Waves (Unit 2)

Question 1

Longitudinal wave

Answer 1

Particle vibration is parallel to direction of wave propagation

Question 2

Examples of a longitudinal wave

Answer 2

Sound waves, seismic p-waves

Question 3

Transverse wave

Answer 3

Particle vibration is perpendicular to direction of wave propagation
Only transverse waves can be polarised

Question 4

Examples of a transverse wave

Answer 4

Electromagnetic radiation, seismic s-waves

Question 5

Particle displacement

Answer 5

The distance of a particle from its equilibrium position in given direction

Question 6

Amplitude

Answer 6

the maximum displacement of a particle (wave) from its equilibrium (or rest) position

Question 7

Frequency

Answer 7

Number of oscillations (of a particle) per second

Question 8

Time period

Answer 8

The time for one complete oscillation

Question 9

Wavelength

Answer 9

Shortest distance between two points in phase

Question 10

Diffraction

Answer 10

Spreading out of a wave (when it passes through a gap or past the edge of an object)

Question 11

Refraction

Answer 11

Wave bends/changes direction when its speed changes

Question 12

Polarisation

Answer 12

(transverse) wave oscillation is in one plane

Question 13

Application of polarisation in sunglasses

Answer 13

• Light reflected from surfaces is (weakly) polarised in one plane (horizontal)
• Polaroid in sunglasses can be orientated to remove this reflected light
• Reducing glare

Question 14

Application of polarisation in tv transmitters and aerials

Answer 14

• Signals from tv transmitter (radio waves) are polarised
• Aerials need to be orientated (rotated) so they are in same plane as the transmitted signal
• For maximum signal strength

Question 15

Superposition

Answer 15

Where two or more waves meet, the resultant displacement equals the vector sum of the individual displacements

Question 16

Conditions for formation of stationary waves

Answer 16

• Two waves travelling past each other in opposite directions
• With the same wavelength (or frequency)
• Similar amplitudes

Question 17

Nodes and antinodes

Answer 17

Nodes – points of no oscillation / zero amplitude

Antinodes – points of maximum amplitude

Question 18

Coherent sources

Answer 18

waves (from two sources) that have:
• a constant phase difference
• same wavelength (or frequency)

Question 19

Monochromatic

Answer 19

Single wavelength

Question 20

Safety with a laser

Answer 20

• Avoid looking along the beam of a laser
• Wear laser safety goggles
• Avoid reflections
• Put up a warning sign that a laser is in use

Question 21

Properties of laser light

Answer 21

• Monochromatic – only a single wavelength
• Coherent – waves have a constant phase difference
• Collimated – produces an approximately parallel beam

Question 22

Appearance of interference fringes from two vertical slit illuminated with yellow light

Answer 22

• Vertical or parallel
• Equally spaced
• Black and yellow bands

Question 23

Fringe width, w, changes

Answer 23

Slits closer together w – increases
Screen further away w – increases
Shorter wavelength (eg blue light) w - decreases

Question 24

Explanation of formation of fringes with Young’s slits

Answer 24

• Interference fringes formed
• Where light from two slits overlaps
• The light from the two slits is coherent
• Bright fringes formed where constructive interference
• because light from the two slits is in phase (path difference equals a whole number of wavelengths)
• Dark fringes formed where destructive interference
• Because light from the two slits is in anti-phase (path difference equals a whole number + 0.5 wavelengths)

Question 25

Appearance of white light through Young’s slits

Answer 25

* Central fringe would be white * Side fringes are (continuous) spectra * Bright fringe would be blue on the side nearest the central fringe. * Bright fringes merge further away from centre.

Question 26

appearance of diffraction pattern from a single slit

Answer 26

* Central bright fringe has twice width of other bright fringes * The other bright fringes have a much lower intensity * and are equally spaced

Question 27

Single slit pattern changes

Answer 27

Narrower slit width • Wider pattern / increased separation • Reduced intensity Shorter wavelength • Narrower pattern / reduced separation

Question 28

Lines per mm of a grating

Answer 28

Spacing, d, of slits on a diffraction grating given by: | d = 1/(number of lines per mm) in mm

Question 29

Applications of gratings to spectral analysis of light from stars

Answer 29

* Dark lines in spectrum from a star (absorption spectrum) | * Reveal the composition of (elements present in) the star’s atmosphere

Question 30

How does light change moving from air to glass

Answer 30

* speed – decreases (slows down) * wavelength – decreases (gets shorter) * frequency – remains constant (stays the same)

Question 31

Conditions for total internal reflection

Answer 31

* Angle of incidence is greater than the critical angle * The refractive index of the material light is going from is greater than the refractive index of the material the light is going to.

Question 32

Total internal reflection

Answer 32

Where all the light is reflected back into the material

Question 33

Critical angle

Answer 33

Angle of incidence which produces an angle of refraction of 90 degrees.

Question 34

Structure of an optical fibre

Answer 34

Central core, surrounded by cladding. Refractive index of core must be greater than refractive index of cladding (to ensure total internal reflection)

Question 35

Purpose of cladding

Answer 35

* prevents crossover of signal/data to other fibres * prevents scratching of the core * reduces pulse broadening/dispersion

Question 36

Use of optical fibres

Answer 36

* Communication – improve transmission of data/high speed internet * Endoscopes – improved medical diagnosis

Question 37

How do pulses of light change travelling down optical fibres

Answer 37

* reduced amplitude due to absorption/energy loss and scattering within fibre * pulse broadening due to multipath dispersion from rays taking different paths and different times to travel down same fibre

Question 38

How is multipath dispersion reduced

Answer 38

Core of fibre is made very narrow/thin.

Question 39

Sketches of stationary waves for first 4 harmonics

Answer 39

See sheet

Question 40

Derivation of n(lambda) = d sin(theta)

Answer 40

See sheet