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Diffraction and the single slit questions
Waves spread out when they pass an edge or a gap. It happens at every gap, at every size, but the spreading only becomes dramatic when the gap shrinks towards the size of the wavelength.
16 original questions · 45 marks · the diffraction and the single slit notes · Waves
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Describe what is meant by diffraction, and state when the spreading of a wave at a gap is greatest.
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Diffraction is the spreading of a wave as it passes through a gap or around an obstacle (1). The spreading is greatest when the gap width is about the same size as the wavelength (1).Sketch the intensity pattern produced when monochromatic light passes through a single narrow slit, and describe its main features.
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A bright central maximum with much dimmer maxima either side (1). The central fringe is twice as wide as the others and far brighter; the outer fringes decrease in brightness with distance from the centre (1).State what happens to the single-slit diffraction pattern when the slit is made narrower.
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The fringes spread out (1): the central maximum (and all the fringes) become wider (1).Sea waves of wavelength 12 m pass through a harbour entrance 15 m wide. State and explain how the waves behave beyond the entrance.
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The entrance is about the same size as the wavelength (1), so the waves diffract strongly, spreading into the harbour as near-circular wavefronts (1).State two ways in which the appearance of a single-slit diffraction pattern differs from the double-slit fringe pattern produced with the same light.
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In the single-slit pattern the central maximum is twice the width of the other maxima, whereas double-slit fringes are all the same width and evenly spaced (1). In the single-slit pattern the side maxima are much dimmer than the centre, whereas double-slit fringes are (nearly) equally bright (1).Explain why sound is clearly diffracted through an open doorway but light passing through the same doorway is not noticeably diffracted.
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Diffraction is significant when the gap is comparable to the wavelength (1). Sound wavelengths are of the order of the door width (about a metre), so sound diffracts strongly (1); the wavelength of light is around 10−7 m, far smaller than the doorway, so its diffraction is negligible (1).Describe how the single-slit diffraction pattern changes when the light is switched from blue to red, for the same slit.
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Red light has a longer wavelength than blue (1), so it diffracts more (1). The fringes, including the central maximum, become wider and more spread out for red than for blue (1).Describe the single-slit diffraction pattern produced with white light.
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The central maximum is white (all wavelengths overlap there) (1). The side maxima are spread into colours, with blue/violet closest to the centre and red furthest out, because longer wavelengths diffract more (1); the fringes overlap and become less distinct further out (1).Explain why the central maximum of a single-slit pattern is both wider and brighter than the other fringes.
A student illuminates an adjustable slit with a red laser and slowly narrows the slit, starting from a width of several millimetres. Describe how the pattern on a distant screen changes as the slit narrows.
Microwaves of wavelength 2.8 cm pass through a vertical gap between two metal plates whose separation can be adjusted. State the approximate gap width that gives the greatest spreading of the microwaves, and describe and explain what a detector moved across the region beyond the gap records when the gap is 50 cm wide.
In a single-slit experiment, the slit width and the wavelength of the light are both doubled. Explain what happens to the width of the central maximum.
Two single slits of different widths are each illuminated with the same monochromatic light. Explain which slit produces the wider central maximum, and what happens to the brightness.
Explain why a laser is well suited to demonstrating diffraction patterns.
A teacher wants to project the widest possible single-slit central maximum. Three arrangements are available.
Arrangement W: laser of wavelength 650 nm, slit width 0.10 mm.
Arrangement X: laser of wavelength 532 nm, slit width 0.10 mm.
Arrangement Y: laser of wavelength 650 nm, slit width 0.20 mm.
Deduce which arrangement the teacher should choose.A single slit is illuminated with white light. A student then places a red filter between the lamp and the slit. Describe and explain how the pattern on the screen changes.
The same practice on paper: the printable workbook for this topic, questions and a worked answer book.
Practise diffraction and the single slit one question at a time
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