1.	A laser beam of wavelength 690 nm is directed normally at parallel slits as shown below.

	Calculate the fringe spacing at the screen.
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Fringe spacing =……….........……...............
	Sketch the pattern which would be observed on the screen.
(4)
	This laser beam is replaced by one with a wavelength of 460 nm. Describe how the appearance of the fringes would change.
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(2)

	The two laser beams are now directed simultaneously at the slits. Which fringes exactly overlap?
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(2)
(Total 8 marks)

2.	The diagram shows an arrangement to produce interference fringes by Young's two slits method.

State suitable values for s and D if clearly observable fringes are to be produced.
s ...........................................................................................................................................
D ..........................................................................................................................................
Explain how the bright fringe labelled P is formed.
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(4)
What would be the effect on the fringe width x of
(i)	increasing the slit separation s,
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(ii)	illuminating the slits with blue light?
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(2)

	To obtain an interference pattern the light from the two slits must be coherent.  What is meant by the term coherent?
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(1)
(Total 7 marks)

3.	The photograph shows the interference pattern produced when monochromatic light falls on a pair of slits 0.5 mm apart.  The pattern was produced on a screen 1.5 m from the slits.

	The photograph has been magnified by a factor of × 3.  Use the photograph to obtain a value for the fringe spacing.
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(2)
Calculate the wavelength of the light used.
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 Wavelength = .........................................................
(2)
	Mark with an X on the photograph the fringe or fringes where light from one slit has travelled a distance of two wavelengths further than the light from the other slit.
	Explain why the fringes near the centre of the photograph are clearer than those near the edges of the photograph.
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 (4)

	In the space below sketch the pattern which would be obtained on the screen if one of the slits were covered up.  Label the bright and the dark regions.
(An accurate scale diagram is not expected.)

(2)
	What additional measurement would you need in order to draw an accurate diagram for this case?
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(1)
(Total 11 marks)

4.	Explain the term plane polarised wave.
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(2)
	Describe an experiment using light or microwaves which tests whether or not the waves are plane polarised.
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(2)

	For each of the statements below, indicate whether the statement is true (Ö) or false (X).
Statement | True/False
The speed of sound in air is less than the speed of sound in water. |
Since sound waves are longitudinal they cannot be diffracted. |
Sound waves transmit pressure but not energy. |
A sound wave of frequency 436 Hz travelling at 331 ms^–1 has a wavelength of 75 cm ± 1 cm |
(4)
(Total 8 marks)

5.	A beam of microwaves is directed at two slits in a metal sheet.  The diagram below shows two adjacent positions P and R where a microwave detector would register maximum readings.

	Use the diagram to determine the wavelength of the microwaves.
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Wavelength = .......................................
	Calculate the frequency of the microwaves.
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Frequency = ........................................
(4)
	On the diagram mark with a Q a position where another maximum reading would occur. On the diagram mark with a D a position where a minimum reading would occur.
(2)

	In a similar experiment, sound waves were directed at the same metal sheet.  The speed of sound is 330ms^–1 and the frequency of the sound waves was 1100Hz.  Explain why a maximum reading would not be detected at P in this experiment.
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(2)
(Total 8 marks)

St Mary's College