Unit 2 Physics – Wave-like Properties of Light Test Answers
Answer all questions in the space provided, showing all working and
reasoning. Answers should be written with 3 significant figures.
--------------------------------------------------------------------------------Questions 1-4 refer to the following in formation:
A continuous longitudinal wave [ frequency of 4.0 Hz ] travels north
along a stretched spring. It is noted at a particular moment that the
spring has compressions in it every 18.0 cm along its length. A small
piece of red ribbon is attached to a certain spot along the spring.
18 cm
Question 1.
What is the wavelength of the wave?
0.18 m
Question 2.
What is the speed of the wave along the spring?
0.72 ms-1
Question 3.
What is the time interval between successive compressions reaching the spring
end?
0.25 s
Question 4.
If you concentrated on the small piece of red tape, describe in detail how it would
be moving as the longitudinal wave travels northward along the spring.
Ans: It would move north-south then back to north again, back and forwards,
oscillating as each wave passes
[1+2+2+2 = 7]
Questions 5-7 refer to the displacementdistance graph for a continuous wave
displayed below. Point P represents the position of a particle in the medium.
Displacement
[mm]
4
P
2
1
2
3
4 distance [cm]
-2
-4
Question 5.
What is the wavelength of this periodic wave?
2
P’
cm
Question 6.
What is the amplitude of this wave?
4 mm
Question 7.
On the graph above, sketch in the displacementdistance graph for this
wave, recorded ½ a period later. Include the new position of point P on your
graph.
[ 1+1+2 = 4 marks ]
--------------------------------------------------------------------------Question 8 refers to the following diagram.
The diagram below shows a wave pulse travelling in a string towards an
end. In the space provided, sketch the pulse shape after it has reached the
end if:(i) the end was fixed
(ii) the end was free to move
END
Question 8.
Fixed end
Free end
]
[ 1+1 = 2 marks
Questions 9-10 refer to the displacementtime graph shown below.
The graph below shows the displacement of a particular point in a medium
over time as a continuous transverse wave travelled through it.
Displacement
[mm]
1
2
3
4
5
Time [milliseconds]
Question 9.
What is the period of the wave?
2 ms
Question 10.
What is the frequency of the continuous wave?
500 Hz
[ 1+2 = 3 marks ]
--------------------------------------------------------------------------Questions 11 refers to the following in formation:
The diagram below displays the instant that two continuous transverse
waves travelling in opposite directions superimpose and interfere.
displacement [cm]
Distance [m]
Question 11.
On the diagram, sketch the resultant waveform for the superposition of these
waves.
(Join the dots with a smooth curve)
[ 2 marks ]
-------------------------------------------------------------------------------
Questions 12-13 refer to the following diagram and information:
A ray of light travelling in air hits a horizontal slab of glass, 10 cm thick,
at an angle of incidence of 28o. The refractive index of the particular glass
is 1.54 and the speed of light in air is 3.0x108 ms-1.

 = 28o
Question 12.
What is the angle of refraction of the ray travelling through the glass?
1 x Sin 28 = 1.54 Sin r so r = Sin-1 (Sin28/1.54) = Sin-1(0.4695/1.54) =
Sin-1(0.3049) = 17 o 45’
o
17 o 45’
Question 13.
What is the speed of light travelling through the glass?
3 x 108 ÷1.54 =
1.95 x 108ms-1
[ 2+2 = 4 marks ]
--------------------------------------------------------------------------Questions 14-15 refer to the following:
The Australian flag has areas of red, white and blue on it.
Question 14.
What colours of white light would be absorbed by the red dye in the flag?
Everything except red, so blue, green etc
Question 15.
If the Australian flag was viewed when pure green light was shone on it, what
colours would be seen for each of the areas on the flag?
Red area
Bbbb black
Blue area
Bbbb
blue
White area
Bbbb
green
[ 1+3 = 4 marks ]
Questions 16-19 refer to the following diagram and information:
glass fibre [n = 1.554]
cladding material [n = 1.528 ]
Monochromatic light pulses from a laser are fired into the glass fibre at an
angle of incidence of about 80o. The fibre is part of communication
network.
Question 16.
What is the critical angle for light travelling from the glass fibre into the cladding?
1.554 Sin (critical angle) = 1.528 Sin 90
Critical angle = Sin-1 (1.528/1.554) = 79 o 30’
79o 30’
Question 17.
Explain why the cladding has to have a refractive index that is slightly smaller than
the glass fibre.
This is so that any light incident on the junction will not exit the cable, but remain
in the fibre optic “tube”, being carried along by successive reflections back into the
cable.
Question 18.
Explain why it is important that the light pulses are sent into the fibre with a large
angle of incidence [ > 80o ].
This enables the light to be totally internally reflected and to continue to be
reflected along the tube. It also enables long paths and fewer reflections for the
length of the tube. The tube can bend around things and the light will still be
carried along the tube rather than escaping.
Question 19.
Explain why it is important for a communication through optic fibre to utilise
monochromatic [ single wavelength ] laser light pulses.
If light of mixed requencies were used, the refractive index for the different
wavelengths would be different, meaning that different colours would arrive at the
other end at different times. This would mess up the signal so it made no sense.
[ 3+2+2+2 = 9 marks ]
------------------------------------------------------------------------------Question 20. refers to the following information.
A 1.8 m tall women stands 2.40 m in front of a plane mirror on the wall
so that she just manages to see a full length image of herself.
Question 20.
Fully describe the nature of her image, including type, orientation, location and
size.
It is virtual, upright and the same size as the woman. It is also the same distance
behind the mirror as she is in front of it.
[ 2 marks ]
Questions 21-23 refer to the following:
Consider two curved mirrors, both with a focal length of 10 cm. One of
the mirrors is a concave mirror, the other is a convex mirror.
Question 21.
Give 1 practical use for each of the mirrors.
Concave mirror use = magnifying an object – makeup mirror
Convex mirror use = increased field of view (around corners)
Question 22.
If the convex mirror was used to observe an object, describe the image seen …
[ ie. real or virtual, upright or inverted, magnified or diminished ].
Virtual, upright, magnification greater than 1
Question 23.
A 12 mm tall object was placed 4.0 cm in front of the concave mirror [ f = 10 cm
].
Use ray tracing to find the nature, position, size and magnification of the
image.
Nature of image = upright, virtual, enlarged
Position = 6.5 cm behind the mirror
Size = 3 x 6.7 mm
Magnification = 3 x 1.67 = 5.01
[ 2+2+3+2+1+1+1 = 12 marks ]
Questions 24-26 refer to the following:
Consider two glass lenses, both with a focal length of 5.0 cm. One of
them is a biconcave lens, the other is a biconvex lens.
Question 24.
Give 1 practical use for each lens.
Concave lens use = telescope in an achromatic doublet
Convex lens use
= magnifying glass
Question 25.
Which lens could produce an image on a screen? [ give a reason for your choice ]
The biconvex lens could produce a real image on a screen. The biconcave lens can
only produce virtual images, so cannot be used to project onto a screen.
Question 26.
A 6 cm tall object was placed 9.0 cm in front of the double convex lens [f = 5.0
cm].
Use ray tracing to find the nature, position, size and magnification of the
image.
Nature of image = Real, inverted, enlarged
Position = 11.5 cm from lens
Size = 7.6 cm
Magnification = 7.6/6 = 1.27
[ 2+2+3+2+1+1+1 = 12 marks ]
END OF WAVE-LIKE PROPERTIES OF LIGHT SAC TEST