The spreading of a wave as it passes through a gap or around an obstacle.

When is diffraction most noticeable?

When the gap width is similar in size to the wavelength.

Describe the single-slit pattern with monochromatic light.

A wide bright central maximum with much less intense, narrower maxima on either side.

What happens to the central maximum when the slit is made narrower?

It becomes wider (more diffraction).

What is seen when white light passes through a single slit?

A white central maximum with coloured edges: violet inside, red outside.

What is a diffraction grating?

A plate with many equally spaced parallel slits that gives sharp, bright maxima.

What does n represent in d sin θ = nλ?

The order of the maximum: n = 0 is the central maximum, n = 1 the first order, and so on.

How do you find the highest order that can be seen?

Use sin θ ≤ 1, so n ≤ d/λ, and take the whole number below.

Why are grating maxima better for measurement than double-slit fringes?

They are much narrower, sharper and brighter, so the angle is measured more precisely.

What happens to the angles of the orders if the grating has more lines per mm?

They increase, because d is smaller and sin θ = nλ/d.

Give an application of diffraction gratings.

Analysing line spectra to identify elements, for example in astronomy.

Diffraction and diffraction gratingsAQA A-Level Physics: Flashcards

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Define diffraction.

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Define diffraction.: The spreading of a wave as it passes through a gap or around an obstacle.
When is diffraction most noticeable?: When the gap width is similar in size to the wavelength.
Describe the single-slit pattern with monochromatic light.: A wide bright central maximum with much less intense, narrower maxima on either side.
What happens to the central maximum when the slit is made narrower?: It becomes wider (more diffraction).
What happens to the central maximum when the wavelength increases?: It becomes wider.
What is seen when white light passes through a single slit?: A white central maximum with coloured edges: violet inside, red outside.
What is a diffraction grating?: A plate with many equally spaced parallel slits that gives sharp, bright maxima.
How is the slit spacing d found from lines per metre N?: d = 1/N
State the diffraction grating equation.: d sin θ = nλ
What does n represent in d sin θ = nλ?: The order of the maximum: n = 0 is the central maximum, n = 1 the first order, and so on.
How do you find the highest order that can be seen?: Use sin θ ≤ 1, so n ≤ d/λ, and take the whole number below.
Why are grating maxima better for measurement than double-slit fringes?: They are much narrower, sharper and brighter, so the angle is measured more precisely.
What happens to the angles of the orders if the grating has more lines per mm?: They increase, because d is smaller and sin θ = nλ/d.
Give an application of diffraction gratings.: Analysing line spectra to identify elements, for example in astronomy.

Exam questions on Diffraction and diffraction gratings

Monochromatic light of wavelength 600 nm is directed at a single narrow slit of width 0.10 mm. The light that passes through forms a diffraction pattern on a screen several metres away.
The slit is replaced by a gap 10 mm wide. Explain why little diffraction is now seen.2 marks
Light of wavelength 589 nm from a sodium lamp is incident normally on a diffraction grating that has 600 lines per millimetre.
Calculate the angle between the straight-through direction and the first-order maximum.2 marks
A laser of unknown wavelength is directed at normal incidence at a diffraction grating that has 300 lines per millimetre. The second-order maximum is observed at an angle of 22.3° to the straight-through direction.
Calculate the wavelength of the laser light.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).