Optics SAEED Flashcards
Where a bright fringe is formed….
The light from one slit reinforces the light from the other slit
In other words the light waves from each slit arrive in phase with each other
Where a dark fringe is formed….
The light from one slit cancels the light from the other slit
In other words the light waves the two slits arrive 180* out of phase
1nm =
10*-9m
The fringe separation is….
The distance from the centre of a bright fringe to the centre of the next bright fringe
This depends on the slit spacing (s) and the distance (d) from the slits to the screen in accordance with the equation
What is the equation?
Fringe separation = wavelength of light x distance from screen / slit spacing
How does the equation for fringe separation show that the fringes become more widely spaced?
The distance (d) from the slits to the screen is increased
The wavelength of the light used is increased
The slit spacing (s) is reduced - note that the slit spacing is the distance between the centres of the slits
What is the difference between distances referred to?
The path difference
Where’s best to measure the fringe separation?
From the centre of a dark fringe to the centre of the next dark fringe (easier to locate than the centre of brighter fringes)
What are double slits describe as and why?
Coherent sources because they emit light waves of the same frequency with a constant phase difference provided we illuminate the double slits with laser light or with light from a narrow single slit of not using non laser light
How do double slits emit wavefronts with a constant phase difference?
Each wave crest or wave trough from the single slit always passes through one of the double slits a fixed time after it passes through the other slit
How is the double slit interference seen in the ripple tank experiment?
Straight waves from the beam vibrating on the water surface diffract after passing through the two gaps in the barrier and produce an interference pattern where the diffracted waves overlap
If one gap is closer to the beam than the other each wavefront from the beam passes through the nearer gap first
However the time interval between the same wavefront passing through the two gaps with a constant phase difference
Would light from two nearby lightbulbs be able to form an interference pattern? Why?
It could not form an interference pattern
This is because the two light sources emit waves at random
The points of cancellation and reinforcement would change at random so no interference pattern is possible
In the double slit experiment what does the fringe separation depend on?
The colour of light used
What is light from a filament lamp or from the sun composed of and what sort of range do they have?
Composed of the colours of the spectrum and therefore covers a continuous range of wavelengths (350nm-635nm)
What would happen if a beam of white light was directed at a colour filter?
The light from the filter is a particular colour because it contains a much narrower range of wavelengths than white light does
Which two main ways does the light from a laser differ from non-laser light?
Laser light is highly monochromatic
A laser is a convenient source of coherent light
What is meant if a laser light is highly monochromatic?
We can then specify it’s wavelength to within a nanometre
Wavelength depends on the type of laser that produces it
What is meant if a laser is a convenient source of coherent light?
When we use a laser to demonstrate double slit interference we can illuminate the double slits directly - we don’t need to make the light pass through a narrow single slit first as we would with non laser light sources
What is a photon?
A packet of electromagnetic waves of constant frequency
Inside a laser (convenient source of coherent light) what does a photon do after being emitted by an electron?
Causes more photons to be emitted as it passes through the light emitting substance
These stimulated photons are in phase with the photon that caused them
Talk about the photons in a non-laser light source
The atoms emit photons at random so the photons in such a beam age random phase difference
What does each component colour of white light produce?
Produces its own fringe pattern and each pattern is centred on the screen at the same position
As a result of each component colour of white light producing its own fringe pattern it’s own fringe pattern and each pattern being centred on the screen in the same position….
Central fringe is white
Inner fringes are tinged with blue on the inner side and red on the outer
Outer fringes merge into an indistinct background of white light becoming fainter with increasing distance from the centre
Why is the central fringe white?
White light fringes
Every colour contributes at the centre of the pattern
Why are inner fringes tinged with blue on the inner side and red on the outer side?
White light fringes
Red fringes are more spaced out than the blue fringes and the two fringe patterns do not overlap exactly
Why do the outer fringes merge into an indistinct background of white light becoming fainter with increasing distance from the centre?
White light fringes
Where the fringes merge different colours reinforce and therefore overlap
What is diffraction?
The spreading of waves when they pass through a gap or by an edge
Diffraction of water waves through a gap can be observed using a ripple tank , this arrangement shows diffracted waves spread out more if….
The gap is narrower
OR
The wavelength is made larger
Close examination of diffracted waves should reveal what? Why do they show this?
Should reveal that each diffracted wave has breaks either side of the centre
These breaks are due to waves diffracted by adjacent sections on the gap being out of phase and cancelling each other out in certain directions
Diffraction of light by a single slit can be demonstrated by directing a parallel beam of light at a slit, what should this show?
Diffracted light forms a pattern that can be observed on a white screen
Pattern shows a central fringe with a further two fringes
The intensity of the fringes is greatest at the centre of the central fringe
For an inference pattern to be shown what must be done?
Each slit must be narrow enough to make the light passing through it diffract sufficiently
Two slits must be close enough so the diffracted waves overlap on the screen
What does diffraction grating consist of?
A plate with many closely spaced parallel slits ruled on it
When a parallel beam of monochromatic light is directed normally at a diffraction grating, light is transmitted by the grating in certain directions only, why is this?
Light passing through each slit is diffracted diffracted
The diffracted light waves from adjacent slits reinforce each other in certain directions only including the incident light direction and cancel out in all other directions
The angle of diffraction between each transmitted beam and the central beam increases if….
