Unit 2.6 - Refraction of light Flashcards
When does light refract?
When light waves cross a boundary between 2 materials
What do light waves do as they cross a boundary between 2 materials?
Refract
Where do we measure angles of refraction from?
From the normal
Draw and label a refracted light beam, showing θ1, θ2, normals, incident ray, refracted ray and emergent ray
(See notes)
θ2
Angle of refraction
θ1
Angle of incidence
What does the angle of refraction, θ2, depend on?
The incident angle θ1
The refractive index of the material
What is the refractive index of a boundary?
sin θ1
——. = constant
sin θ2
This is the refractive index of a boundary
Example of a boundary
From air into glass
What does the refractive index of a boundary depend on?
The wavelength of the ray of light
Is the refractive index of a boundary the same both ways? Why?
No
It’s only a constant at the boundary between, say, air and glass
The constant for a ray moving from glass into air is different
How is the refractive index of the boundary for going from air into glass different to the refractive index of the boundary for going from glass into air?
The inverse (one over) of each other
Why is using the refractive index of a boundary not ideal? What idea was developed instead?
It’s a bit too inconvenient that this refractive index is dependant on the materials on either side of the boundary
Absolute refractive index
Absolute Refractive Index
The refractive index of every material as compares to the index of absolute vacuum
What’s the absolute refractive index of an absolute vacuum?
1
Absolute refractive index of glass
1.50
(But it depends on the type)
Absolute refractive index of water
1.33
Absolute refractive index of air
1
(The same as a vacuum for all purposes)
What are the values for the absolute refractive indexes of different materials more than and why?
Always more than 1, as this is the value for an absolute vacuum, and other materials are of course going to be denser than a vacuum
Inequality to represent n (the refractive index of a material)
n ≥ 1
What does n represent ?
The refractive index of a material
What happens to light as it moves into a more optically dense material?
It slows
Describe the index of refraction for an optically dense material
Higher
What happens to light as it moves into a medium of higher index of refraction?
It slows
When does light have to travel into a substance at in order to refract?
At an angle
What happens to light as it moves into a less optically dense medium?
Speeds up
Describe the index of refraction of a not very optically dense medium
Low
What happens to light as it moves into a medium of a lower index of refraction?
Speeds up
What does a lower index of refraction mean for a material?
Less optically dense
What does a higher index of refraction mean for a material?
More optically dense
What does light do as it slows when defracting?
Bends towards the normal
When does diffracted light bend towards the normal?
As it slows
When does diffracted light bend away from the normal?
As it speeds up
What does light do as it speeds up whilst defracting?
Bends away from the normal
Describe the indexes of refraction travelled through when light slows
Low n to high n
Describe the indexes of refraction travelled through when light speeds up
High n to low n
Wht are refraction angles to do with?
The speed of light
The materials travelled through
Equation for calculating the refractive index of a material
n = c
—
v
Define the symbols in n = c
—
v
c = speed of light in a vacuum (constant)
v = speed of the wave in the medium
n = refractive index
Define the symbols in n1v1 = n2v2
n1 = refractive index of initial medium
v1 = speed of the wave in the initial medium
n 2 = refractive index of the second medium
v2 = speed of the wave in the second medium
Derive n1v1 = n2v2
n = c/v can be rearranged to give nv = c
Since c is constant…
n1v1 = n2v2
Snell’s law
n1sinθ1 = n2sinθ2
Explain, with the aid of a diagram, Snell’s law
(See notes there’s no point in writing it all out without a diagram)
What’s the unit of refractive index?
(No unit)
Why does the refractive index have no unit and what does this make it?
c = ms^-1
———
V = ms^-1
Dimensionless unit
Dimensionless unit
e.g - refractive index
No unit
Angle of incidence
Angle between a ray striking a surface and the normal at the same point
Angle of refraction
Angle with the normal made by the ray travelling away from the boundary
In which material is the angle ALWAYS smaller, no matter in which direction light is travelling?
The optically denser material
If light is travelling form glass to air, which angle is biggest and why?
The angle of refraction in the air
The angle in the optically denser medium is always the smaller one
What happens to the angle of refraction as the angle of incidence increases?
Also increases
Which angle reaches 90 degrees first - the angle of refraction or the angle of incidence?
Angle of refraction
What can the angle of refraction not exceed?
90 degrees
If light is travelling from glass into air, and the angle of incidence surpasses the angle of refraction being 90 degrees, what cannot happen?
Light can’t emerge into the air
What happens when the angle of incidence surpasses the angle at which the angle of refraction is 90 degrees?
Light travels back into the optically denser medium - total internal reflection
In what type of medium does total internal reflection occur?
The optically denser medium
Which law is obeyed during total internal reflection?
