Past papers- 3. waves

Question 1

short‑sightedness and long-sightedness which lens is used and why

Answer 1

diverging lens in front of eye lens
rays meeting on the retina

converging lens reduces focal length of eye OR converging lens brings focal point forward OR without lens, rays converge behind back of eye
(so that) rays converge / focus on back of eye / retina

Question 2

compression and rarefaction

Answer 2

(region where) particles are close(r) together (than normal) OR (region where) there is a great(er) pressure (than normal)

(region where) particles are further / far apart (than normal) OR (region where) there is a low(er) pressure (than normal)

Question 3

We can see light from the Sun but we cannot hear any sound from it. State the reason for this.

Answer 3

light does not need a medium to travel through OR sound needs a medium to travel through (and there is no medium
between Sun and Earth)

Question 4

State what is meant by monochromatic.

Answer 4

(light / electromagnetic radiation) of a single frequency

Question 5

State two reasons why microwaves are used for mobile phone (cell phone) signals.

Answer 5

(microwaves) only need short aerials / antennas
(microwaves) penetrate (some) walls

Question 6

Describe, with the aid of a diagram, how a digital signal differs from an analogue signal.

State two advantages of using digital signals rather than analogue signals

Answer 6

1. digital (signal) consists of two values
   analogue (signal) varies over a range (of values)
   digital (signal) with blocks of high (1) and low (0) AND labelled diagram of analogue with continuously variable signal
2. faster (data) transmission rate OR data can be compressed

data / signal transmitted over long(er) distances (as signal can be regenerated)

noise easily removed (from signal / data) OR signal can be regenerated

Question 7

State the types of wave that P-waves and S-waves can be modelled as.

Answer 7

P-waves: longitudinal
S-waves: transverse

Question 8

Give two reasons why these two types of electromagnetic radiation are used in glass optical fibres for high-speed broadband

Answer 8

glass is transparent to visible light and (some) IR
(visible light and some IR) can carry high rates of data / information

Question 9

how do you know if an image is virtual

Answer 9

Cannot be projected on a screen / light doesn’t pass through image

light / rays do not pass through I or
light / rays only seem to come from I or
produced by diverging rays
actual light rays do not meet at image OR light rays do not converge

Question 10

Describe one use of X-rays in medicine

Answer 10

X-rays for detecting broken bones
X-rays pass through soft tissue AND not through bone

Question 11

State two ways in which transverse waves differ from longitudinal waves.

Answer 11

particles (in transverse waves) vibrate perpendicular to the direction of travel (of the wave) OR
particles in longitudinal waves vibrate parallel to the direction of travel of the wave

 longitudinal waves have compressions and rarefactions
 transverse waves have troughs and crests

Question 12

Explain why sound travels faster in water than in air

Answer 12

molecules closer together / water has greater density

Question 13

State two properties of the image formed by the mirror.

Answer 13

virtual
upright
same size as object
laterally inverted

Question 14

In the plastic, blue light travels slightly slower than red light and so the critical angle for blue light is smaller than the critical angle for red light. The laser that emits red light is replaced by one that emits blue light. Now blue light
enters the block at P and hits the straight edge at M.

Explain what happens to the blue light after it hits the straight edge at M

Answer 14

critical angle (for blue light) < 37° or critical angle for red (light) is 37°

angle of incidence (of blue light) greater than its critical angle (in plastic)

total internal reflection or all the (blue) light reflects or no (blue) light leaves the glass / refracts / travels in air along the straight edge

Question 15

The frequency of the wave is increased to a value many times greater than the value obtained in (b).

Describe and explain two ways in which a diagram representing the wave with the greater frequency differs from Fig. 6.2.

Answer 15

compressions / rarefactions closer or more compressions / rarefactions (in same distance)

less diffraction / spreading out

(because of) smaller wavelength or ratio wavelength / gap width smaller ((wavelength large so diffraction large))

Question 16

Another harbour has a much wider gap between its walls.
Describe and explain how the pattern of wave crests in this harbour is different from the pattern you have drawn in (i).

Answer 16

waves pass through gap remaining straight
less / no diffraction occurs

Question 17

Iodine-131 emits γ-radiation. It has a half-life of 8 hours.
Explain why this emission and this half-life make iodine-131 a suitable material for a tracer in medical diagnosis.

Answer 17

• γ can be detected outside body
• needs long enough half-life to be detected / reach part of the body required
• needs short enough half-life to soon have very little activity
• gamma weakly ionising or pass out of body without harm

Question 18

Explain, in terms of the behaviour of light rays, what is meant by principal focus for a thin converging lens.

State what is meant by focal length

Answer 18

1. (point) where (parallel) rays (of light) meet (after passing through lens)
   on principal axis

point) where parallel rays (of light) meet / are focussed (after passing through lens) or (point) through which rays (of light)
that emerge parallel pass (before reaching lens)

2. distance between principal focus / focal point and optical centre / lens

Question 19

Describe one medical use of X‑rays.
State one reason why it is necessary to take safety precautions when X‑rays are used.

Answer 19

1. stated medical use (e.g. treating cancer / X-ray shadowgraph / sterilising equipment)

statement of what happens to the X-rays (e.g. absorbed by tumour / bones / bacteria)

stated consequence (e.g. tumour killed or image / picture / shadow / photograph produced)

2. can cause burns / (cell) mutation / cell damage / tumours / cancer / damages DNA etc.

Question 20

Describe an experiment to determine the speed of sound in air. State the apparatus you need, details of how to take measurements and how to calculate the speed of sound in air.

