Revision for Unit3 NAB Test 3.1
1. What is transferred by a wave?
Energy
2. The amplitude of a wave is increased by a factor
of three.
What will happen to the intensity of the wave?
It increases by a factor of 9. (I α a2)
3. What type of wave can be used to describe any
repeating wave using superposition?
Sine or cosine.
4. What do all the symbols represent below:
y = a sin 2 π (f t - x / λ )
y is the displacement of medium. ‘a’ is the
amplitude. ‘f’ is the frequency. t is the time. ‘x’
position along x axis. ‘λ’ is wavelength.
5. For a wave y = 20 sin 2 π (5 t - x / 8 )
a) Is the wave moving to the left or the right?
Right
b) Calculate the speed of the wave.
V = f λ = 5 x 8 = 40 m/s
6.
The above wave is moving to the left at 21 m/s.
State the wave equation that would describe this
wave.
(f = v / λ = 21 / 7 = 3 Hz )
Y = 25 cos 2π (3 t + x / 7 )
7.
a) In what type of wave do the particles oscillate at
right angles to the direction of travel?
Transverse
b) What type of wave is a sound wave?
Longitudinal
8. What type of wave is produced by the
interference of two coherent waves travelling in
opposite directions?
Standing wave.
9.
a) Which points are nodes? X, Z
b) Which points are antinodes? W, Y
10. What is the amplitude at a node?
Zero
11.
Minimum levels of sound are marked with a blue
dot.
a) Calculate the wavelength of the sound.
20 / 5 = 4m = λ / 2, So λ = 8 m
b) Calculate the frequency if v = 340 m/s.
f = v / λ = 340 / 8 = 42.5 Hz
12.
Points
W to Y
U to V
U to Y
V to Y
Phase
Wavelengths
0.5
0.25
1.0
0.75
Difference
Degrees
180
90
360
270
Radians
π
π/2
2π
3π/2
13. A travelling wave has a wavelength of 40 cm.
Calculate the phase difference from the origin to a
point 27.6 cm along the wave.
Φ / 2π = X / λ
Φ / 2π = 27.6 / 40
Φ = 4.34 rad
14. What is the phase difference in radians on the
next nearest wave that is “in phase”?
2π rads = 6.28 rad
(Two nearest points that are in phase e.g. 1λ)
15. What name is given to the change in frequency
when a source is moving relative to an observer?
Doppler Effect
16. A siren emits a note at 800 Hz as the ambulance
travels away from the observer at 30 m/s.
Calculate the frequency heard by the observer.
(Speed of sound = 340 m/s)
fobs = fs v /( v + vs )
fobs = 800 x 340 /( 340 + 30 )
fobs = 735 Hz
17. A stationary ice-cream van emits a note at 1000
Hz as a car approaches at 20 m/s.
Calculate the frequency heard by the car driver.
fobs = fs ( v + vo ) / v
fobs = 1000 ( 340 + 20 ) / 340
f = 1059 Hz
18. A source of sound waves is moving towards a
stationary observer with speed vs. The sound has a
frequency of fs and a speed v.
Derive an expression for the frequency heard by the
observer.
λ = v / fs
for observer λobs = v / fs – vs / fs = 1/fs ( v – vs)
fobs = v / λobs = fs v / ( v - vs)
19. What is required to make two light beams
coherent?
Same frequency and constant phase difference.
20. Explain why it is difficult to make light waves
coherent unlike sound or microwaves.
Light is produced by electrons falling in energy level
in an atom. This process is spontaneous or random.
Different sources will not always be in phase even if
they are the same frequency.
21. When light enters glass why do we have to
consider the optical path length rather than the
actual length?
Being optically more dense means that there are n
times more wavelengths in the same distance so we
must multiply the length by the refractive index.
22. Two coherent light waves of wavelength 500 nm
meet after a path difference of 250 nm.
Calculate the phase difference.
Φ=2πx/λ
Φ=2π x 250/500 = π rad. (180°) Out of phase.
23. For a ray of light striking a thin film, where will a
phase change of π rad occur?
At the point of entering the film (a more optically
dense medium).
24. A soap bubble is 1 x 10 -6 m thick with a
refractive index of 1.2. For light of wavelength 600
nm will the reflected ray be constructive,
destructive or otherwise?
2nt = m λ
2 x 1.2 x 1 x 10 -6 = m x 600 x 10-9
m = 4 (destructive)
25. For a wedge fringe, glass slides of length 8 cm
are used with light of wavelength 550 nm. The
fringe spacing is calculated to be 2.2 x 10-4 m.
Calculate the thickness of a human hair at one end
between the slides.
ΔX = λL/2D
2.2 x 10-4 = 550 x 10-9 x 0.08 / 2x D
D = 0.1 x 10-3 m
26. A camera lens has a coating with n = 1.25 to
reduce reflected rays of wavelength 500 nm.
Calculate the thickness of this coating.
d = λ/4n = 500 x 10-9 / 4 x 1.25 = 1 x 10-7 m
27. Laser light passes through a grating to produce
an interference pattern on a screen 8 m away. The
grating has 400 lines/mm and between the second
order maximum on one side and the other second
order maximum measures 8.32 m.
a) Calculate the wavelength of light used.
ΔX = λD/d
d = 1 x 10-3/400 = 2.5 x 10-6
8.32/4= λx 8/2.5 x 10-6
λ = 650 nm
b) What type of interference is this?
Division of wavefront.
28. Why do some camera lenses have a magenta
hue when they reflect white light?
The coating can only transmit one wavelength
completely. If the green wavelengths are chosen to
transmit then red and blue will still reflect to
produce magenta.
29. What is polarised light?
Light that oscillates in only one plane.
30. Can sound waves be polarised?
Explain your answer.
No. They are longitudinal. Only transverse waves
can be polarised.
31. How would you polarise light?
Pass it through a polaroid filter or reflect it off an
electrical insulator. Any process that absorbs the
vibrations all planes except one.
32. Explain how a combination of “polariser” and
“analyser” can prevent the transmission of light.
The polariser will allow only light oscillating in one
plane to pass through. The second polariser will also
allow this light to pass through if it is aligned in the
same plane. As the second polariser (the analyser) is
rotated it gradually reduces the light level to zero
when it is rotated by 90°.
33. Explain how Polaroid glasses can remove glare.
Any reflections from an insulating surface are partly
polarised. Maximum polarisation occurs at
Brewster’s angle, n = tan ip. If the polarised glasses
act as an analyser, when rotated at 90° to the first
polarising filter the reflected light will not pass
through.
34. Complete the diagram below for when ip is
equal to Brewster’s angle.
35. Prove n = tan ip
n = sin ip / sin r
However, r + ip = 90°
So, r = 90° - ip
n = sin ip / sin (90° - ip)
n = sin ip / cos ip = tan ip
n = tan ip
36. For water n = 1.33 calculate the polarising angle
and the angle of refraction.
n = tan ip
1.33 = tan ip
ip = 53°
r = 90 - 53° = 37°