Waves Flashcards
State the difference between transverse and longitudinal waves.
Longitudinal: the direction of oscillation is parallel to the direction of energy transfer. Consists of compressions and rarefactions.
Transverse: the direction of oscillation is perpendicular to the direction of energy transfer.
What does wavelength tell you on a displacement-time graph?
Time period (time taken for one wave to pass). Can be used to calculate frequency (number of waves which pass a point every second).
What is the range of wavelengths of visible light?
400nm (blue) – 750nm (red)
How do frequency and amplitude affect sound?
- Higher frequency = higher pitch
- Greater amplitude = louder (I∝A²)
Why does refraction occur, and how does the change in speed affect the wave?
When waves enter a new medium, their speed changes, as does their direction. If the wave slows down, it bends towards the normal. If the wave speeds up, it bends away from the normal.
How is wavespeed, frequency and wave length affected by refraction?
Wavespeed and wavelength decrease by the same proportion. Frequency remains constant.
Why do prisms disperse spectrums of light?
Different wavelengths of light have slightly different refract indices. Blue light has highest n, so it refracts more than red.
What is total internal reflection (TIR), and what conditions does it require?
TIR is when all light is reflected at a boundary between two different mediums (no refraction).
1. The light must be in the medium with the higher refractive index.
2. The angle of incidence must be greater than the critical angle.
How do optic fibres work?
TIR is used to send light signals down thin, glass optic fibres. The cladding surrounding the core has a lower refractive index, allowing TIR to occur. The fibres are protected by a sheath.
Outline what multiplexing is and how it can be solved.
Sending multiple signals using different wavelengths down the same fibre, so faster wavelengths of the second pulse can catch up with slower wavelengths of the first pulse. Solved by using monochromatic light.
Outline what modal dispersion is and how it can be solved.
Many different paths a light ray could take through a fibre. Light from the second pulse could catch up with light from the first pulse. Solved by using a narrow core and similar refractive indices for core and cladding so long path lengths aren’t available.
Why does absorption occur in optic fibres, and how can it be solved?
Light is scattered from the core by impurities in the material, reducing the brightness/ amplitude of the pulse and making the signal harder to read. Solved by using very pure glass and placing signal repeaters along the length of the fibre (can slow down rate of data transfer).
How do polarising filters work?
Absorb or transmit transverse waves parallel to their axis.
How are stationary waves formed?
Two waves travelling in opposite directions will interfere/ superpose; their displacements will sum at every point. If they have the same frequency and wavelength, they produce a stationary wave.
What are the characteristics of nodes and antinodes on stationary waves?
- At a node, there is only destructive interference.
- At an antinode, there is the maximum amount of constructive interference, resulting in a maximum amplitude.
- Points on a stationary wave are either in phase or antiphase.
How was Young’s double slit experiment carried out, and what were the results?
Coherent (constant phase difference) light source passed through double slit.
- Central maximum: path difference = 0
Arrive in phase: constructive bright fringe
- Minimum: path difference = λ/2
Arrive out of phase: destructive dark fringe
- Maximum: path difference = λ
Arrive in phase: constructive bright fringe
How is central maximum width calculated using a single slit interference pattern graph?
Double width of subsequent fringes.
What should you do with lines per mm in a diffraction grating calculation?
Convert to lines per m, than use the reciprocal as the grating spacing.
How can you find the highest visible order?
Set θ = 90, so n = d/λ
If calculation yields decimal, round down.
List the variables in the first harmonic equation.
- f: frequency
- l: length
- T: tension
- μ: mass per unit length
