Wave And Particle Nature Of Light PPQs

Question 1

A ray of unpolarised light is incident on a polarising filter. The polarising filter transmits light of intensity, I.
The polarising filter is then rotated 90degrees.
What is the intensity of light transmitted after rotation [1]

Answer 1

I, intensity is independent from rotation

Question 2

In an experiment, laser light is shone through a diffraction grating so that a series of bright dots appears on a screen. The equation n(lambda)=dsin0 can be used to determine the wavelength of the laser light. Which of the following is a correct description of how the experiment should be performed? [1]

A the angle 0 is measured using a protractor
B the diffraction grating is set up so that it is parallel to the laser light beam
C the diffraction grating is set up so that it is parallel to the screen
D the distance between the bright dots is measured with a micrometer

Answer 2

C

Question 3

Where does minimum displacement of the particles in a longitudinal wave occur? [1]

Answer 3

At the compressions and rarefactions

Question 4

When laser light is shone through a diffraction grating a series of maxima is formed on a screen. Assuming all other factors remain constant, which of the following changes would increase the distance between adjacent maxima?
A decreasing the distance between the diffraction grating and the screen
B decreasing the distance between the lines on the diffraction grating
C decreasing the intensity of the laser light used
D decreasing the wavelength of the laser light used

Answer 4

B

Question 5

A string is stretched between two fixed points and set into oscillation.
The frequency of the vibrating string is not dependent on
A the amplitude of the string’s vibration
B the length of the stirng
C the mass per unit length of the string
D the tension in the string

Answer 5

A

Question 6

On a violin, a stationary wave is created when a string is plucked. A violin string has a fixed length but the tension can be adjusted.
When a string is plucked, it produces a sound with a frequecny of 432Hz. This string is adjusted to produce a sound of frequency 440Hz.
Calculate the percentage increase in the tension in the string [3]

Answer 6

f(lambda)= square root of T/mu
So T= kf^2
F (fish symbol) sqaure root T

440^2 - 432^2 /432^2 x 100 = 3.7%

Question 7

A teacher set up a signal generator connected to two loudspeakers. This apparatus was used to determine the speed of sound in air in an open space.
A measuring tape was positioned along the ground at XY, which was a perpendicular distance of 3 metres from the two loudspeakers.
The two loudspeakers are 5 metres apart.
O is the midpoint.
The signal generator was set to a frequency of 160Hz. The students walked along the line XY. As they walked, they heard a series of loud and quiet sounds. At O the students heard a loud sound.
Explain why the students heard a loud sound at O. [3]

Answer 7

• path difference is zero/ waves travelled the same distance from speakers to O
• waves are in phase
• constructive interference / superposition takes place

Question 8

A teacher set up a signal generator connected to two loudspeakers. This apparatus was used to determine the speed of sound in air in an open space.
A measuring tape was positioned along the ground at XY, which was a perpendicular distance of 3 metres from the two loudspeakers.
The two loudspeakers are 5 metres apart.
O is the midpoint.
The signal generator was set to a frequency of 160Hz. The students walked along the line XY. As they walked, they heard a series of loud and quiet sounds. At O the students heard a loud sound.
As the students moved from O towards Y, the sound became quieter until a minimum was reached. One student stood still at the minimum point while another student recorded from the measuring tape the distance from O at which this occurred.
He recorded this as 84cm.
Determine the speed of sound in air [5]

Answer 8

O = 5/2
5/2- 0.84 = 1.66
5-1.66 = 3.34
Use Pythagoras
(Root of) 3^2 + 3.34^2 = 4.49
(Root of) 3^2 + 1.66^2 = 3.43
Path difference = 4/49 - 3.43 = 1.06 metres
Therefore wavelength = 2 x 1.06 = 2.12
v = f(lambda)
2.12 x 160 = 339m/s

