waves

• Which two wave properties must be equal for a stationary wave to form?

  Frequency

  Wavelength
• What is the refractive index of air?

  1
• Give the critical angle equation

  Where n1 is the more optically dense material and n2 is the less optically dense material.


  See image

  
• What is the refractive index?

  ratio of the speed of light in a vacuum, c, to the speed of light in that material, cs. It is given by:

  n = c/cs

  It can never be less than 1
• Which of these objects are NOT examples of polarisers?

  Aerials

  Convex lens

  3D glasses

  Polaroids

  Convex lens
• A diffraction grating has a slit width of 700 nm. Which type of EM wave will experience diffraction?

  X-ray: λ = 1 nm

  UV: 20λ = 20 nm

  Gamma ray: 1λ = 1 pm

  Green light: = 500λ =500 nm

  Green light: = 500λ =500 nm
• If we increase the frequency of the incident light, what happens to the width of the central maximum?

  Decreases
• Which material only allows transverse waves of a specific angle through?

  Polariser
• If we increase the width of the diffraction slit, what happens to the width of the central maximum?

  Decreases
• What is diffraction used to determine in crystals?

  Atomic spacing
• Diffraction gratings can be used to _______ wavelengths in light from different substances.

  Separate
• What is the range of allowed values of the polarisation angle?

  0 - 360 degrees
• As we increase λ, sin θ increases so θ increases. This means the pattern becomes more spread out.


  If we increase the distance between slits, d, sin θ decreases so θ decreased. This means the pattern becomes less spread out.
• Not all orders exist

  Remember that sin θ cannot be greater than 1.

  If you are using the equation and find that sin θ is larger than 1, the order you are looking at must not exist.
• In the stationary wave of a string practical, what is the relationship between mass per unit length of string and frequency and why?

  The larger the mass per unit length, μ, the lower the resonant frequency.

  For the same length string, waves travel more slowly through a heavier string, so the frequency must be lower.
• Explain what effect increasing the wavelength of incoming light in the single slit diffraction experiment will have on the diffraction pattern produced.

  If we increase the wavelength of the incident light, the width of the central maximum will increase.

  This is because diffraction effects will increase.

  The intensity of the central maximum will decrease because the photons are more spread out.
• In the single slit diffraction experiment, what effect does decreasing the wavelength of incoming light have on the width of the central maximum?

  Decreases
• Sharper patterns with sharper fringes are produced by having ______slits.

  more
• How do you find the mass per unit length of the string?

  Divide the mass by the length
• The longer the wavelength of light, the larger the diffraction effect.


  Of the visible spectrum, red has the longest wavelength and blue has the shortest. Remember they all travel at the same speed.
• The slit must be a similar size to the ___________ of the light in order to produce a ____________ pattern.

  wavelength

  diffraction
• How can you get higher harmonics?

  Increasing number of nodes and antinodes
• What is the zeroth order maxima?

  The bright central fringe
• Wavelength

  If we increase the wavelength of the incident light, the width of the central maximum will increase.

  This is because diffraction effects will increase. Think of longer wavelength light (red) being on the outside of white light fringes compared with shorter wavelength light (blue) being on the inside.


  The intensity of the central maximum will decrease because the photons are more spread out.
• The width of the central diffraction maximum varies with slit width and wavelength.

  Slit width

  If we increase the slit width, the width of the central maximum will decrease.


  This is because the diffraction effects will decrease.Imagine if the width becomes very large. The light will just pass straight through without being diffracted at all.

  The intensity of the central maximum will increase because the photons are less spread out.
• A closed end in pipes will always be....

  A node
• White light diffraction pattern

  If we use white light instead of a laser, we see a different but similar diffraction pattern.

  White light is made up of all the visible colours of light. It is not monochromatic.

  Each colour is diffracted by different amounts.


  Red has the longest wavelength, so is diffracted the most and appears on the outside of the fringes.

  Blue has the shortest wavelength, so is diffracted the least and appears on the inside of the fringes.
• Laser light is monochromatic and coherent.

  What does that mean?

  Monochromatic means that the light is all the same wavelength.

  Coherent means the light is in phase and has the same frequency.

  
• An open end in pipes will always be....

  An Antinode
• Name one phenomenon which shows the particle nature of light:

  The photoelectric effect
• What term describes light of equal wavelength?

  Monochromatic
• Which property of waves is NOT required to be the same for two coherent waves?

  Wavelength

  Frequency

  Phase difference

  Amplitude

  Amplitude
• State the 3 types of pipes that exists

  Closed open

  Open open

  Closed closed
• Laser safety requirements:

  Give 3


  1

  Don't point it in people's eyes

  2

  Wear protective clothing and eyewear

  3

  Laser use clearly signposted
• Danger of lasers

  We need to be safe with lasers because they are highly luminous over a very small area.

  The major risk with lasers is eye injury. With high powered lasers, the body's natural aversion reflex (the eye will automatically look away from a bright source) is too slow to prevent damage to the retina.

  How can we be safer with lasers?


  To be safe with lasers you should:

  Not point the laser in people's eyes.

  Wear protective clothing and eyewear as necessary.

  Make sure that internationally agreed signage is used outside and inside laboratories.

  Stick to institutional safety requirements.
• TRUE OR FALSE: reflections must occur at a solid boundary

  FALSE

  The transition from a high to low pressure air behaves as a boundary and reflects the wave back along the pipe
• What kind of interference is produced when waves have a path difference of (n+1/2)λ?

  Destructive interference
• What are the conditions for interference?

  The light should have the same wavelength as slit

  The light should have a fixed phase difference
• Which two quantities is the fringe spacing w directly proportional to?

  Wavelength

  Distance from the slit to the screen
• Using white light

  We can use white light instead of laser light in a double slit experiment.


  Instead of clear bright and dark fringes:

  The middle fringe is just bright white light.

  All fringes are more spread out.

  Side fringes have a spectrum of visible colours. Blue light diffracts less than red so is nearer the centre of the screen.