The law of reflection
Critical angle
The size of the angle in the denser medium, the angle of incidence, at which the external angle becomes 90 degrees
The size of the angle in the denser medium, the angle of incidence, at which the external angle becomes 90 degrees
Critical angle
Clearly explain what the critical angle is
When θ2 = 90 degrees
θ1 = critical angle
What happens when the angle of incidence is less than the critical angle?
Light is refracted into the less dense medium
What happens when the angle of incidence is greater than the critical angle?
Total internal reflection takes place
Draw a diagram to show the critical angle and then total internal reflection
(See notes)
Why does partial internal reflection happen?
Obeys the law of reflection
Where does light reflected via partial internal reflection reflect to?
Remains in the optically denser medium
Partial internal reflection
A small proportion of the light reaching a boundary between an optically denser medium and a less dense medium is always reflected by the surface, and obeys the law of reflection
The reflected light remains in the optically denser medium
What’s the name for a small proportion of light reaching a boundary between an optically denser medium and a less dense medium always being reflected by the surface, obeying the law of reflection?
Partial internal reflection
Equation involving critical angle and the refractive index
n1sinθc = n2
θc
Critical angle
Critical angle symbol
θc
What is the equation n1sinθc = n2 basically a re-write of? Explain this
Snell’s law
n1sinθc = n2sinθ2
Sin 90 = 1 (this must be true about θ2 for this to be the critical angle)
Therefore n1sinθc = n2
What is the critical angle to a material?
A property of it
Refractive index and critical angle of water
n = 1.33
Critical angle = 48.7 degrees
Refractive index and critical angle of glass (borosilicate)
n = 1.47
Critical angle = 42.9 degrees
Refractive index and critical angle of diamond
n = 2.42
Critical Nile = 24.4 degrees
Compare images produced by total internal reflection compared to those produced by mirrors
Much brighter
Why are images produced by total internal reflection much brighter than those produced by mirrors?
Much less absorption of light at a totally internally reflecting surface than a mirror surface
What type of optical devices make use of total internal reflection and why?
Cameras and binoculars
Images are much brighter than those produces by mirrors (much less absorption of light)
What do optical devices such as cameras and binoculars make use of and why?
Total internal reflection within glass prisms rather than reflection by mirrors
Much brighter mage due to less absorption of light at a totally internally reflecting surface than a mirror surface
Is it easy to achieve total internal reflection with glass? Why?
Yes
Critical angle of glass is less than 45 degrees
Why does diamond sparkle like it does?
Exceptionally high refractive index
Exceptionally low critical angle
Total internal reflection within the diamond + the ultimate escape of light into the air
Why will glass never sparkle like a diamond?
It has a lower refractive index
Why is there so much internal reflection in a diamond and what does this cause?
Exceptionally high refractive index
Exceptionally low critical angle
Causes diamond to sparkle
How can information be transferred in optic fibres?
As a series of “ons” and “offs” as electrical or, more often, optic signals
What do optic fibres allow to occur? How?
Information to be transferred long distances
A series of “ons” and “offs” as electrical or, more often, optic signals
What makes optic fibre communication possible?
Total internal reflection
What does total internal reflection make possible?
Optic fibre communication
How is optic fibre communication made possible by total internal reflection?
A fibre of glass is long and thin, so light can travel along and hit the surface of the glass at large angles of incidence (much larger than the critical angle)
Under which circumstance would light hit the surface of glass in an optic fibre at an angle of incidence that ISN’T greater than the critical angle and what would this cause?
If the cable is bent too sharply
Total internal reflection can no longer occur
Describe the refractive index of the cladding of an optic fibre compared to the core
Lower
What is the refractive index of the boundary between the central core and the boundary in an optic fibre?
Close to 1
Why is the refractive index of the cladding of an optic fibre lower than the core?
For total internal reflection to occur…
Light must travel form a material with a higher refractive index to a lower refractive index
The angle of incidence must be greater than the critical angle
What happens in an optic fibre if the cable is bent too sharply?
The angle of incidence becomes smaller than the critical angle
Retraction through the boundary takes place
What would cause refraction to take place through the boundary of an optic fibre?
The angle of incidence being smaller than the critical angle (when the cable is bent)
What happens when light is totally internally reflected by a perfectly smooth surface?
No absorption
When does no absorption take place in an optic fibre?
When light is totally internally reflected by a perfectly smooth surface
Why does no absorption take place when light is totally internally reflected by a smooth surface?
At each reflection in the optic fibre, the light intensity stays the same
What do scratches in an optic fibre do?
Change the local angle of incidence
What changes the local angle of incidence in an optic fibre
Scratches
What happens to the intensity of light if an optic fibre is badly scratched and why?
Reduces
The local angle of incidence changes
What changes the local angle of incidence in an optic fibre and what does this do?
Scratches
Reduces the intensity of light travelling long the fibre
How are optic fibres designed to avoid?
Designed so that the total internal reflection does not take place in the glass-air surface
What are the 2 ways in which an optic fibre can be built?
Step index fibre
Graduated index fibre
What does a step index fibre constist of?
