Section 2 - Waves and Optics Exam Questions

Question 1

Define monochromatic (1)

Answer 1

Single wavelength

Question 2

Define coherent (1)

Answer 2

Constant phase difference

Question 3

State a safety precaution when using a laser (1)

Answer 3

Do not look into laser

Question 4

State and explain the effect of using a laser with a shorter wavelength on the maxima spacing in Young’s double slit (2)

Answer 4

Maxima closer together

use w=𝜆D/s

Question 5

Use wave theory to explain how the fringe pattern is formed (4)

Answer 5

Slits act as coherent sources
Waves diffract at slits
Waves superpose
Bright patches: constructive
Dark patches: destructive

Question 6

State two requirements for two light sources to be coherent (2)

Answer 6

Same wavelength

Same phase

Question 7

Explain how Young’s double slit arrangement produces interference fringes, refer to width of first slit and coherence of second slits (6)

Answer 7

Narrow single slit gives wide diffraction to ensure that both second slits are illuminated
Paths to second slits are of constant length giving constant phase difference
Light diffracted at slits and overlap and interfere
Where path lengths differ by whole number of wavelengths, constructive interference occurs producing a bright fringe

Question 8

If Young’s double slit was carried out with red light and then with white light, how would the two differ? (3)

Answer 8

Central fringe would be white
Dark fringes would be closer together
Side fringes are spectra

Question 9

State two ways a diffraction pattern on single slit would change if the slit became narrower (2)

Answer 9

Increased separation

Lower intensity

Question 10

State and explain what happens to angle θ in λ = d sin θ when wavelength decreases? (2)

Answer 10

Angle θ gets smaller

As path difference gets smaller

Question 11

How can the appearance of a first order beam from light from a star be used to deduce information about the outer layers of the star? (1)

Answer 11

Absorption spectrum reveals the composition of the star’s atmosphere

Question 12

Why will total internal reflection occur when light ray travels from water into glass? (refractive index) (1)

Answer 12

TIR only occurs when ray travels from higher n to lower n

Question 13

What is the name for the part of an optical fibre that is around the core? (1)

Answer 13

Cladding

Question 14

State and explain an advantage of a smaller diameter core (2)

Answer 14

Reduce multipath

Which would cause poor resolution

Question 15

State one application of optical fibres and its benefit to society (2)

Answer 15

Communications

Improved transmission of data

Question 16

Explain why optical fibres used for communication need to have cladding (2)

Answer 16

Keeps signal secure

Keeps most light rays in

Question 17

State and explain two physical properties of the light produced by a laser which makes it different to that produced by a lamp (4)

Answer 17

Monochromatic
Waves of single wavelength
Coherent
Waves produced in constant phase

Question 18

Explain how glass cladding around the optical fibre’s core improves the security of data being transmitted through it and give a reason (3)

Answer 18

Light doesn’t enter cladding so can’t pass across from one fibre to a neighbouring fibre
Fibres without cladding can allow light to pass between fibres when scratched or linked by moisture
Personal data must be transmitted along fibres where there is no danger of light leakage

Question 19

Define longitudinal wave (1)

Answer 19

A wave with a direction of vibration that is parallel to the direction of propagation of the wave

Question 20

Define transverse wave (1)

Answer 20

A wave with a direction of vibration that is perpendicular to the direction of propagation of the wave

Question 21

Explain why it is important to correctly align the aerial of a TV to receive the strongest signal (2)

Answer 21

Transmitted radio waves are often polarised

Aerial rods must be aligned in the same plane of the wave

Question 22

Define amplitude of a wave (1)

Answer 22

The maximum displacement of the wave from the equilibrium position

Question 23

Describe the vertical displacement of a point over the next cycle of a transverse wave (3)

Answer 23

Vertically downwards 1/4 cycle to negative amplitude
Then upwards 1/4 cycle to equilibrium point and 1/4 cycle to positive amplitude
Down 1/4 cycle to equilibrium position

