Chapter 15 Seeing the light

15.1 The electromagnetic spectrum
The frequency of a wave sequence observed in a vibrating string is 200 Hz. If the velocity
of the waves is 30 m s–1, what is the wavelength?
A boat on a lake bobs up and down due to ripples on the water surface. The person in the
boat notices that the crests of the ripples are 6 m apart and that a ripple raises the boat
every 3 s. Calculate the speed of the ripples.
The distance between compressions for a particular sound source is 1.3 m, and the
compressions travel at a speed of 330 m s–1 through the air. What is the frequency with
which the sound source vibrates?
15.3 Behaviour of light
Light travels at a speed of 2.1 × 108 m s–1 through medium X. What is this medium’s
refractive index?
What will be the frequency of violet light (λ = 420 nm) as it passes through window
Calculate the speed at which light travels through diamond.
15.6 Refraction
Light travelling from water into glass (nglass = 1.53) is refracted at an angle of 49°. At
what angle was the light incident upon the glass?
If a laser light is shone onto a pool of water at an incident angle of 15° to the normal,
what will be its angle of refraction?
A beam of light shines onto a glass slab (nglass = 1.51) which has a thickness of 4 cm. If
the beam makes an angle of 30° with the slab surface, how far horizontally will the beam
exit the block from where it entered?
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Review questions
A marker buoy bobs up and down as the waves move past it. How is the frequency of the
buoy’s motion related to the number of waves that pass it in one second?
In what ways do transverse waves differ from compression waves? How are they similar?
Why is the front lettering on an ambulance printed in reverse?
Explain why you can see your face in a mirror, but not in a white piece of paper.
Which of the following are luminous and which are illuminated?
The Sun
A projector screen
A star
A campfire
A red-hot piece of iron
A television screen
Explain how you are able to use two mirrors to see your view from behind.
Phuong placed a coin in the bottom of an opaque mug. From where she is sitting, she
can’t quite see the coin. However, when she pours some water into the mug, she finds
that she can now see the coin. How is this possible?
One type of plastic has a refractive index equal to that of water (n = 1.33). If you placed a
lump of this plastic into water, would you be able to see it?
A wave travelling at 4 m s–1 has a frequency of 10 Hz. What is its wavelength?
10. Mark stands at the edge of a fish pond and sees a large fish in the water. From where he is
standing, the fish is 2 m horizontally from the pond’s edge and appears to be 50 cm
below the surface. How far below the surface of the pond is the fish actually located?
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Assume that Mark’s eyes are 1.5 m above pond level.
11. An object is placed at an equal distance a from two plane mirrors which are placed at
right angles as shown in the figure below. How many images of the object are formed in
the mirrors?
12. (a)
What is the angle of refraction in water (n = 1.33) of a light ray that has an
incident angle of 30°?
By how much will the angle of refraction increase if the incident angle is increased
by 10°?
13. What is the minimum length of mirror that will allow Joel to see his entire body if he is
1.9 m tall and his eyes are located 10 cm below the top of his head?
14. A laterally symmetrical object will look exactly the same when placed in front of a
mirror. The capital letter ‘A’, for example, will still be recognisable when reversed in a
mirror. What other letters of the alphabet are laterally symmetrical when written in
15. As part of an experiment on Snell’s Law, a student measures the angle of refraction (θ2)
obtained when an incident light ray enters a clear plastic block for a number of different
incident angles (θ1). Her results are shown in the table below.
Draw a graph of this data.
Use your graph to determine the refractive index of the plastic.
16. Draw a configuration of plane mirrors that will allow you to see around a 90° corner.
Indicate angles on your diagram.
17. When you look into a plane mirror, your left and right sides appear reversed. This is
called lateral inversion. How could you position a series of plane mirrors so that you see
your image upside down?
18. Immiscible liquids are liquids that do not mix. When poured into the same container,
immiscible liquids will settle in layers in order of their density, with the densest liquid on
the bottom and the least dense at the top. The figure above right shows a beaker in which
three transparent immiscible liquids have been placed and left to settle. The beaker has a
circular bottom (radius = 10 cm) and parallel sides. (Assume the thickness of the glass is
negligible.) Each layer is 1.5 cm deep.
A ray of light enters the beaker and strikes the surface of the acetone at an angle of 25° at
a point 9 cm from the centre of the surface. How far from the centre of the beaker’s
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bottom will the refracted ray leave? (Assume the beaker is of negligible thickness and the
ray’s plane of refraction is radial.)
19. Calculate the sideways deflection x as a ray of light passes through a parallel-sided plastic
block (n = 1.42) as shown in the figure below.
20. Calculate the angle of deviation as the light ray goes through the triangular prism shown
in the figure below.
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