4.4.1 Progressive Waves

Question 1

What are the approximate wavelengths for:

Radio waves

Microwaves

Infared

Visible light

Answer 1

10^-6 - 10^-1

1-0^-1-10^-3

10^-3 - 4x10^-7

4x10^-7 - 7x10^-7

Question 2

What are the approximate wavelengths for:

Ultraviolet

X-Rays

Gamma Rays

Answer 2

7x10^-7 - 10^-8

10^-8 - 10^-13

10^-10 - 10^-16

Question 3

summarise the properties of:
transverse waves
longitudinal waves

Answer 3

transverse:
- oscillations perpendicular to propagation of energy transfer
- has peaks & troughs
longitudinal:
- oscillations parallel to propagation of energy transfer
- has compressions & rarefactions

Question 4

define:
period
frequency
and provide an equation relating them

Answer 4

period: time taken for one full oscillation to pass a given point
frequency: number of oscillations passing a given point per unit time
F=1/T

Question 5

explain how to determine frequency

Answer 5

• feed an oscilloscope a signal
• set time-base on x axis for time, y axis for amplitude
• measure time taken for one oscillation and calculate frequency

Question 6

define & explain:
reflection
refraction

Answer 6

reflection:
- direction changes between boundary of two media, remains in original medium,
- Θincidence=Θreflection, wavelength & frequency unchanged
refraction:
- direction and speed change, enters new medium, frequency unchanged
- soundwaves ^ in denser media, EM waves v
partial reflection at boundary

Question 7

define & explain:
diffraction
polarisation

Answer 7

diffraction:
- wave front spreads through gap, wavelength and frequency unchanged, max diffraction -> gap size = wavelength
polarisation:
- oscillation is restricted to one plane, only occurs in transverse waves

Question 8

recall how to demonstrate wave effects with polarising filters

Answer 8

place two polarising filters on top of one another, one rotated 90deg, light intensity v, all light polarised

Question 9

recall how to demonstrate wave effects with a ripple tank

Answer 9

adjust depth to show refraction, add a slit to show diffraction

Question 10

recall how to demonstrate wave effects with metal grilles

Answer 10

microwave transmitter & receiver on opp. sides of grille, vertical grille = max signal, horizontal = min signal (polarisation?)

Question 11

define the intensity of a progressive wave and relate it to amplitude

Answer 11

intensity = the radiant power passing at right angles through a surface per unit area
intensity is directly proportional to the amplitude squared

Question 12

provide some properties of electromagnetic waves

Answer 12

• they are transverse, progressive waves
• they consist of magnetic and electric fields oscillating at right angles to each other
• they all travel at a speed of 3.00x10^8 ms^-1 in a vacuum

Question 13

explain the refraction of light, and how to find the angle of refraction

Answer 13

• the speed of light changes as it enters a new medium
• the refractive index (n) can be used to find the angle of refraction
• n=c/v, n1sinΘ1=n2sinΘ2

Question 14

explain total internal reflection

Answer 14

• light is reflected back into it’s original media
• the first material must have the higher n
• the Θ of incidence > critical angle
• sinC=1/n
• original material must be air

Question 15

define superposition

Answer 15

• 2+ prog waves overlap to form a single wave
• when two waves meet at a point the resultant displacement = the sum of the displacements of the two waves at that point

Question 16

compare constructive and destructive interference

Answer 16

constructive interference: displacements in same direction, path diff = nλ
destructive interference: displacements in opposite direction, path diff = (n+1/2)λ

Question 17

define phase difference and coherence

Answer 17

• the difference in degrees or radians between the same point on two wave cycles
• waves with a constant phase difference

Question 18

explain how to investigate superposition & wavelength with sound waves

Answer 18

• connect two speakers to one oscillator in line with each other
• walk in front of them slowly
• loud sound = maximum = path diff is λ
• quiet sound = minimum = path diff is λ/2

Question 19

explain how to investigate superposition & wavelength with a laser

Answer 19

• shine laser through two slits approx. the same size as wavelength
• the slits act as two coherent point sources
• an interference pattern will form
• x=λD/a
• x = fringe spacing, D= distance from screen, a= slit spacing
  (for x measure several fringes/number of spacings)

Question 20

explain how to investigate superposition & wavelength with microwaves

Answer 20

• attach two microwave transmitter cones to one signal generator
• move a receiver probe along a straight line parallel to the generator
• you will get an alternating pattern of strong/weak signals

Question 21

explain diffraction gratings

Answer 21

• diffraction grating produces sharper lines
• therefore the interference pattern is a lot sharper
• there is a maximum brightness at the centre (zero order line)
• lines on either side = first order, carrying on outwards
  dSinΘ=nλ
  d=slit spacing, Θ= angle between first & zero order line
  Θ=x/D
  x= fringe width, D= dist. between grating & screen

Question 22

explain what happens when you put white light in a diffraction grating

Answer 22

• spectra are produced
• zero order = white
• other orders = spectrum, with violet on the inside