Central core of high optical density, which carries the signal
Outer sheath of optically less dense glass
Where does total internal reflection take place in a step index fibre?
At the ou dairy between the central core and the outer sheath
Which part of a step index fibre carries the signal?
Central core
What does a graduated index fibre consist of?
High optical density at the centre, decreasing gradually, it’s low refractive index material in the outer layers
Which type of optic fibre is the mos expensive to make?
Graduated index fibre
Draw a light signal in a step index fibre
(See notes)
Draw light in a graduated index fibre
(See notes)
Draw and label an optical fibre
(See notes)
Pulse
Consists of a series of waves, all at the same amplitude
What are digital signals?
A series of blocks representing “1” and “0”, as if light were rectangular
Mode
The number of paths that light can travel down an optical fibre
What can light do in a multimode fibre?
Light from a single pulse can travel along different paths
What from light can travel along different paths in a multimode fibre?
A single pulse
Describe the light of a laser
1 wavelength
Why is laser light used in optic fibres?
To avoid wavelength dispersion, as different lights can travel at different speeds
Which type of fibre has the thickest core?
Multimode
What happens in a multimode fibre and why?
A series of total internal reflections, as they have a thicker core, meaning that several waves can travel along the fibre at any time
Which light would have a shorter distance to travel than another in a multimode fibre?
Light which travels in a straight line compares to light which travels at an angle near the critical angle
What’s similar between all of the different paths of light in a multimode fibre?
Their speeds
In what type of fibre is a pulse of light smeared when it’s received and why?
Multimode
Multiple paths, which travel at the same speed but over different distances
What does the fact that the paths of light travel different distances in a multimode fibre lead to?
The pulse of light is smeared when it’s received
In what type of optic fibre could data become muddled and why?
Multimode fibres
When the coded pulses are sent, the smearing of each pulse could cause them to overlap and muddle the data
Which type of optic fibres aren’t ideal for long distances and why?
Multimode fibres
Data gets muddled
Why are multimode fibres called this?
There’s a number of paths that light can travel down the optical fibre
How can we reduce the smearing effect in multimode fibres?
Make the range of possible path lengths smaller
Use infared light
How do we make the range of possible path lengths smaller in a multimode fibre?
The range of possible angles where reflection can happen have to be reduced
This is done by increasing the critical angle, which requires the refractive index of the cladding to be only slightly smaller than the refractive index of the core
Why do we use Infared light in multimode fibres?
Has wavelength similar to the cladding
Loses less energy in the transmission
Using one colour (wavelength) gets rid of the problem of pulses travelling at different speeds
Which type of fibres have the most narrow cores?
Monomode fibres
How big are the diameters of monomode fibres?
Only a few wavelengths
Which type of optic fibres are easily broken and difficult to manufacture and why?
Monomode fibres
They’re very thin
Why are monomode fibres called this?
Only 1 possible path that light can follow through the fibre, straight down the core
What can pass through a monomode fibre at a given time?
1 wave
How can signals be sent in monomode fibres and why?
Over a greater distance at a greater rate as there’s only 1 possible path that light can take through the fibre
What is avoided in monomode fibres? Why?
Dispersion
There’ only 1 possible path that light can follow through the fibre
Where is the 1 possible path that light can travel down a monomode fibre?
Down the core
In what type of fibres can signals be sent over a greater distance, at a greater rate?
Monomode
Dispersion
Pulse spreading
What causes signals to become muddled in a fibre?
Dispersion (pulse spreading)
What happens when the angle of incidence is equal to the critical angle?
Light bends along the boundary
What does the area underneath a light power against time graph represent?
Energy
How do we calculate the number of pulses per second in a wire?
- Work out how far apart in terms of time 2 paths of light take to reach a certain point
- Pulses per second =
1
—
This time
What do we assume when calculating the number of pulses per second?
That all the light travels at the same wavelength
Water has a refractive index of 1.33. What is meant by this?
The speed of light in water is c/1.33
Where c is the speed of light in a vacuum
what type of fibre gives the fastest rate of data transfer?
monomode
How do we calculate whether overlapping will occur in a multimode fibre
- Speed in wire (n = c/v equation)
- Times for both the normal and zig-zag route (std)
- Difference in time
- Is this time higher than the time between intervals? If so —> overlapping
How do we calculate all of the angles at which bright beams emerge from a diffraction grating?
Work out nmax
If its 5 for example, use the diffraction grating with n=1, and repeat with n=2 for the second angle
What should always be referenced when describing mono mode fibres?
That they have very narrow cores
Compare the paths travelled in mono mode and multi mode fibres
Monomode - parallel to the axis
Multi mode - zig zag pattern
What do we do in questions that ask us to show that a certain path is a certain amount longer than another?
Trigonometry
State Snell’s law
At the boundary between 2 given materials, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant
If something says the refractive index of _____ , where does the light come from?
The material asked
