Waves

Question 1

Define frequency and give its units.

Answer 1

The number of waves passing through a point per second. (hertz) Hz

Question 2

Define wavelength.

Answer 2

shortest distance between two points in phase

Question 3

Define amplitude.

Answer 3

The maximum displacement of the wave from its equilibrium position.

Question 4

What is a longitudinal wave?

Answer 4

A wave in which the oscillation of the particles is parallel to the direction of energy transfer. There are rarefactions (areas of low pressure) and compressions (areas of high pressure).

Question 5

What is a transverse wave? Give examples.

Answer 5

Waves where the particle oscillations are perpendicular to the direction of energy transfer.
For example: electromagnetic waves

Question 6

True or False? The magnetic field and electric field in a electromagnetic wave are parallel to each other.

Answer 6

False.
The electric and magnetic field are at right angles to each other

Question 7

What does a polarising filter do?

Answer 7

Only allows oscillations in one plane.

Question 8

How is polarisation used as evidence of the nature of transverse waves?

Answer 8

Polarisation can only occur if a wave’s oscillations are perpendicular to its direction of travel (as they are in transverse waves).

Question 9

How are polarisers used in antennas?

Answer 9

TV and radio signals are usually
plane-polarised by the orientation of the rods on the transmitting aerial, so the receiving aerial must be aligned in the same plane of polarisation to receive the signal at full strength.

Question 10

What is a stationary wave?

Answer 10

superposition of two progressive waves with the same frequency, wavelength and amplitude, moving in the opposite direction

transfers no energy

positions of maximum and minimum amplitude are constant

Question 11

What is a node?

Answer 11

Where the waves meet completely out of phase, total destructive interreference occurs and regions of zero displacement form.

Question 12

What is an antinode?

Answer 12

Where the waves meet in phase, constructive interference occurs so regions of maximum amplitude form.

Question 13

Define coherence.

Answer 13

Waves are said to be coherent if they have:
The same frequency
A constant phase difference

Question 14

Why is a laser useful in showing interference and diffraction?

Answer 14

It produces monochromatic (same wavelength / colour) light so diffraction and interference patterns are more defined.

Question 15

What was Young’s double-slit experiment?

Answer 15

A single light source is directed towards two slits, which each act as a coherent light source, the light interferes constructively and destructively to create an interference pattern

Question 16

Describe the interference pattern created using white light.

Answer 16

A bright white central maximum flanked by alternating spectral fringes of decreasing intensity with violet closest to the zero order and red furthest

Question 17

Why does an interference pattern form when light is passed through a single slit?

Answer 17

The light diffracts as it passes through the slit, where the waves are in phase constructive interference occurs making bright fringes and where the waves are completely out of phase destructive interference occurs making a dark fringe

Question 18

Increasing the slit width increases the width of the central diffraction maximum. True or False?

Answer 18

False, the slit is not so close to the wavelength in size so less diffraction occurs - the central maximum becomes narrower and more intense

Question 19

When light enters a more optically dense medium does it bend towards or away from the normal?

Answer 19

Towards the normal.

Question 20

When does total internal reflection occur?

Answer 20

When light is at a boundary to a less optically dense medium and the angle of incidence is greater than the critical angle.

Question 21

What is the purpose of the cladding in a step index optical fibre?

Answer 21

● Protects core from scratches which would allow light to escape and degrade the signal.
● Allows TIR as it has a lower refractive index than the core.

Question 22

How does signal degradation by absorption in an optical fibre affect the received signal?

Answer 22

Part of the signal’s energy is absorbed by the fibre so its amplitude is reduced.

Question 23

What is pulse broadening?

Answer 23

When the received signal is wider than the original, this can cause overlap of signals leading to information loss.

Question 24

How does modal dispersion cause pulse broadening?

Answer 24

Light rays enter the fibre at different angles so they take different paths along it, some may travel down the middle while others are reflected repeatedly, so the rays take different times to travel along the fibre, causing pulse broadening.

Question 25

What is material dispersion?

Answer 25

When light with different wavelengths is used some wavelengths slow down more than others in the fibre so they arrive at different times causing pulse broadening.

Question 26

How can modal dispersion be reduced?

Answer 26

Use a single mode fibre (very narrow fibre) so the possible difference in path lengths is smaller.

Question 27

How can material dispersion be reduced?

Answer 27

Use monochromatic light.

Question 28

How can both absorption and dispersion be reduced?

Answer 28

Use a optical fibre repeater to regenerate the signal now and then.

Question 29

State the advantages of optical fibres over traditional copper wires

Answer 29

● Signal can carry more information as light has a high frequency.
● No energy lost as heat. ● No electrical interference. ● Cheaper. ● Very fast

Question 30

What path does a light ray take when the angle of incidence is equal to the critical angle?

Answer 30

It goes along the boundary ie. the angle of refraction is 90°

Question 31

State 2 applications of diffraction gratings.

Answer 31

● Splitting up light from stars to make line absorption spectra- used to identify elements present in the star.
● X-ray crystallography, a crystal sheet acts as the diffraction grating the X-rays pass through, used to find the spacing between atoms

Question 32

When light passing through a diffraction grating is changed from blue to red, do the orders get closer together?

Answer 32

The wavelength of light has increased so it will diffract more, the orders will become further apart.

Question 33

What is diffraction?

Answer 33

The spreading out of waves when they pass through or around a gap

Question 34

How did Young’s double slit experiment provide evidence for the wave nature of light?

Answer 34

Diffraction and interference are wave properties hence the interference pattern of light shows light has wave properties.

Question 35

What is path difference? How do you calculate it?

Answer 35

The difference in distance travelled by 2 waves.

nλ for constructive

n+1/2 λ for destructive