Waves, Electrons & Photons (Edexcel A Level Physics)

Exam Questions

34 mins5 questions
1a
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5 marks

Light from a laser pointer was passed through a diffraction grating. The light was perpendicular to the diffraction grating as shown. A diffraction pattern was produced on a screen.

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The distance between the first order maximum and the central maximum of the diffraction pattern was x. The distance between the diffraction grating and the screen was D.

Distance x was measured to be 0.500 m with a metre rule. The wavelength of light lambda subscript 1from the laser pointer was 650 nm.

The laser pointer was replaced with one that produced light of a different wavelength. The new distance x was measured to be 0.400 m.

D = 1.45 m

Calculate the wavelength lambda subscript 2 of the light emitted by the replacement laser pointer.

lambda subscript 2= .......................................................

1b
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2 marks

Explain one modification to this method that would decrease the uncertainty in the calculated value of lambda subscript 2.

1c
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3 marks

In another experiment, the light from the laser pointer was not quite perpendicular to the screen.

Explain how this would change the diffraction pattern produced on the screen.

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2
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6 marks

In 1965, Richard Feynman proposed a double slit experiment to investigate the wave properties of electrons.

The experiment was later carried out using the arrangement shown.

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A beam of electrons was directed at a barrier with two slits.

The detector recorded the positions where electrons arrived after passing through the slits.

The following pattern was obtained. Black dots represent points where electrons were detected. A band where electrons were not detected has been labelled X and a band where electrons were detected has been labelled Y.

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The path difference for electrons arriving at band X from the separate slits was 2.5 × 10-11 m.
For electrons arriving at band Y the path difference was 5.0 × 10-11 m.

Explain why this pattern is observed when the electron energy is 9.6 × 10-17 J.

The electrons are travelling at non-relativistic speeds.

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3a
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3 marks

The photograph shows a school spectrometer.  

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The spectrometer allows parallel rays of light to be passed through a diffraction grating and the resulting angles of diffraction to be measured.

In the telescope, light from the grating is focused to make a real image 16.7 mm in front of the eyepiece lens. The eyepiece lens then uses this real image as an object to produce a magnified virtual image for the observer.

Calculate the magnification produced by the eyepiece lens.
focal length of eyepiece lens = 17.9 mm  

Magnification = ........................................

3b
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9 marks

The spectrometer and diffraction grating are used to analyse the light from a sodium lamp. In the sodium lamp, sodium is heated until it becomes a vapour and an electric current is passed through it. The vapour then emits light.

After the light passes through the diffraction grating a line spectrum is observed.

i)
Explain why only certain wavelengths are observed.  

(6)
ii)
Diffraction gratings with the following spacings are available: 

d /10-6 m 1.0 1.7 2.0 3.3

Explain which would be the best spacing to use to measure the diffraction angle for the third order maximum for yellow light of wavelength 589 nm. 

(3)

3c
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4 marks

The diagram shows some of the energy levels in a sodium atom. 

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Add an arrow to the diagram to show the transition involved in the emission of yellow light of wavelength 589 nm.

Show your working below.

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