Answer 20

method of producing sound, e.g. clap for echo method or gun for direct measurement, sig gen or loudspeaker, hammer
on block

apparatus used, e.g. stopwatch, long tape, trundle wheel, wall if using echo method, metre rule, microphones and timer or microphones and oscilloscope

measure distance between person and the wall, measure distance between loudspeaker and microphone or measure distance between two microphones

appropriate time measured, e.g. at one end start stopwatch when smoke seen from gun and stop it when sound heard, start stopwatch when gun heard / clap heard and stop when echo heard, measure time taken between clap and hearing echo, timer starts when first microphone receives signal and stops when second receives signal

speed = measured distance / time for direct method
OR speed = 2 × distance from student clapping to wall / time for echo method OR distance between microphones = wavelength AND v = f × λ

Question 21

Sound waves from a television are diffracted through doorways. Light waves from a television are not diffracted through doorways. Suggest why light waves and sound waves behave differently in this situation.

Answer 21

wavelength of light is (much) smaller than width of doorway or wavelength of sound

wavelength of sound is similar to width of doorway OR
λ ≃ width of gap for diffraction to occur OR larger wavelength
results in greater diffraction

Question 22

Explain the term total internal reflection.

Answer 22

light travelling from optically dense medium to optically less dense medium (ight (must pass) from medium where it travels slower or to medium where it is faster or from medium with larger refractive index or to medium with smaller refractive index)

all light reflected OR no light refracted

angle of incidence is greater than the critical angle

Question 23

(a) Explain why the direction of the wavefronts changes in the way shown in Fig. 6.1 (deep to shallow)

Answer 23

speed changes or (wave) speed is smaller in right-hand part of tank or waves slow down or bottom (on the page)
section of wave hits the boundary first

(wave) speed is smaller in right-hand part of tank or waves slow down or bottom (on the page) section of wave hits the
boundary first

bottom (on the page) / one part / one side / one section of wave slows down first (and different sections are delayed by
different amounts)

Question 24

Describe and explain the action of optical fibres in communication technology. You may draw a diagram in your answer.

Answer 24

light / infrared travels in fibre

total internal reflection at inner surface or within (graded-index) fibre

light carries information / signal / data / message or signal / light encoded

Question 25

State and explain the use of optical fibres in medicine

Answer 25

description of fibre passing to site to be examined / treated

light passes down fibre (to site) AND (image) returns (to sensor / observer) OR alternative use to endoscopy

extra detail, e.g. laser light source, illuminated organ, image, camera / type of sensor

Question 26

State two uses for infrared radiation

Answer 26

remote controls
(infrared) sensors / alarms
specific electrical appliances
thermal imaging

Question 27

X-rays are used in hospitals to help treat patients. Suggest and explain three precautions for the safe use of X-rays.

Answer 27

• shielding of operator behind screen / lead apron / out of room AND to absorb radiation
• shielding of other parts of patient with lead / shielding of other parts of patient AND to absorb radiation
• distance from source AND reduces intensity / amount of radiation / exposure
• limit time of exposure / not too frequent / max number of X-rays per year AND to limit dose
• limit strength / intensity of X-ray beam AND to limit dose

Question 28

The light travels more slowly in the transparent solid.

(a) Explain, in terms of the wavefronts, why the light changes direction as it enters the solid. You may draw on Fig. 6.1 as part of your answer.

Answer 28

idea of one side of wavefront enters / hits solid first OR wavefront does not all hit the solid all at once;

idea of this side slowed down first OR this side delayed relative to other side

angle of wave(front) changes OR different parts of wavefront delayed by different amounts

Question 29

real image

Answer 29

light passes through it OR can be projected/seen on a screen OR refracted rays cross/meet

Question 30

Describe, with a labelled diagram, an experiment using water waves that shows the reflection of wavefronts that occur at a straight barrier.

Answer 30

Diagram to show: labelled barrier, incident straight or curved waves

Diagram shows appropriately reflected waves
Water surface e.g. tank of water/ripple tank/pond/acceptable alternative

How waves are produced: e.g., moving end or length of solid rod dipping into surface OR small solid object thrown in.

Detail of barrier: made of metal, glass or wood fixed in position

How observed: by eye, video, film, stroboscope

Question 31

A dentist takes an X-ray photograph of a patient’s teeth. Explain why it is safe for the patient to be close to the source of X-rays, but the dentist must stand away from the source.

State, with a reason, why microwave ovens are designed only to work with the door closed.

Answer 31

X-rays ionising/harmful/dangerous (to humans)
Any one from:
patient rarely exposed
low total dose on patient
meaningful comment about benefit outweighs danger
dentist frequently exposed
total dose on dentist would be high if stayed in room

microwaves harmful/dangerous (to humans)
microwaves would pass through open door

Question 32

Fig. 8.2 shows a spherical fishbowl, full of water, by a window. A black curtain hangs behind the fishbowl.

When full of water, the fishbowl can act as a converging lens.
Suggest one possible hazard of leaving the fishbowl next to the window in bright sunlight

Answer 32

(focused rays) set fire to curtain

Question 33

State two uses of optical fibres.

Answer 33

• to carry (telephone) signals / communications
• for medical diagnosis / imaging
• specified artistic (display)
• specified lighting

Question 34

State one other similarity between sound of frequency 750Hz and ultrasound.

State one way in which sound of frequency 750Hz is different from ultrasound.

Answer 34

longitudinal OR vibration parallel to wave travel direction OR transfer energy

frequency / pitch less OR below 20 000 Hz OR audible (to someone with normal hearing)

Question 35

Explain what is meant by the term transverse wave motion.

Answer 35

transmission of energy (through medium) and no transfer of matter

(direction of) vibration of particles or (direction of) vibration of medium

perpendicular to direction of energy travel / wave / propagation

Question 36

State the two types of radiation that are emitted by the heater.

Answer 36

infra-red and visible light