The diffraction grating
Cambridge A-Level Physics (9702) · Unit 8: Superposition · 8 flashcards
The diffraction grating is topic 8.4 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 8 — Superposition , alongside Stationary waves, Diffraction and Interference. In one line: The formula is d sin θ = nλ. This equation relates the grating spacing, angle to the maxima, the order number of the maximum, and the wavelength of the incident light.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 8 flashcards — 2 definitions, 4 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Formula that relates the diffraction grating spacing (d), the angle to the nth order maximum (θ), the order number (n), and the wavelength of light (λ)
The formula is d sin θ = nλ. This equation relates the grating spacing, angle to the maxima, the order number of the maximum, and the wavelength of the incident light.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Recall d sin θ = nλ and use it.
- Describe how to measure the wavelength of light with a diffraction grating. How a spectrometer is built and used is not required.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in The diffraction grating
- › Calculate the number of bright dots using the formula (2n + 1), where n is the maximum integer order, to include the zero-order and both sides.
- › In the equation d sinθ = nλ, always use the angle θ relative to the original path of the light.
State the formula that relates the diffraction grating spacing (d), the angle to the nth order maximum (θ), the order number (n), and the wavelength of light (λ).
The formula is d sin θ = nλ. This equation relates the grating spacing, angle to the maxima, the order number of the maximum, and the wavelength of the incident light.
What does 'd' represent in the formula d sin θ = nλ and what are its common units?
'd' represents the grating spacing, which is the distance between adjacent slits on the diffraction grating. It is commonly measured in meters (m), but can also be given in mm or μm. Ensure you convert to meters before using it in the formula.
Explain how a diffraction grating produces a series of bright fringes (maxima) when illuminated by monochromatic light.
A diffraction grating produces bright fringes due to the interference of light waves that have diffracted through the slits. Constructive interference occurs when the path difference between waves from adjacent slits is equal to an integer multiple of the wavelength (nλ), leading to bright fringes at specific angles.
A diffraction grating has 500 lines per mm. Calculate the grating spacing 'd'.
The grating spacing, d, is the inverse of the number of lines per unit length. d = 1 / (lines per unit length). Here, d = 1 / (500 lines/mm) = 1/(500 x 10^3 lines/m) = 2 x 10⁻⁶ m.
Describe how you would use a diffraction grating to determine the wavelength of monochromatic light.
Shine the light through the grating and measure the angle θ to a specific order maximum (n). Knowing the grating spacing (d) and order number (n), use the formula λ = d sin θ / n to calculate the wavelength λ.
Monochromatic light of wavelength 600 nm is incident on a grating with a spacing of 2.0 x 10⁻⁶ m. Calculate the angle of the first-order maximum.
Using d sin θ = nλ, rearrange to get sin θ = nλ / d. For the first order (n=1), sin θ = (1 * 600 x 10⁻⁹ m) / (2.0 x 10⁻⁶ m) = 0.3. Therefore, θ = arcsin(0.3) = 17.46 degrees.
What is the maximum number of orders of diffraction that can be observed for a given wavelength λ and grating spacing d?
The maximum number of orders is determined by the condition that sin θ ≤ 1. Therefore, n_max = d / λ, rounded down to the nearest whole number. If d/λ = 3.5, then a maximum of three orders can be observed either side of the zero order.
Why are diffraction gratings preferred over double slits for measuring the wavelength of light?
Diffraction gratings produce sharper and brighter fringes than double slits, making it easier to measure the angle θ accurately. The greater number of slits leads to increased intensity at the maxima and a more precise determination of wavelength.
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Read full revision notes on The diffraction grating — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 8 — Superposition
The diffraction grating sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this The diffraction grating deck
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