Waves

Question 1

Explain what is meant by a progressive wave. (2)

Answer 1

It is a wave that transfers energy from one point to another without transferring material

Question 2

Explain what is meant by particle displacement

Answer 2

Particle displacement is the distance of particles from mean position

Question 3

Explain what is meant by amplitude

Answer 3

Amplitude is the maximum displacement from the equilibrium position

Question 4

Explain what is meant by wavelength

Answer 4

Wavelength is the shortest distance between two points in a phase

Question 5

Explain what is meant by the frequency of wave

Answer 5

The number of waves passing a point in 1 second

Question 6

State the characteristic features of longitudinal waves and transverse waves.(3)

Answer 6

Longitudinal waves : Particle vibrations are parallel to direction of energy transfer
Transverse waves: Particle vibrations are perpendicular to direction of energy transfer

Question 7

State one difference between a transverse wave and a longitudinal wave. (1)

Answer 7

Transverse waves oscillate perpendicular to wave travel

Transverse waves can be polarised

Question 8

State what is meant by polarisation. (2)

Answer 8

The restriction of a wave so that it can only oscillate in a single plane

Question 9

Explain why a stationary wave is formed on the string.(3)

Answer 9

Two waves travel in opposite directions along the string
The waves have the same frequency
The waves have the same amplitude
Both waves interfere with each other

Question 10

State the conditions that are necessary for a stationary wave to form on the string.(3)

Answer 10

2 waves must travel in opposite directions
They must have a similar amplitude
They must have a constant frequency or same frequency

Question 11

Explain the difference between an undamped progressive wave and a stationary transverse wave, in terms of amplitude, phase and energy transfer.(5)

Answer 11

Each point along the wave has the same amplitude for progressive waves but it varies for stationary waves.
In progressive waves, adjacent points vibrate with different phase but in stationary waves, between nodes all particles vibrate in phase.
In progressive waves, energy is transferred through space but in stationary waves, energy is not transferred through space

Question 12

State the 2 conditions required for the sources to be coherent.(2)

Answer 12

Constant phase difference
Same frequency

Question 13

Explain the meaning of the term monochromatic light

Answer 13

Single frequency (NOT SINGLE COLOUR)

Question 14

State and explain one precaution that should be taken when using laser light

Answer 14

Don’t shine towards a person
Avoid reflections
Wear laser safety goggles
Stand behind laser

Eye/Skin damage could occur

Question 15

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

Answer 15

Core is the transmission medium for light waves to progress
Cladding prevents the scratching of the core and has a lower refractive index than the core to provide total internal reflection

Question 16

An optical fibre consists of a core, cladding and an outer sheath. State the purpose of the outer sheath in an optical fibre.

Answer 16

Prevents damage to fibre

Question 17

Explain why dispersion occurs in an optical fibre

Answer 17

Different wavelengths result in different speeds
Spreading of pulse into different wavelengths

Question 18

Explain what is meant by modal dispersion in an optical fibre

Answer 18

Light takes different length paths through the fibre
Different paths take different times, so the pulse is spread out

Question 19

Explain what is meant by material dispersion in an optical fibre

Answer 19

Different wavelengths have different speeds in the material, so red light reaches the end faster than blue light

Question 20

State the two conditions necessary for a light ray to undergo total internal reflection at the boundary between two transparent media.(2)

Answer 20

n1 > n2

Incident angle > critical angle

Question 21

Explain how a bright line is formed by the diffraction grating at the first-order diffraction angle.(3)

Answer 21

Lights from each slit superpose
Light from adjacent slits have a path difference of one wavelength
At this angle all the waves are in phase
Constructive interference occurs

Question 22

State the Principle of Superposition

Answer 22

When two or more waves interfere, the resultant displacement at any point is equal to the vector sum of the displacements due to each individual wave at that point

Question 23

Define interference

Answer 23

Interference is the superposition of two or more waves in similar direction to give a resultant wave whose amplitude is given by the principle of superposition

Question 24

What is constructive interference

Answer 24

When two waves arriving at a point in phase with each other, resultant displacement would be equal to the sum of displacements of the two original waves.

Question 25

What is destructive interference

Answer 25

If two waves arriving at a point in anti- phase with each other, resultant displacement would be equal to the difference of displacements of the two original waves

Question 26

What is path difference

Answer 26

The path difference between two waves is the difference in distance that they have travelled

Question 27

Conditions for constructive interference

Answer 27

■ For constructive interference
o path difference=nλ ( 0, λ, 2λ, 3λ, etc.)
o Therefore phase difference is zero o Constructive interference occurs

Question 28

Conditions for destructive interference

Answer 28

■ For destructive interference
o path difference=nλ+ 1/2λ=(n+1⁄2) λ
o Therefore phase difference is 1800(OR π radian)
o Destructive interference occurs

Question 29

Define wavefront

Answer 29

A wavefront is a surface through crests and normal to the direction of energy transfer of the wave. Lines in the direction of energy transfer of the wave are called rays