Waves and Optics Flashcards
Waves, Optics.
Define monochromatic (1)
Single wavelength
Define coherent (1)
Constant phase difference
State a safety precaution when using a laser (1)
Do not look into laser
State and explain the effect of using a laser with a shorter wavelength on the maxima spacing in Young’s double slit (2)
Maxima closer together
use w=𝜆D/s
Use wave theory to explain how the fringe pattern is formed in Youngs double slit experiment (4)
Slits act as coherent sources Waves diffract at slits Waves superpose - Bright patches: constructive - in phase Dark patches: destructive - 180 degrees out of phase
State two requirements for two light sources to be coherent (2)
Same wavelength
Same phase
Explain how Young’s double slit arrangement produces interference fringes, refer to width of first slit and coherence of second slits (6)
Narrow single slit gives wide diffraction to ensure that both second slits are illuminated
Paths to second slits are of constant length giving constant phase difference
Light diffracted at slits and overlap and interfere
Where path lengths differ by whole number of wavelengths, constructive interference occurs producing a bright fringe
Where path lengths differ by 0.5 x n wavelengths, destructive interference occurs producing a dark fringe
If Young’s double slit was carried out with red light and then with white light, how would the two differ? (3)
Central fringe would be white
Dark fringes would be narrower together
Side fringes are spectra
State two ways a diffraction pattern on single slit would change if the slit became narrower (2)
Increased separation - see single slit equation
Lower intensity - less light could get through
State and explain what happens to angle θ in λ = d sin θ when wavelength decreases? (2)
Angle θ gets smaller
As path difference gets smaller
Why will total internal reflection occur when light ray travels from water into glass? (refractive index) (1)
TIR only occurs when ray travels from higher n to lower n
as long as the incident angle is greater than the critical angle
What is the name for the part of an optical fibre that is around the core? (1)
Cladding
State and explain an advantage of a smaller diameter core (2)
Reduce multipath dispersion
Which would cause poor resolution
State one application of optical fibres and its benefit to society (2)
Communications
Improved transmission of data
Explain why optical fibres used for communication need to have cladding (2)
Keeps signal secure
Keeps most light rays in
State and explain two physical properties of the light produced by a laser which makes it different to that produced by a lamp (4)
Monochromatic
Waves of single wavelength
Coherent
Waves produced in constant phase
Explain how glass cladding around the optical fibre’s core improves the security of data being transmitted through it and give a reason (3)
Light doesn’t enter cladding so can’t pass across from one fibre to a neighbouring fibre
Fibres without cladding can allow light to pass between fibres when scratched or linked by moisture
Personal data must be transmitted along fibres where there is no danger of light leakage
Define longitudinal wave (1)
A wave with a direction of vibration that is parallel to the direction of propagation of the wave
Define transverse wave (1)
A wave with a direction of vibration that is perpendicular to the direction of propagation of the wave
Explain why it is important to correctly align the aerial of a TV to receive the strongest signal (2)
Transmitted radio waves are often polarised
Aerial rods must be aligned in the same plane of the wave
Define amplitude of a wave (1)
The maximum displacement of the wave from the equilibrium position
If there are two polarising filters inbetween an observer and light, if one is turned about 360 degrees, what would the observer see? (2)
Variation in intensity between max and min
Two maxima in 360 degree rotation
Give three examples of waves that are transverse (2)
Electromagnetic radiation
Surface of water
Rope
State one application of a polarising filter and a reason for its use (2)
Camera/sunglasses
Reduce glare
A microwave transmitter directs waves towards a metal plate. A detector is placed between and moved along a line, what causes the maxima and minima of waves detected? (3)
Superposition
reflection from metal plate
two waves of same frequency
travelling in opposite directions
How can a guitarist raise the fundamental frequency of vibration in their string? (1)
Tighten the string
shorter length of string
Use a string of lower mass per unit length
Describe the structure of a step-index optical fibre outlining the purpose of the core and cladding (3)
Core is transmission medium for electromagnetic waves to progress by total internal reflection
Cladding has a lower refractive index to allow total internal reflection to occur
Cladding offers protection from scratching that could lead to light loss
Blue light has a higher refractive index fibre than red light, explain how this difference results in a change in a pulse of white light by the time it leaves the fibre (2)
Blue travels slower than red due to greater refractive index
Red reaches the end of the fibre before blue leading to material pulse broadening
Discuss two changes to reduce material pulse broadening of white light(2)
Monochromatic source so speed is constant
Shorter distance between repeaters so pulse is reformed before significant broadening occurs
Suggest an experiment to demonstrate the wave nature of sound (1)
Diffraction through a door
Which quantities are changed when monochromatic light passes from air into glass? (1)
Speed and wavelength only, frequency remains constant
Of the electromagnetic waves, which has the greatest energy? (1)
Gamma rays
How do we know waves transfer energy?
