3.1 & 3.2 Flashcards
Progressive Waves
A progressive wave transfers energy without transferring material
What is an example of a transverse wave?
All electromagnetic (EM) waves
How can transverse waves be demonstrated?
through the waves seen on a string when it’s attached to a signal generator.
What is a longitudinal wave?
a wave in which the oscillation of particles is parallel to the direction of energy transfer.
What is an example of a longitudinal wave?
Sound wave
What is polarisation?
Polarisation is when a wave oscillates in only one plane and only transverse waves can be polarised
What is an application of polarisation?
Polaroid sunglasses as They reduce glare by blocking partially polarised light reflected from water and tarmac.
What is superposition?
When 2 waves meet and overlap.
Amplitude
The maximum displacement (of the wave or medium) from the equilibrium position.
Frequency
Number of waves/complete cycles/wavelengths (passing a point/produced) per second.
Wavelength
Distance between 2 identical points in succession on a wave.
Time period
The time for one complete oscillation
What is constructive interference?
occurs when two waves have displacement in the same direction.
standing/stationary
wave
Stationary waves are formed when two identical waves travelling in opposite directions meet and superpose. This usually happens when one wave is the reflection of the other. It has no net flow of energy.
when does constructive interference occur
Where the waves meet in phase, constructive interference occurs so antinodes are
formed
when does destructive interference occur
Where the waves meet completely out of phase, destructive interference occurs and
nodes are formed
What is an example of a stationary wave and how is it formed
Stationary microwaves can be formed by reflecting a microwave beam at a metal plate, to
find the nodes and antinodes use a microwave probe.
Path difference
The proportion of a wavelength by which two waves are “out of sync”.
What does Young’s double slit experiment demonstrate?
demonstrates interference of light from two-sources.
How can you perform the YDS experiment?
use two coherent sources of light or one coherent source and a double slit to form an interference pattern. If you don’t have a coherent source of light, you can use a single slit before the double slit to make the light have a fixed path difference and a filter to make it monochromatic.
How does the interference pattern form from the YDS experiment ?
Each slit acts as a coherent point source making a pattern of light and dark fringes. Light fringes are formed where the light meets in phase and interferes constructively. Dark fringes are formed where the light meets completely out of phase and interferes destructively.
What are the safety precautions to be followed while using lasers?
● Wear laser safety goggles
● Don’t shine the laser at reflective surfaces
● Never shine the laser at a person
what does Young’s double slit experiment provide evidence for?
wave nature of light because
diffraction and interference are wave properties, and so proved that EM radiation must act as a
wave
Node
A point of zero amplitude along a stationary wave caused by destructive interference.
Antinode
A point of maximum amplitude along a stationary wave caused by constructive interference.
What is diffraction?
Diffraction is the spreading out of waves when they pass through or around a gap
When does the greatest diffraction occur?
when the gap is the same size as the wavelength.
What happens when a wave meets an obstacle?
, you get diffraction round the edges, the wider the obstacle compared to the wavelength, the less diffraction.
What is the interference pattern formed by monochromatic light diffracted through a single slit?
a pattern of light and dark fringes, with a bright central fringe that is double the width of all other fringes, with alternating dark and bright fringes on either side.
What happens when white light is diffracted through a single slit?
the different wavelengths of light are all diffracted by different amounts so you get a spectrum of colour in the diffraction pattern with a central white maximum
Using white light instead of monochromatic laser light
gives wider maxima and a less intense diffraction pattern with a central white fringe with alternating bright fringes
How can you vary the width of the central maximum in a diffraction pattern?
by increasing the slit width, which decreases the amount of diffraction, or increasing the light wavelength, which increases the amount of diffraction.
What happens to the central maximum if you increase the light wavelength
central maximum becomes wider and its
intensity decreases.
What happens to the central maximum if you Increasing the slit width
central maximum
becomes narrower and its intensity increases.
What is diffraction grating
A set of slits containing many equally spaced slits very close together for light waves to pass through
Difference between single and double slit pattern?
single slit - central max. fringe that is twice the width of the other fringes* double slit pattern has equally spaced fringes
What happens when white light is shone on the diffraction grating instead of monochromatic?
white light yields less clear patterns (as position of dark bands depends on λ)*
colours appear;
only central band is white
What is the difference between the interference pattern formed by a diffraction grating and a double slit using monochromatic light ?
diffraction grating - pattern is much sharper and brighter than double slit because there are many more rays of light reinforcing the pattern.
Total Internal
Reflection
The complete reflection of a light ray reaching an interface with a less dense medium when the angle of incidence exceeds the critical angle.
Critical angle
The angle at which there is a transition between refraction and total internal reflection.
Refraction
The change of direction when a wave travels from one medium to another and changes speed.
Intensity
The wave power transmitted per unit area at right-angles to the wave velocity
What happens, in terms of refraction, if light passes across a boundary at 90° to a surface?
It doesn’t refract
What is a refractive index?
A measure of the optical density of a material relative to air
What is the approximate refractive index of air?
1
What is the word definition of Snell’s law?
The ratio of the sines of the angles of incidence and refraction are constant when it passes between two given media
What are the two conditions for total internal reflection?
- light passes from more to less dense medium
- angle of incidence > critical angle
How does light travel along an optical fibre?
By total internal reflection, only escaping when it reaches the other end
What is an endoscope?
A medical instrument that uses optical fibres to look inside the body
What do endoscopes consist of?
a coherent bundle of fibres (lens system)
* an incoherent bundle of fibres (light delivery system)
What happens if a fibre is bent too tightly?