Question 9

A teacher set up a signal generator connected to two loudspeakers. This apparatus was used to determine the speed of sound in air in an open space.
A measuring tape was positioned along the ground at XY, which was a perpendicular distance of 3 metres from the two loudspeakers.
The two loudspeakers are 5 metres apart.
O is the midpoint.
The signal generator was set to a frequency of 160Hz. The students walked along the line XY. As they walked, they heard a series of loud and quiet sounds. At O the students heard a loud sound.
The teacher suggested using a microphone connected to an oscilloscope to determine where the loud and quiet zones were located along the line XY.
She said that this method would result in much less uncertainty than when students walked along the line XY.
Explain one reason why this is a suitable suggestion [2]

Answer 9

• hard for a person to judge when sound is quietest/loudest
• amplitude on an oscilloscope can be measured more accurately

Question 10

A teacher set up a signal generator connected to two loudspeakers. This apparatus was used to determine the speed of sound in air in an open space.
A measuring tape was positioned along the ground at XY, which was a perpendicular distance of 3 metres from the two loudspeakers.
The two loudspeakers are 5 metres apart.
O is the midpoint.
The signal generator was set to a frequency of 160Hz. The students walked along the line XY. As they walked, they heard a series of loud and quiet sounds. At O the students heard a loud sound.
A student suggested that equally valid results would be obtained if the experiment was performed in the classroom. Criticise this suggestion. [2]

Answer 10

• reflections/echoes from walls/ceilings would occur
• more than two waves meet and interfere/ superpose

Question 11

Using diffraction grating and a laser pointer, only the first order maxima and the central maximum are produced. Explain how the teacher, using the same laser pointer, could improve his value for the number of lines per millimetre [3]

Answer 11

• increase the distance between the diffraction grating and the screen.
• reduces the percentage uncertainty because larger values
• measuring distance from first order maxima on one side of the central maxima to the first order maxima on the other side

Question 12

A student investigated stationary waves on a stretched string. The string was attached to a vibrator and a mass M.
Explain how a stationary wave forms on the string
[3]

Answer 12

• wave reflected
• at the pulley
• superposition/interference takes place

Question 13

Active Noise Reduction (ANR) is a system used to reduce unwanted noise. In ANR a second sound wave is produced that cancels the first. ANR is used in aircraft to reduce the noise heard from the engine.
For a particular passenger in an aircraft, the noise from the engine travels as a wave towards the passenger from the front. An ANR system creates a second sound that is directed to towards the passenger from the side.
Explain how this can lead to the sound being cancelled for one ear but louder for the other. [6]

Answer 13

• sound from ANR has to diffract to reach the furthest ear
• sound cancelled when destructive interference takes place
• sound louder when constructive interference takes place
• destructive interference is where waves are in antiphase
  Constructive interference is where waves are in phase
• distance between ears is half a wavelength

Question 14

When laser light is directed through a small circular gap, a diffraction pattern can be observed on a screen. Expllain, using Huygens’ construction, how diffraction occurs as waves pass through a gap [2]

Answer 14

• waves spread out
• each point on the wavefront acts as a source of new/secondary wavelets.

Question 15

The frequency of sound in an oilbird’s click is 2.0kHz and a click is 2.0ms in duration.
Determine whether these clicks can be used to locate obstacles at a distance of 50cm.
Speed of sound = 330m/s
[5]

Answer 15

Displacement = velocity times time over two
330 x 2e-3 / 2
s = 0.33metres
Lambda = v/f
330/2e3
= 0.165

Half pulse length needs to be less than distance and half wavelength needs to be less than distance.
Both are
So therefore it is suitable.

