secondary maxima; the correction is that the central maximum is the widest and brightest part of the pattern.
• Believing that white light produces a white pattern throughout; the correction is that only the central maximum is white, while the secondary maxima are coloured.
• Assuming that the pattern is the same for all slit widths; the correction is that the pattern becomes narrower and less spread out as the slit width increases.
• Stating that increasing the slit width makes the central maximum wider; the correction is that increasing the slit width makes the central maximum narrower.
• Stating that increasing the wavelength makes the central maximum narrower; the correction is that increasing the wavelength makes the central maximum wider.
• Confusing the width of the central maximum with its brightness; the correction is that brightness and width are separate features, and the question asks about width.
• Using the single-slit equation for minima when analysing a grating; the correction is to use d sin θ = nλ for grating maxima.
• Confusing the slit spacing d with the number of lines per metre N; the correction is that d = 1 ÷ N.
• Assuming the zero-order maximum is dispersed into a spectrum; the correction is that at θ = 0° all wavelengths overlap, so only non-zero orders show dispersion.
• Using the angle from the grating surface instead of the angle from the normal; correct this by always measuring θ between the diffracted ray and the straight-through direction.
• Treating d as the number of lines per metre rather than the slit spacing; correct this by using d = 1 / N, where N is the number of lines per metre.
• Forgetting that n must be an integer for a maximum; correct this by checking that the calculated order is a whole number and rejecting non-integer values.
• Assuming a grating produces a continuous rainbow like a prism; correct this by stating that a grating gives discrete orders at specific angles for each wavelength.
• Thinking that all wavelengths diffract by the same angle; correct this by using d sinθ = nλ to show that θ depends on λ.
• Confusing the order number n with the number of slits; correct this by defining n as the order of the maximum and the number of slits as N.
• Students often think diffraction only occurs when the slit width equals the wavelength. In fact, diffraction occurs for all slit widths, but it is most noticeable when the slit width is similar to the wavelength.
• Many students believe that in a diffraction grating, increasing the number of slits per metre makes the maxima wider. Actually, more slits per metre (smaller d) makes the maxima sharper and more widely spaced.
• Students sometimes confuse the diffraction pattern of a single slit with the double slit interference pattern. A single slit gives a broad central maximum with weaker secondary maxima, while double slit gives equally spaced fringes modulated by a single slit envelope.

1. 1Day 1: Review the basics of wave behaviour and superposition. Watch a video or read a textbook section on diffraction and make notes on the conditions for diffraction.
2. 2Day 2: Study single slit diffraction. Draw diagrams of the pattern for different slit widths and wavelengths. Learn the relationship between central maximum width and slit width.
3. 3Day 3: Learn the diffraction grating equation and practice calculations for wavelength, grating spacing, and order number. Include unit conversions (mm to m, lines per mm to lines per m).
4. 4Day 4: Work through exam-style questions on single slit and diffraction gratings. Focus on explanation questions and calculations. Check your answers against mark schemes.
5. 5Day 5: Create a summary sheet with key equations, definitions, and common pitfalls. Test yourself with active recall prompts and past paper questions.

• 📋Calculation questions using d sin(theta) = n(lambda): You will be asked to find wavelength, grating spacing, or order number. Always convert units and rearrange carefully.
• 📋Explanation questions on single slit diffraction: Describe how the pattern changes when slit width or wavelength changes. Use terms like 'central maximum', 'secondary maxima', 'wider', 'narrower'.
• 📋Practical questions on measuring wavelength with a diffraction grating: Describe the procedure, including how to measure angles accurately and reduce uncertainty.
• 📋Comparison questions between single slit and double slit patterns: Explain the differences in the pattern and the reasons for them.