8.4 The Diffraction Grating | CIE AS Physics Topic Questions 2025

1a

4 marks

The diffraction grating equation is given by:

$d \sin θ = n λ$

State the definition of the following variables and an appropriate unit for each.

(i)

$d$

[1]

(ii)

$θ$

[1]

(iii)

$n$

[1]

(iv)

$λ$

[1]

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1b

2 marks

A diffraction grating has 720 × 10³ lines per metres.

Calculate the distance between adjacent slits of the grating.

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1c

3 marks

A narrow beam of light of wavelength 635 nm is incident normally on the diffraction grating, as shown in Fig. 1.1

Fig. 1.1

The first‑order diffraction maximum of the light is at an angle of θ to the direction of the incident light beam.

Calculate the value of θ.

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1d

1 mark

The highest order observed on the screen is the second-order diffraction maximum.

State how many minima are seen on the screen.

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1a

6 marks

Students in a laboratory have created the following set-up shown in Fig 1.1. Parallel rays of monochromatic light from two adjacent slits A and B of wavelength $λ$ are incident normally on a diffraction grating with a slit separation $d.$

9-3-ib-hl-hsq2a-q

Fig. 1.1

Use the diagram to derive the equation nλ = dsinθ where θ is the angle of diffraction of a maxima order n visible on a screen.

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1b

5 marks

The monochromatic light in the set-up in part (a) has a wavelength of 499 nm. The graph in Fig. 1.2 shows the variation of sinθ with the order n of the maximum. The central order corresponds to n = 0.

9-4-ib-hl-hsq2b-q

Fig. 1.2

Determine a mean value for the number of slits per mm of the grating.

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1c

3 marks

The grating is 30 mm wide.

Determine the number of slits required to obtain a maximum of five bright fringes on the screen.

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1d

2 marks

The students claim that they observed the following diffraction pattern in Fig. 1.3 on the screen for the grating from part (c).

9-3-2

Fig. 1.3

State two reasons why the interference pattern obtained cannot be correct.

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1a

3 marks

A teacher explains a diffraction investigation to a group of physics students.

'In this investigation a laser will be used to shine monochromatic light of wavelength 581.9 nm onto a diffraction grating. The light passing through the grating will therefore be coherent.'

Define the following terms used by the teacher

(i)

Monochromatic

[1]

(ii)

Coherent

[2]

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1b

6 marks

During the investigation, a second-order maximum is produced at an angle of 35° measured from the normal to the grating.

(i)

Calculate the number of lines per metre on the grating.

[3]

(ii)

Calculate the highest order which is observable.

[3]

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1c

3 marks

Calculate the angular separation between the highest order and second order maxima.

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1d

2 marks

The investigation is repeated using the same grating but a different monochromatic source.

In this experiment, the second-order maximum is observed at an angle of 25.7°.

Calculate the wavelength of this second source.

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The Diffraction Grating (CIE AS Physics)

Topic Questions