Angle of incidence will be less than critical angle and light will escape
What can endoscopes be used to look at?
Digestive, respiratory and female reproductive systems
What are the positives of endoscopes?
Can diagnose patients without an incision, often without anesthetic
In an optical fibre, when will total internal reflection occur?
As long as θ is larger than the critical angle
In medicine, what are the uses of optical fibres?
endoscopes
* lasers - burn tissue to heal wound
What is a coherent bundle of fibres?
Where the fibres stay in the same relative position along their length
What are some of the problems for optical fibres? (3)
- scratches can cause light to leak
- two fibres touching can cause light to pass from one to the other - ‘cross talk’
- dispersion
How can scratches and cross talk be resolved when using optical fibres?
Using cladding
Does cladding have a lower or higher refractive index than the core?
Lower
How is light sent down an optical fibre?
In ‘pulses’ or ‘bursts’
How can a pulse be distorted in an optical fibre?
And how is it reduced?
- absorption - some energy absorbed so pulse has lower amplitude, but doesn’t affect the frequency. Reduced by using an optical fibre repeater to boost the
signal at periodic positions along the fibre. - dispersion - causes pulse broadening. 2 types:
modal- light enters at different angles and hence takes a different path.
material- signal contains several different
wavelengths so light travels at different speeds. solved by using monochromatic light.
What are the two types of dispersion?
- modal (chromatic) dispersion - light taking different paths
- material (multipath) dispersion- light being different speeds
How does modal (multipath) dispersion occur?
A pulse can take a variety of different paths through a fibre due to the different angles of incidence in the original pulse , meaning a single pulse can spread out over time
How can modal (multipath) dispersion be decreased?
- use monomode fibres with a core diameter of only a few wavelengths, so light travels via one path ( core needs to be narrow)
- cladding
How can cladding help to reduce modal (multipath) dispersion?
- Refractive index of cladding is only slightly lower than the refractive index of the core, so the critical angle is larger than is would be at a glass-air boundary → only small range of angles that can be transmitted ?)
- It prevents scratching of the core. It keeps the core away from adjacent fibre cores hence preventing crossover of information to other fibres
How can material dispersion be reduced?
Using monochromatic light (red will travel faster than blue)
What colour of light should be used in an optical fibre?
Red - it travels faster
What is it called when in a prism, white light is split into a spectrum of colours?
Dispersion
a prism, what colour light is refracted more: red or blue?
Blue
Why is blue refracted more than red in a prism?
Blue light travels more slowly in glass than red light
When is pulse distortion more of a problem?
When the pulses are very short and close together
When are single fibres used?
In communications
When are bundles of fibres used?
In endoscopes
Is violet the closest to the central maximum and red the furtherst from Central maximum ? TRUE OR FALSE
True
Explain what you would observe if you moved a microwave probe in a straight line parallel to the line of two coherent microwave transmitters
The probe would detect alternatic areas of maximum and minimal signal strength
How do TV and radio signals make use of wave polarisation
- Broadcasting aerial has rods which emit polarised waves
- TV aerials on homes have horizontal rods
- These rods must line up in order to get maximum signal strength
- The same thing happens with radio aerials
How does Material dispersion occur?
when white light is used instead of monochromatic light .This is because different wavelengths of light travel at different speeds
What happens to an object when it absorbs an electromagnetic wave?
Its temperature increases
When are superposed waves easier to ‘see’?
- the waves are of similar amplitude (↑ contrast between maxima and minima)
- the waves have similar frequencies - otherwise the interference patterns create change so fast that they are difficult to detect
- the waves have a constant phase difference i.e. they are phase linked
Examples of coherent sources?
- light produced by a laser
- sound from two loudspeakers connected in parallel
- light emerging from two apertures illuminated by the same source
Comparison of the energy of particles in stationary and travelling waves?
- stationary - energy stored and not transferred
* progressive - energy transferred
What can increase the pitch of a note on a guitar string?
- ↑ tightness/tension
- ↓ length of string
- ↓ thickness of string
The equation to calculate the frequency of the 1st harmonic is found on the data sheet. what do the symbols stand for?
f = 1/2l x √T/μ
f = 1/2l x √T/μ
f= frequency
l= length (m)
T= tension (m)
μ= mass per unit length( kg per meter)
What does the double slit interference pattern consist of?
Equidistant parallel fringes alternating between:
- maxima (constructive interference)
- minima (destructive interference)
The equation for the double slit interference pattern is found on the data sheet. what do the symbols stand for?
w = λD / s
w = λD / s
w= fringe spacing (m)
λ= wavelength (m)
D= slit to screen distance (m) (capital d is always the bigger distance)
s=spacing between slits (m)
What happens to the double slit interference pattern if green light is used instead of red?
Wavelength is decreased so distance between adjacent fringes decreases
What happens to the double slit interference pattern if both slits are made narrower?
Wider interference so there are more dots, but fainter as there is less light through(x ↑)
What happens to diffraction when the gap width ↓?
Diffraction ↑
What happens, when light is shone on a diffraction grating, when the wavelength is increased?
- short λ (e.g. blue light) - narrow diffraction pattern
* long λ (e.g. red light) - broad diffraction pattern
What is the equation for the maximum number of orders?
n = d/λ
Why is the diffraction grating more accurate than the double slits?
- double slits - fringes formed are slightly blurred → large errors
- diffraction grating - images are clear and measurements accurate, also final result is an average of several calculations
What happens when two polarising filters are arranged at right angles to each other?
No light will get through