Question 16

Ultrasonic testing can be used for detecting corrosion inside metal pipes.
Describe how the ultrasound travels through a metal.
[3]

Answer 16

• particles in metal oscillate
• parallel to direction of travel
• as a longitudinal wave

Question 17

A steel pipe was manufactured with a wall thickness of 4.0cm.
After several years of use this pipe is tested for corrosion. A pulse of ultrasound is sent into the steel from the outer surface and the reflection from the inner surface is detected after a time of 5.1e-6s.
Determine whether the steel is corroded at this point.
Speed of sound in steel = 5900m/s
[4]

Answer 17

Displacement = 5900 x 5.1e-6
= 0.03m / 2
= 0.015m = 1.5cm
Therefore steel is corroded

Question 18

How the duration of the pulse affects thickness of the pipe wall that can be accurately measured
[1]

Answer 18

Shorter pulses means smaller thickness can be measured.

Question 19

In an investigation to determine the speed of sound in air, a student sets up an oscilloscope to display the waveform of a sound wave. The timebase is set to 25 microseconds / division. The student sets the timebase on the oscilloscope to a lower value per division. Describe any changes to the appearance of the wavefrom on the screen. [1]

Answer 19

Fewer cycles are shown on display. Stretched

Question 20

A student was studying musical instruments. A guitar has metal strings under tension. When a string is plucked it vibrates, producing a sound wave in the air. Describe how the vibrating string produces pressure variations in the air. [3]

Answer 20

• vibrating string pushes on particles in air
• this creates area of high and low pressure
• as particles are displaced from equilibrium

Question 21

Explain what is meant by plane polarised light [3]

Answer 21

• oscillations
• in one plane only
• plane includes direction of energy transfer

Question 22

3D glasses. The light form the screen reaching each eye passes through a different filter so each eye sees a different image. The filter for one eye has a plane of polarisation of 45 degrees and the filter for the other eye has a plane of polarisation of 135 degrees. Explain this choice of angles [2]

Answer 22

There is a 90 degree difference
Light aligned for one filter will be blocked for the other

Question 23

One complaint about 3D films seen through polarising filters is that they appear darker compared to ordinary films.
Suggest why this is the case [2]

Answer 23

• polarisation absorbs the unaligned part of radiation
• so intensity/amplitude is reduced

Question 24

3D film viewing is no longer done with plane polarised glasses because these require the viewers to keep their heads exactly level for the whole film. Tilting of the head causes partial viewing of the left image by the right eye and vice versa. Explain why one eye would see a faint image intended for the other eye if the head is tilted slightly. [2]

Answer 24

The angle between the plane of polarisation of the light and filter has changed.
The light for one eye has component in plane of polarisation of the other filter.

Question 25

A student views a laptop screen through a polarising filter. Initially the screen appears normal brightness. He rotates the filter to 90 degrees and observes that the screen appears dark.
Explain what the student observes as he gradually rotates the filter to 180 degrees and then to 270 degrees. [6]

Answer 25

He will be able to see the screen again at 180 degrees but not at 270.
180 - bc when oscillations are parallel to the filter all the light is transmitted
270 - when oscillations are perpendicular to the filter, all the light is absorbed
- idea of a gradual change as the filter is rotated
- as light from the screen is partially polarised.

Question 26

Describe how ultrasound travels through a metal. [3]

Answer 26

• particles in metal oscillate
• parallel to direction of travel
• as longitudinal waves

Question 27

Explain why pulses are used rather than a continuous wave and how the duration of the pulse affects the thickness of the pipe wall that can be accurately measured. [3]

Answer 27

• pulses are used so that you can tell how long it takes for a wave to return as you can record this before sending the next pulse.
• shorter pulses means smaller thickness can be measured.

Question 28

Which of the following provides evidence that light has a wave nature?
A the emission of light from an energy-level transition in a hydrogen atom
B the diffraction of light passing through a narrow opening
C the absorption of UV radiation in the photoelectric effect
D the reflection of light from a mirror

Answer 28

B

Question 29

A glass tube, closed at one end, has a loudspeaker placed at the other end. This is used to create a vibrating column of air, producing sound.
The wave in the tube is best described as
A longitudinal and progressive
B longitudinal and standing
C transverse and progressive
D transverse and standing

Answer 29

B

Question 30

Explain how excited atoms emit radiation [2]

Answer 30

• electrons move to lower energy levels
• emit photons

Question 31

Explain why only certain frequencies of radiation are emitted [3]

Answer 31

• atoms exist in discrete energy levels
• only certain energy changes are possible
• E=hf

Question 32

Some of the radiation is UV radiation which the human eye cannot detect. The phosphor coating absorbs the ultraviolet radiation and emits visible light. Suggest why the phosphor coating emits different wavelengths from the mercury. [1]

Answer 32

has different energy level spacings.

