Optics Flashcards
What is a Coherent Light Source
Coherent light source produces waves with constant phase difference and identical frequency
Why is using a laser useful for showing interference and diffraction
It produces monochromatic light at a fixed wavelength which shows more defined interference patterns
Describe Youngs Double Slit Experiment
A Single Light Source is directed towards two slits, which each act as a coherent light source.
A bright central maximum is produced
The light constructively interferes every path difference of a full wavelength producing a bright light
The light destructively interferes every path difference of 1/2 of the wavelength producing a dark fringe
This produces dark and bright alternating fringes of decreasing intensity either side of the bright central maximum
Describe the interference pattern created when a single slit experiment uses white light
A bright white central maximum’
Flanked by alternating spectral fringes of decreasing intensity
With Violet being closest to order 0
Indigo
Blue
Green
Yellow
Orange
Red being the furthest
The pattern the. repeats
Why does an interference pattern form when light is passed through a single slit
The light Diffracts as it is passed through the slit of same magnitude to the wavelength
The waves that are in phase constructively interfere making bright fringes
The waves that are out of phase destructively interfere making dark fringes
A secondary bright fringe is formed either side the bright central maximum when two waves constructively interfere. That is 1/2 the width and a lot less intense each further successive bright fringe is a lot less intense and narrower
What would increasing the slit width do to the central diffraction maximum (single slit)
as W = (Landa * D)/s
W and S are inversely proportional hence if you increase s W will decreasing making the separation between the fringes smaller and in tern the central diffraction maximum narrower and more intense
What is the refractive index of air?
1
When light enters a more dense medium does it bend towards or away from the normal?
Faster
Away
Slower
Towards
more dense -> more friction -> moves slower
Towards the normal
When does total internal reflection occur
When the angle of incidence is higher then the critical angle
Happens when entering a substance with a lower refractive index
What’s the purpose of cladding in optical fibres
Protects the core
Allows total internal reflection as it has a lower refractive index then the core
What is modal Dispersion
When some of the signal reaches the end of the fibre before other parts due to each wave taking a different number of reflections.
- use monochromatic light
What is pulse dispersion
White light is used vs monochromatic
Light is detracted into different colours with different wavelengths while under going total internal reflection, this means that different colours have different retention times , due to internally reflecting a different number of times, to the end of the fibre compared to other colours causing delays and information loss.
Explain Bundle coherence
In application such as endoscopes
it is important the image produced is identical to that of the start
So it is essential that the hundreds of fibres are in the same coherent pattern at either side of the bundle.
How can absorption of light and dispersion be reduced in optical fibres
Use an optical fibre repeater to regenerate a correct signal
Why are optical fibres better then traditional copper wires
. Signal can carry a higher rate of information due to the use of light over electrons
. No energy lost as heat
. No electrical interference
What happens when the angle of incidence is equal to the critical angle
The light goes along the boundary
Angle of refraction = 90 degrees
Critical angle =
arcsine ( n2/n1)
where n1>n2
n= refractive index
What is the water to air critical angle
Water has a refractive index of 1.33
What is snells law?
Refractive index =
sin (Angle of incidence ) / sin (angle of refraction)
Speed of light in a vacuum / speed of light in substance
n1(sin(i) = n2(sin(r)
A higher refractive index =
More optically dance material
State two applications for diffraction gratings
Splitting up light from stars to make line absorption spectra -used to identify element present in the star
X Ray crystallography, a crystal sheet is used as a diffraction grating
used to find the spacing between atoms
Explain the formula d(sin()) = n * Landa
1) First order maximum - path difference between 2 adjacent rays is 1 wavelength
2) A Right angle triangle is formed
3) For the first maximum Sin() = Landa/ d
4) The other maxima occur when the path difference between the two rays of light is n* Landa
5) Replace n*landa with it to get
d(sin()) = n * Landa
d = distance between slits
When light passing through a diffraction grating is changed from blue to red do the orders get closer?
The wavelength has increased
d(sin()) = n * Landa
so dSin() has increased resulting in larger values of the angle so the orders have gone further apart
What is a Diffraction
The Spreading of waves when they pass through or around a gap
Hoe did Youngs double slit provide evidence for waves nature of light
Shows Diffraction and interference which both are theoretically impossible with particle theory of light.
List 4 Safety precautions when using a laser
Wear safety goggles
Don’t shine laser at reflective surfaces
Display warning signs
Always shine laxer in the same direction
What formula is associated with young’s double slit experiment
Fringe spacing =
(Landa * Distance from screen to split )/ Slit Separation
Can Sound waves and em waves interfere?
Yes
How does Light refracting as it entered a substance prove wave theory
“Bending effect” occurs when part of the wave reaches the medium boundary before the other
. If light is moving towars a material of higher refraction index it refracts towards the normal at the boundary
.If light is moving towards a material of lowe refraction index it refacts away from the normal at the boundary
If light were a particle the only factor that would change is the speed and direction would stay constant
How would you investigate Diffraction?
Add water to a clear plastic tray so the ripple generator is touching the surface of the water
Use Metal L junctions to create a narrow slit
A suitable distance form the ripple generator
Using tracing paper tho copy the pattern shown on the screen when lid is closed and measure the angle to the normal
Repeat for multiple slit sizes
For optical fibres, would a larger core diameter or a shorter core diameter allow for a greater transmission rate.
Shorter
Narrower core leads to less modal dispersion - as the range of paths lengths between reflections decrease (pulse broadening)
This means that the pulses are shorter in length, faster in reaching the end and therefore producing a greater rate of transmission
For any material red light has a lower refractive index then green light, and blue has a higher refractive index
If a green light ray is incident on a boundary at its critical angle
Explain what would happen to the red light, and what would happen to the blue light if both were shone at the same angle of incidence
Refractive indices change by the same factor ie Sin(Critical angle) = n2/n1 n2 and n1 decrease by the same scale factor
Hence Path by blue and red light are the same as the blue light
because the critical angle is the same
Explain how the following diffraction pattern supports the idea that electrons within the beam are behaving as waves.
Successive bright rings around the central maximum are caused by the superposition and constructive interference between 2 in phase waves.
This interference pattern is only produced by waves that undergo diffraction
Particle behaviour would only produced a small spot of light at the centre (or scatter randomly)
A single slit diffraction using green monochromatic light
The monochromatic light is changed to red light
Describe the change in pattern observed on the screen.
Fringe Spacing increases with Larger Wavelengths of light
Producing a wider Central Maximum
and Subsequent maxima are further apart from each other
A filter only lets waves with 600-700nm of wavelength through
Suggest how this affects a single slit diffraction.
Successive orders of maxima more broader fringes then if monochromatic light were used,
Different wavelengths create orders of maxima on different points on the screen in the same region
Central maximum width stays unchanged