Light of a longer wavelength is used
A grating with closer slit is used
How could you find the maximum of orders produced?
Substitute theta = 90* in the grating equation and calculate n using n =d / lamda
What are the steps to investigating the refraction of light by glass?
- use a day box to direct a light ray into a rectangular glass block at different angles of incidence at the midpoint P of one of the longer sides
- note that the angle of incidence is the angle between in the incident light ray and the normal point of incidence
- for each angle of incidence at P Mark the point Q where the light ray leaves the block
- the angle of refraction is the angle between the normal at P and the line PQ
Where is the angle of incidence found?
The angle of incidence is the angle between in the incident light ray and the normal point of incidence
Where is the angle refraction found?
The angle of refraction is the angle between the normal at P and the line PQ
Measurements of the angles of incidence and refraction fit different incident rays show that:
- the angle of refraction, r, at P is always less than the angle of incidence, i
- the ratio of sin i/sin r is the same for each light ray. This is know as snells law. The ratio is referred to as the refractive index, n, of glass
For a light ray travelling from air into a transparent substance, how is the refractive index or the substance calculated?
Refractive index of the substance
n = sin i/sin r
What is snells law?
The ratio of sin i/sin r is the same for each light ray
The ratio is referred to as the refractive index, n, of glass
When does partial reflection occur?
Occurs when a light ray in air enters glass or any other refractive substance
If you look at the light angle of refraction of the light ray emerging from the block, it is the same as the angle of incidence of the light ray entering the block, why does this happen?
This is because the two sides of the block at which refraction occurs are parallel to each other
If i1 and r1 are the angles of incidence and refraction at the point where the light ray enters the block, then the refractive index of the glass is …..
CALCULATION
n = sin i 1 / sin r 1
At the point where the light ray leaves the block, i2 = r1 and r2 = i1
So….
CALCULATION
sin i2 / sin r2 = 1 / n
What happens during the refraction of a light ray by a triangular prism?
Path of a monochromatic light ray through a triangular prism
The light ray refracts towards the normal where it enters the glass prism then refracts away from the normal where it leaves the prism
Why does refraction occur?
Because the speed of the light waves is different in each substance
The amount of refraction that takes place depends on the speed of the waves in each substance
What is the equation for calculating the refractive index of a substance? What does this show?
ns = c / cs
This equation for calculating the refractive index of a substance shows that the smaller the speed of light is in a substance the greater is the refractive index of the substance
What happens to the frequency f of waves when refraction occurs?
Frequency does not change when refraction occurs
What does i stand for in the equations?
The angle between the incident ray and the normal
What does r stand for in the equations?
The angle between the refracted ray and then normal
When a light ray passes from a vacuum into a transparent substance of refractive index n ….
CALCULATION
n = sinO 1 / sinO 2
Where O 1 = the angle between incident ray and the normal
O 2 = the angle between the refracted ray and the normal
n = the speed of light in a vacuum / the speed of light in the transparent substance
What is the refractive index of air?
1
Where can the critical angle be found and when does total internal reflection occur?
When a light ray travels from glass into air it refracts away from the normal
If the angle of incidence is increased to a certain value known as the critical angle, the light ray refracts along the boundary
If the angle of incidence is increased further the light ray undergoes total internal reflection at the boundary, the same as if the boundary were replaced by a plane mirror
In general total internal reflection can only take place if:
- the incident substance has a larger refractive index than the other substance
- the angle of incidence exceeds the critical angle
At the critical angle, what is the angle of reflection? Why?
90*
Because the light ray emerges along the boundary
Therefore n@
Describe the of a light ray along an optical fibre
The light ray is totally internally reflected each time it reaches the fibre boundary, even where the fibre bends, unless the radius of the bend is too small
At each point where the light ray reaches the boundary the angle of incidence exceeds the critical angle of the fibre
What does a communications optical fibre allow?
Allows pulses of light that enter at one end from a transmitter to reach a receiver at the other end
Such fibres need to be highly transparent to minimise absorption of light which would otherwise reduce the amplitude of the pulses progressively the further they travel in the fibre
What does each fibre consist of and what does this do?
Consists of a core surrounded by a layer of cladding of lower refractive index to reduce loss from the core
Light loss would also reduce the amplitude of the pulses
Where does total internal reflection take place?
Takes place at the core cladding boundary
At any point where two fibres are in direct contact , light would cross from one fibre to the other if there were no cladding
Such crossover would mean that the signals would not be secure, as they would reach the wrong destination
Why must the core be very narrow for internal reflection? Where does this occur?
To prevent modal (eg/ multipath) dispersion
This occurs in a wide core because light travelling along the axis of the core travels a shorter distance per metre of fibre than light that repeatedly undergoes total internal reflection
A pulse of light sent along a wide core would become longer than it ought to be
If it was too long it would merge with the next pulse
When else does pulse dispersion occur?
Also occurs if white light is used instead of monochromatic light (light of single wavelength)
This material dispersion is because the speed of light in the glass of the optical fibre depends on the wavelength of light travelling through it
Violet light travels more slowly than red light in glass
The difference in speed would cause white light pulses in optical fibres to become longer as the violet component falls behind the faster red component of each pulse
So the light (or infrared) used must be monochromatic to prevent pulse merging
What is mean if a fibre bundle needs to be a coherent bundle?
Means that the fibre ends at each end are in the same relative positions