Question 33

Explain what is meant by refraction 2

Answer 33

Light changes direction as it passes through one material to the other due to change in density/ speed/medium

Question 34

A beaker contains gel balls. When water is added, the gel balls below the surface of the water are no longer visible.
Explain how this shows that the gel has the same refractive index as water [2]

Answer 34

• the light passing through does not change direction
• same/similar wavespeed in water and gel

Question 35

State what is meant by diffraction 2

Answer 35

Spread out of waves
As it passes through a gap or passes around an obstacle

Question 36

State the principle of superposition of waves [2]

Answer 36

Two or more waves meet/overlap
The total displacement at a point is the sum of the individual displacements

Question 37

A beach in england. Waves can be seen passing rocks on their way to the beach. The uneven surface of the sand has formed as a result of diffraction and superposition of these waves. Use ideas of diffraction and superposition to explain why the sand surface becomes uneven. [5]

Answer 37

• when waves pass the rocks, they diffract and spread out
• wavefronts overlap
• at some places waves are in phase
• constructive superposition occurs
• maximum amplitude erodes beach

Question 38

A diagram shows a diffraction pattern (the circles)
This diffraction pattern is produced by electrons passing through a thin sheet of graphite.
State what this suggests about the behaviour of electrons [1]

Answer 38

Electrons have wave like properties

Question 39

A diagram shows a diffraction pattern (the circles)
This diffraction pattern is produced by electrons passing through a thin sheet of graphite.
Suggest why substantial diffraction occurs [1]

Answer 39

Spacing in the graphite

Question 40

Define coherent [2]

Answer 40

A constant phase relationship
If relationship isn’t constant, a point could sometimes be constructive and sometimes destructive

Question 41

Explain why longitudinal waves cannot be polarised [2]

Answer 41

Longitudinal waves oscillate parallel to direction of travel
Plane polarisation makes transverse waves oscillate in only one plane that is perpendicular to the direction of travel

Question 42

State what is meant by the critical angle [2]

Answer 42

• angle of incidence from light travelling from denser medium has an angle of refraction of 90degrees

Question 43

Explain why there is a threshold frequency [4]

Answer 43

• one photon is absorbed by one electron
• photon energy increases with frequency (E=hf)
• there is a minimum energy needed for emission of a photoelectron
• threshold frequency is the minimum frequency for the emission of photoelectrons

Question 44

Phototubes are devices which make use of the photoelectric effect to detect light above a specific frequency.
Explain why
- no current flows when the phototube is in darkness
- current flows in the circuit when the phototube is illuminated by light above a specific frequency
[5]

Answer 44

• when illuminated:
• photons cause emission of photoelectrons
• photoelectrons form a current
• photon energy is greater than or equal to work function
• in darkness:
• no photons so no photoelectrons released

Question 45

Explain how the changing pattern of the soundtrack produces a changing current in the phototube circuit [3]

Answer 45

• size of gap in soundtrack determines the amount of light
• amount of light determines number of photons
• number of photons determines number of photoelectrons released
• number of electrons determines size of current

Question 46

Explain what is meant by plane polarised light [3]

Answer 46

• waves that only oscillate in one plane
• plane includes direction of travel

Question 47

3D glasses. The light from the screen reaching each eye passes through a different filter so each eye sees a different image. The filter for one eye has a plane of polarisation of 45 degrees and the filter for the other eye has a plane of polarisation of 135 degrees
Explain this choice of angles. [2]

Answer 47

• 90 degree difference
• light aligned for one filter will be blocked by the other filter

Question 48

One complaint about 3D films seen through polarising filters is that they appear darker compared to ordinary films. Suggest why this is the case. [2]

Answer 48

• polarisation absorbs the unaligned part of the radiation
• so intensity reaching each eye is reduced.

