Syllabus Edition

First teaching 2020

Last exams 2024

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Diffraction & Interference (CIE AS Physics)

Exam Questions

1 hour33 questions
1a
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3 marks

Fig. 1.1 shows an arrangement for observing the interference pattern produced by laser light passing through two narrow slits S1 and S2.

9-3-hl-sq-medium-q2a-diag

Fig. 1.1

The distance S1S2 is d, and the distance between the double slit and the screen is D, where D d, so angles θ and ϕ are small.

M is the midpoint of S1S2 and it is observed that there is a bright fringe at point A on the screen, a distance fn from point O on the screen. Light from S1 travels a distance S2Y further to point A than light from S1.

The wavelength of light from the laser is 650 nm and the angular separation of the bright fringes on the screen is 5.00 × 10−4 rad.

(i)
Explain the conditions required in the paths of the rays coming from Sand S2 to obtain the bright fringe at A. 
[2]
(ii)
State an equation in terms of wavelength for the distance S2Y.
[1]
1b
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4 marks

Deduce expressions for the following angles θ in terms of S2Y and d in the double-slit arrangement shown in part a.

[2]

1c
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3 marks

Calculate the distance S1S2 between the two slits.

1d
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3 marks

In a different experimental set up the separation of the slits Sand Sis 1.30 mm.

The distance MO is 1.40 m.

The distance fn is the distance of the ninth bright fringe from O and the angle φ is 3.70 × 10−3 radians. 

Calculate the wavelength of the laser light. 

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2a
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6 marks

A student is conducting a series of investigations on diffraction and interference with two slits.

In the first investigation, monochromatic light passes through a double-slit arrangement. The intensity of the fringes varies with distance from the central fringe. This is observed on a screen, as shown in Fig. 1.1.

9-3-hl-sq-medium-q1aFig. 1.1

The intensity of the monochromatic light passing through one of the slits is reduced.

Explain the effect of this change on the appearance of

    
(i)
The dark fringes.
[3]
(ii)
The bright fringes.
[3]
2b
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7 marks

In an investigation, a white light source is incident on an orange filter, a single slit, and then a double-slit, as shown in Fig. 1.2.

An interference pattern of light and dark fringes is observed on the screen.



9-3-hl-sq-medium-q1b

Fig. 1.2 

(i)
The orange filter is now replaced by a green filter.
 
State and explain the change in appearance, other than the change in colour, of the fringes on the screen.
[2]
(ii)
The green filter is now removed.
 
State and explain the change in the appearance of the central maximum fringe, as well as the fringes away from this central position.
[5]
2c
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2 marks

In a third experiment, the white light is replaced by orange light of wavelength 600 nm. The double-slit has a separation of 0.350 mm and the screen is 6.35 m away. 

Calculate the distance between the central and first maximum as seen on the screen.

2d
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3 marks

The light source is now changed to a blue LED of wavelength 450 nm.

Explain the features of the interference pattern that will now be observed on the screen.

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3a
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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]

3b
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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]
3c
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3 marks

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

3d
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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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