Question 24

Why could the point become motionless if a wave of the same amplitude frequency moves along from the opposite direction? (2)

Answer 24

Stationary wave formed
by superposition
Point is at a node

Question 25

If there are two polarising filters inbetween an observer and light, if one is turned about 360 degrees, what would the observer see? (2)

Answer 25

Variation in intensity between max and min

Two maxima in 360 degree rotation

Question 26

Give three examples of waves that are transverse (2)

Answer 26

Electromagnetic radiation
Surface of water
Rope

Question 27

State one application of a polarising filter and a reason for its use (2)

Answer 27

Camera

Reduce glare

Question 28

A microwave transmitter directs waves towards a metal plate. A detector is placed between and moved along a line, what causes the maxima and minima of waves detected? (3)

Answer 28

Superposition
reflection from metal plate
two waves of same frequency
travelling in opposite directions

Question 29

How can a guitarist raise the fundamental frequency of vibration in their string? (1)

Answer 29

Tighten the string

Question 30

State and explain the features of Young’s Double Slit (7)

Answer 30

Light source - multiple wavelength of visible light
Filter - selection of single wavelength, production of monochromatic source
Single slit - diffraction - Allows single source to strike 2 slits
Double slit - source entering is coherent due to previous step
Screen is large distance from double slit - measurable outcome and reduction in size of error

Question 31

Describe the structure of a step-index optical fibre outlining the purpose of the core and cladding (3)

Answer 31

Core is transmission medium for electromagnetic waves to progress by total internal reflection
Cladding has a lower refractive index to allow total internal reflection to occur
Cladding offers protection from scratching that could lead to light loss

Question 32

Blue light has a higher refractive index fibre than red light, explain how this difference results in a change in a pulse of white light by the time it leaves the fibre (2)

Answer 32

Blue travels slower than red due to greater refractive index

Red reaches the end of the fibre before blue leading to material pulse broadening

Question 33

Discuss two changes to reduce material pulse broadening of white light(2)

Answer 33

Monochromatic source so speed is constant

Shorter repeaters so pulse is reformed before significant broadening occurs

Question 34

Suggest an experiment to demonstrate the wave nature of sound (1)

Answer 34

Diffraction through a door

Question 35

Which observation helped to conclude that the speed of sound does not depend on frequency, when measured by firing a gun? (2)

Answer 35

Sound of gun is a mixture of frequencies

All sound reaches observer at the same time

Question 36

Which quantities are changed when monochromatic light passes from air into glass? (1)

Answer 36

Speed and wavelength only

Question 37

A student stretches a guitar string using unit masses to find fundamental frequencies at certain tensions, why might the actual frequency for higher tensions be different than predicted? (2)

Answer 37

Reducing diameter with string is stretched

Lower mass per unit length so frequency is higher

Question 38

Of the electromagnetic waves, which has the greatest energy? (1)

Answer 38

Gamma rays

Question 39

An endoscope contains two bundles of optical fibres

Name each bundle and explain its use in viewing inside the body (4)

Answer 39

Coherent bundle with fixed arrangement at each end
Used to transmit images
Non-coherent bundle with random arrangement of fibres
Used to transmit light into the body

Question 40

Slits in front of microwave transmitter, receiver is moved along a line behind the slits. Explain why the signal strength falls to a minimum at certain points (3)

Answer 40

Path difference for two waves
Gives rise to a phase difference
Destructive interference occurs

Question 41

Slits in front of microwave transmitter, receiver is moved along a line behind the slits. Explain why the minimum intensity between two points is not zero (2)

Answer 41

Intensity decreases with distance

One wave travels further than the other

Question 42

An aerial wire is hung between two masts, when the wind blows stationary waves can be formed. Explain how stationary waves are produced and why only certain frequencies form (4)

Answer 42

Wind produces a wave that travels along the wire
Waves are reflected at each end
Incident and reflected waves interfere
Only certain frequencies since fixed ends have to be nodes