Section 3 - Waves Flashcards
What is the definition of frequency and what are the units?
The number of waves passing through a point per second (hertz) Hz
What is the definition of wavelength?
The distance between 2 adjacent points on a wave
What is the definition of amplitude?
The maximum displacement of the wave from its equilibrium position
How can you find out the time period of a wave using its frequency?
T = 1/f
What is phase difference and what is it measured in?
- How much a particle/wave lags behind another particle/wave
- Measured in radians, degrees or fractions of a cycle
What is a longitudinal wave?
Give an example
- A wave in which the oscillation of the particles is parallel to the direction of energy transfer
- There are rarefractions (areas of low pressure) and compressions (areas of high pressure)
Sound waves
What is a transverse wave? Give 2 examples
- Waves where the particle oscillations are perpendicular to the direction of energy transfer
Electromagnetic waves and waves on a string
How fast do electromagentic waves travel in a vacuum?
All travel at the speed of light - same speed
What is the relationship between the magnetic field and electric field?
Perpendicular
What does a polarising filter do?
Only allows oscillations in one plane
How is polarisation used as evidence of the nature of transverse waves?
Polarisation can only occur if a wave’s oscillations are perpendicular to its direction of travel (in transverse waves)
How is polarisation used in antennas?
- TV and radio signals are usually plane-polarised by the orientation of the rods on the transmitting aerial
- The receiving aerial must be aligned in the same plane of polarisation to receive the signal at full strength
What is a stationary wave?
A wave which transfers no energy and whose positions of maximum and minimum amplitude are constant
What is a node?
A point on a stationary wave where the displacement is 0
What is an antinode?
A point on a stationary wave with maximum displacement
How are stationary waves produced?
A stationary wave is formed from the superposition of 2 progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude - often a wave reflected back on itself
* Where the waves meet in phase, constructive interference occurs so antinodes form
* Where the waves meet completely out of phase, destructive interfernece occurs and nodes form
What does the first harmonic for a stationary wave with 2 closed end look like?
2 nodes at either end and an antinode in the middle
What is the definition of coherence?
Coherent waves have a fixed phase difference and the same frequency and wavelength
Why is a laser useful in showing interference and diffraction?
It produces monochromatic (same wavelength/colour) light so diffraction and interference patterns are more defined
What was Young’s double-slit experiment?
- A single light source is directed towards 2 slits - which each act as a coherent light source
- The light interferes constructively and destructively to create an interference pattern
Describe the interference pattern created using white light
A bright white central maxima flanked by alternating spectral fringes of decreasing intensity with violet closest to the zero order and red furthest
The maxima is wider than the other maxima
Why does an interfernce pattern form when light is passed through a single slit?
- Light diffracts as it passes through the slit
- Where the waves are in phase constructive interference occurs making bright fringes
- Where the waves are completely out of phase destructive interference occurs making a dark fringe
How does increasing the slit width change the width of the central maximum?
The slit isn’t as close as it was to the wavelength in size so less diffraction occurs - the central maximum becomes narrower and more intense
What does a single slit pattern on a graph look like?
Central peak and then tiny tiny peaks either side
What is the approximate refractive index of air?
1
When light enters a more optically dense medium does it bend towards or away from the normal?
Towards the normal
When does total internal reflection occur?
When light is at a boundary to a less optically dense medium and the angle of incidence is greater than the critical angle
What is the purpose of the cladding in a step index optical fibre?
- Protects core from scratches which would alow light to escape and degrade the signal
- Allows TIR as it has a lower refractive index than the core
How does signal degradation by absorption in an optical fibre affect the received signal?
Part of the signal’s energy is absorbed by the fibre so its amplitude is reduced
What is pulse broadening?
When the received signal is wider than the original, this can cause overlap of signals leading to information loss
How does modal dispersion cause pulse broadening?
- 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
- The rays take different times to travel along the fibre, causing pulse broadening
What is material dispersion?
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
How can modal dispersion be reduced?
Use a single mode fibre (very narrow fibre) so the possible difference in path lengths is smaller
How can material dispersion be reduced?
Use monochromatic light
How can both absorption and dispersion be reduced?
Use an optical fibre repeater to regenerate the signal
State the advantages of optical fibres over traditional copper wires
- Signal can carry more information as light has a high frequency
- No energy lost as heat
- No electrical interference
- Cheaper
- Very fast
What path does a light ray take when the angle of incidence is equal to the critical angle?
It goes along the boundary ie the angle of refraction is 90°
What is snells law?
n1sini = n2sinr
What are 2 applications of diffraction gratings?
- Splitting up light from stars to make line absorption spectra - used to identify elements present in the star
- X-ray crystallography to find the spacing between atoms - a crystal sheet acts as the diffraction grating the X-rays pass through
Derive the formula dsinθ = nλ
- Draw this https://commons.wikimedia.org/wiki/File:Youngs_slits.png
- For first order maximum, the path difference between 2 adjacent rays of light is 1λ (as shown), the angle between the normal to the grating and the light ray is θ (the lower angle is 90-θ)
- For the first oder maximum sinθ=λ/d (sinθ = opp/hyp) rearrange to dsin = λ
- Other maxima occur when the path difference between the 2 rays of light is nλ, where n is an integer, replace λ with nλ to get dsin = nλ
When light passing through a diffraction grating is changed from blue to red, do the orders get closer together?
The wavelength of light has increased so it will diffract more, the orders will become further apart
The central maxima will be wider
What is diffraction?
The spreading out of waves when they pass through or around a gap
How did Young’s double slit experiment provide evidence for the wave nature of light?
Diffraction and interfernece are wave properties hence the interference pattern of light shows light has wave properties
What are 4 safety precautions that must be followed when using a laser?
- Wear laser safety goggles
- Don’t shine the laser at reflective surfaces
- Display a warning sign
- Never shine the laser at a person
How could you investigate stationary sound waves?
- Place a speaker at one end of a closed glass tube
- Lay powder across the bottom of the tube
- It will be shaken from the antinodes and settle at the nodes
- The distance between each node is half a wavelength
What is phase?
The position of a certain point on a wave cycle, (units are radians, degrees or fractions of a cycle)
When a wave travels from air into a medium how do you ensure the light doesn’t deviate?
Enters the medium at the normal
When a wave travels between 2 mediums, how do you ensure there is no deviation?
- The 2 mediums have the same refractive index
- The 2 mediums are fitted together without any gaps
When a question says discuss what shoud you do?
- Describe what could happen
- Comment on how likely that is to happen and why