Electromagnetic waves heats things up when they are absorbed
X-rays and gamma rays knock electrons out of their orbits (ionisation)
Loud sounds cause oscillations of air particles which make things vibrate.
Waves power can be used to generate electricity.
Define wavelength
The distance from a point on the wave to the same point on the adjacent wave. It is measured in meters
Define frequency
Is the number of waves that pass a point every second. It is measured in hertz (Hz)
Define period
It is the time taken for each one whole wave to pass a fixed point. It is measured in seconds (s)
Define phase difference
the amount by which one wave lags behind another wave.
What units can phase difference be measured in?
Phase and phase difference can be measured in degrees, radians or fractions of a cycle.
Describe an experiment to measure the speed of sound
Start with signal generator + microphones in phase.
Move microphone and measure the distance for 1 wavelength.
Measure the time delay using the picoscope and collect a range of distance and time measurements.
Draw a graph of distance against time.
Use the gradient to find the speed of sound.
Define polarisation
Restriction of a wave to a single plane
What waves can be polarised?
Transverse
Define interference
Interference – When two waves arrive at the same point and at the same time, the resultant displacement is given by the algebraic sum of the two individual displacements.
Define constructive interference
If two waves meet exactly in phase the amplitudes add up to produce large crests and troughs
Define destructive interference
If two waves meet exactly out of phase the amplitudes cancel to produce no crests and troughs
State the difference between a progressive wave and a standing wave.
Progressive (travelling wave) – transfer energy from one region to another
Standing (stationary waves) – Are fixed in space and do not travel. They are caused by the superposition of two identical waves.
How are stationary waves formed?
Vibrations cause waves to travel along the string towards the pulley.
The waves reflect from the pulley and travel back to the vibration generator.
The two waves meet and constructively and destructively interfere.
What are the conditions required for standing waves to be formed?
Same frequency and wavelength. Similar amplitude in the medium. Same speed
Define a node
NO DisplacEment – Positive displacement from one wave is cancelled by an equal negative displacement from the other wave. (Destructive interference)
Define an antinode
A point of maximum displacement (constructive interference)
For a string that is fixed at both ends what are the rules for nodes and antinodes?
There must be a node at each end.
State 3 ways to increase the fundamental frequency of a guitar string
Shorter string
Higher tension
Lower mass per unit length
For a closed pipe what are the rules for nodes and antinodes
The closed end of the pipe is always a node, the open end of the pipe is always an antinode
For an open pipe what are the rules for nodes and antinodes
There must be an antinode at each end
Define diffraction
The spreading of waves on passing through a gap or near an edge
When is diffraction noticeable?
When the wavelength is similar in size to the gap/obstacle
Using the single slit equation state 2 ways to increase the angle of diffraction
Larger wavelength
Smaller gap size
Define coherence
The spreading of waves on passing through a gap or near an edge
Using the double slit equation state 3 ways to increase the width between the fringes
- Larger wavelength
- Increased distance to screen
- Decrease the slit separation
In fibre optics cables why is absorption a problem?
Absorption is where some of the signal’s energy is absorbed by the material of the fibre. This energy loss results in the amplitude of the signal being reduced.
In fibre optics cables why is dispersion a problem?
dispersion cause pulse broadening. The received pulse is broader than the initial signal. Broadened pulses can overlap each other leading to information loss.
What is modal dispersion?
Is caused by light rays entering the cable at different angles. This causes them to take different paths and therefore take different amounts of time to travel.
How can modal dispersion be reduced?
To reduce modal dispersion by using a single mode fibre in which light is only allowed to follow a very narrow path.
What is material dispersion
Is caused by different amounts of refraction expienced by different wavelengths of light. This will cause some wavelengths to be slowed down more and take longer to travel down the cable.
