Waves Flashcards
The higher the refractive index of a material the higher its
Optical density
The more optically dense a material the more light
Slows down when it travels through it.
How is the refractive index of a material, s, calculated?
N = refractive index = C / Cs
C = speed
What is the speed of light
3 x 10^8 ms^-1
The refractive index of air can be assumed to be … Because …?
1 because the speed of light in air is only a very small amount slower than in a vacume.
When light is refracted what stays the same and what changes?
It’s speed and wavelength change but its frequency stays the same.
Light bends away from the medium when it moves from a
More to less optically dense medium.
Total internal reflection happens when
Light going into a less optically dense object is at the critical angle or higher.
Can total internal refraction happen going from a less to more optically dense medium?
No, only from more to less.
In an optical fiber what is more optically dense, the core of cladding?
The core
Why are optical fibres narrow?
So that the light always reflects (It won’t be bellow the critical angle)
What are some of the advantages of using optical fibres for tv and phones rather than electricity?
Light has a higher frequency so light can carry more information.
Does not heat up fibre, little energy loss.
No electrical interference
Can travel very quickly
What is the difference between a mechanical and electromagnetic wave?
Mechanical waves need a medium to travel through for their propagation while electromagnetic waves do not - they can travel through a vacume.
Examples of mechanical waves are …
Water waves
Sound waves
Waves in a string
Step index optical fibres refers to …
The step down of the refractive index in stages from the core to the cladding.
Information sent down optical fibres is a …… Signal.
Digital
The signal in optic fivers broaden as they travel why could this be a problem?
If separate signals broaden and merge together this could reduce the quality.
The digital signal in an optical fibre degrades due to
Absorption and scattering from contact with the material in the fiber.
A signal in an optical fiver broadens as it travels because …
Light that travels straight down the middle (an axial ray) takes less time than one that bounced of the sides (a non-axial ray)
The principle of superposition states that …
When two or more waves cross, the resultant displacement equals he sum of the individual displacements.
Two points on a wave are exactly out of phase if they are …
An odd number of half-cycles apart.
To be in phase waves need to have the same what?
Frequency and wavelength.
Why is a stationary wave?
It is the superposition of two progressive waves with the same frequency (or wavelength) and the same amplitude moving in opposite directions.
Is any energy transferred by a stationary wave?
No
What can be used to make waves on a piece of string?
A Oscillator
The frequencies at which a wave happens to produce a multiple of half a wavelength an forms a oscillating loop that does not change shape is called?
Resonant frequencies
What are resonant frequencies?
The frequencies at which an exact number of half a wavelength are across the string.
An anti node is where amplitude in a standing wave is at its
Maximum
A node on a standing wave is where the amplitude is at a
Minimum
At which of these is there constructive or destructive interference?
Node
Antinode
Node - destructive
Antinode - constructive
Resonant frequents can be named according to their antinodes, how?
The Ath harmonic
A = number of antinodes
The second harmonic has how many nodes and why is its other name?
It has 3 nodes
AKA the first overtone
How many wavelengths fit on the third harmonic, how many nodes does it have and what is it also known as!
1 1/2 wavelengths
4 nodes
AKA the second overtone
As each you progress to the next harmonic what happens to?
Wavelengths that fit on?
Frequency ?
Nodes?
Extra 1/2 a wavelength
Frequency increases by one fundamental frequency.
Nodes increase by 1
The heavier the string the ….. The fundamental frequency, why?
The lower because the wave travels slower. The wavelength is constant and c=f x lander. F decreases.
The loser the string the …… the fundamental frequency, why?
Lower because the wave is slower.
Can microwaves form stationary waves by reflecting I metal?
Yes
What is diffraction?
When light spreads out after passing through a narrow gap.
An wave will diffract most when it passes through a gap …
That is of a similar size to the waves wavelength
To observe a diffraction pattern the light should be
Monochromatic (the same frequency)
Describe a single slit diffraction pattern.
A bright central fringe with alternating dark an bright fringes on ether side of it.
What does coherent mean?
The frequency and wavelength are the same.
They have a fixed phase difference..
For two source interference to take place the waves must be …
Monochromatic (one frequency)
Coherent :
Same wavelength, frequency constant phase difference
What are advantages of using a laser to demonstrate interference.
Light form a laser is monochromatic Light from a laser is coherent. Light from a laser is concentrated into a beam. Easy to use Inexpensive.
Laser can be very damaging to eyes. (Burn the retina) what safety precautions can be taken when using a laser?
Never shine at a person Wear laser safety goggles Avoid shining the laser beam at a reflective surface. Have a warning sign. Turn the laser of when not needed.
What is the criteria for the light interference for
Single source?
Double slit?
Single - monochromatic
Double - monochromatic + coherent
Any point that is in equal distant from the two sources what will happen? What is it called? (3)
The light will be in phase
Constructively interfere
This is called a maxima
What will happen at any point where the the path difference is half wavelength e.g 1 1/2 or 4 1/2?
What are these points called?
The waves will be out of phase
Total destructive interference occurs
These dark points are called minima.
In two source interference? What happens at
Minima and maxima
At maxima the light is in phase and constructive interference occurs.
At minima the light is out of phase and destructive interference occurs.
In terms of wavelength when do constructive and destructive interference happen. From two slit interference?
Constructive when path difference
= n wavelengths
Destructive when path difference
= (n+1/2) wavelengths
Why for two source interference of light can’t you have two different sources?
The light might not be coherent
By using two slits the light is coherent.
How can you work out fringe spacing (w) using
Distance between the slits (D) ?
An slit spacing?
W=D wavelength / S
What is the equation for frying spacing in double slit interference?
W= (D x wavelength) / s
D = distance between source and fringes. s = slit spacing.
What is the law of refraction between two boundaries n1 and n2?
N1sin(@1) = n2sin(@2)
@1 = angle of incidence @2 = angle of refraction
Light bends towards the normal when travelling from a …
Less to more optically dense medium.