Question 49

3D film viewing is no longer done with plane polarised glasses because these require the viewers to keep their heads exactly level for the whole film. Tilting of the head causes partial viewing of the left image by the right eye and vice versa.
Explain why one eye would see a faint image intended for the other eye if the head is tilted slightly.
[2]

Answer 49

• angle between one filter and plane of polarisation of the light intended for the other filter has changed
• the light for one eye has component in plane of polarisation of the other filter and passes through to the other eye.

Question 50

The image (circles of diffraction) shows a diffraction pattern observed when a beam of electrons is fired at thin gold foil.
What property of electrons does this observation demonstrate?
A they exist in discrete energy levels
B they have a negative charge
C their small mass
D their wave nature

Answer 50

D

Question 51

The light emitted from a laptop screen is plane polarised. Explain how the plane of polarisation of the emitted light can be demonstrated using a polarising filter. [3]

Answer 51

• rotate filter
• when no light passes, filter is perpendicular to the plane of polarisation
• brightness of screen goes bright to dark every 90

Question 52

Light can be modelled as a wave. Describe how light is transmitted as a transverse wave. [2]

Answer 52

Oscillations are perpendicular to direction of travel.
Light is an EM wave

Question 53

A graph shows how the intensity of UV radiation varies with height above the surface of the earth.
An aeroplane made of the same metal as the spacecraft is flying at a height of 10km.
Explain why the aeroplane charges at a slower rate than the spacecraft due to the photoelectric effect. [6]

Answer 53

• intensity of UV greater at higher altitudes
• bc UV radiation is absorbed in the atmosphere
• higher intensity of radiation increases number of photons per second
• E is proportional to frequency so energy of photon doesnt depend on altitude, so photoelectric effect still occurs.
  One electron to one photon.
  Rate of released electrons from space craft is greater.

Question 54

In an experiment to determine the speed of sound in air, a poweder is sprinkles over the base of a horizontal glass tube. One end of the tube is closed. A sound source is placed at the open end of the tube.
Sound waves travel along the tube and reflect from the closed end.
Explain why the powder forms into small piles at regular intervals along the length of the tube. [5]

Answer 54

Interference
Nodes and antinodes formed
Constructive interference occurs when amplitude is max
Destructive interference occurs when amplitude is min
Powder is displaced from nodes to antinodes.

Question 55

‘Atoms can be promoted into an excited state when they absorb energy. This results in the release of radiation at a random time. When several atoms are close together a quantum effect can occur. When one atom emits radiation this affects all the other nearby excited atoms. The excess energy of many of the atoms is released simultaneously and an intense flash of light is produced. This effect is called superadiance and can be used to produce lasers that emit a narrower range of frequencies than conventional lasers.

When superradiance occurs, the atoms all absorb the same amount of energy.
Explain how this results in all the atoms emitting radiation of a particular frequency.
[5]

Answer 55

• atoms contain discrete energy levels
• the atom/electron loses energy and falls back down energy levels emitting a photon.
• with energy equal to the difference in energy levels.
• energy of photon is proportional to frequency
• so emitted frequency of radiation corresponds to the difference in energy levels of a particular atom.

Question 56

Explain why a monochromatic light source is important in diffraction experiments [3]

Answer 56

• emits a very small range of frequencies / wavelengths
• so smaller variation at each diffraction angle
• producing a clearer/sharper interference pattern

Question 57

The microphones are placed at equal distances from the loudspeaker. The signals are in phase. One of the microphones is moved further away from the loudspeaker. Initially the signals become out of phase. After moving the microphone a distance d the signals are back in phase.

Which of the following expressions gives the speed of sound?
A 2fd
B fd
C 26/f
D d/f

Answer 57

B

Question 58

State what is meant by a real image [1]

Answer 58

Light rays converge to a point where the image is formed

Question 59

A thicker string produces a note with a lower fundamental frequency than a thinner string of the same material.
Justify this statement [5]

Answer 59

• thicker string has a greater mass per unit length
• wavelength is same in each string
• v = sqrt T/μ. And v = f lambda
• f is proportional to 1/sqrtμ

Question 60

describe what is meant by wavefront [1]

Answer 60

A plane along which all the points are in phase

Question 61

When light is incident on an LDR, electrons move to a higher energy level where they become conduction electrons. This causes the resistance of the LDR to decrease. A student suggests that this is an example of the photoelectric effect. The student is incorrect.
Compare and contrast the photoelectric effect with the effect of radiation incident on an LDR
[6]

Answer 61

Similarities
- an electron absorbs a photon
- photons need a minimum amount of energy
- so light must be above a certain frequency
- increasing the light intensity increases the number of electrons released per second.
- evidence for the particle model of light
Differences
- in the photoelectric effect, electrons are released from the surface
- but electrons remain within the LDR
- photoelectric effect occurs in metals/ LDR is a semiconductor

Question 62

If the distance from object to the lens is less than a certain value, no image is produced on the screen.
Explain why [3]

Answer 62

• only a real image will be produced on the screen
• object cannot be any closer than f for a real image
• because light diverges after passing through lens

Question 63

The effect of diffraction is more noticeable, in everyday life, with sound than with light. This is because…
A sound has a much longer wavelength than light
B sound is a longitudinal wave, light is a transverse wave
C sound is a mechanical wave, light is an electromagnetic wave
D sound travels more slowly in air than light does

Answer 63

A

Question 64

Midpoint between lens surface and principle axis is termed as
A midway center
B focal center
C focal point
D optical center

Answer 64

D

Question 65

A converging lens of power 10D produces a magnified image of a small object. The image is 0.25m from the centre of the lens and is the same way up as the object.
State one other property of the image. [1]

Answer 65

Image is virtual

Question 66

Descrive the effect on the pattern of diffraction if the distance betwqeen the two slits is gradually increaased to one cm [2]

Answer 66

• get closer together
• gap becomes too large for overlap to occur

Question 67

What is meant by ‘energy level’? [1]

Answer 67

• discrete energy of atom
• one of the energies of atom

Question 68

What is a photon [1]

Answer 68

Discrete package of EM particle of light

Question 69

When oil floats on water, coloured interference patterns are often seen. The interference patterns are formed because of the thin film of oil. A thin film of oil can also produce interference patterns with monochromatic light. Explain why interference patterns maybe be seen. [5]

Answer 69

• two rays of light
• which are coherent/ same frequency/ come from same source.
• superpose, constructive and destructive interference occurs.
• if they meet in phase / n(lambda) - constructive interference, bright fringe
• if they meet in antiphase, (n+0.5)(lambda) , destructive interference, dark fringe

Question 70

What is meant by a photon [2]

Answer 70

• discrete packet of
• electromagnetic energy
  Of particles of light

Question 71

A device for making a vertical slit with variable width.
When the slit is fully open a laser beam is shone through it and a single point of light is seen on a screen. As the slit is reduced in width the point of light becomes a horizontal line that gets longer as the slit gets narrower. Explain this observation [3]

Answer 71

• diffraction occurs
• light spreads (sideways) as it passes through the slit
• narrower slit causes more spreading OR diffraction increasing as gap width gets closer to wavelength

Question 72

Ultrasonic testing can be used for detecting corrosion inside metal pipes. Describe how the ultrasound travels through a metal.
[3]

Answer 72

• atoms vibrate
• along direction of propagation / parallel to direction of wave travel
• making compressions and rarefactions as a longitudinal wave