Refraction Diffraction And Interference Flashcards
what is path difference
is the difference in the distance travelled by two waves
what is a coherent light source
has the same frequency and wavelength and a fixed phase difference
what is an example of a coherent and monochromatic light
lasers as they emit a single wavelength of light
what does the youngs double slit experiment demonstrate
interference of light from two sources
what do you do in the youngs double slit experiment
she a coherent light source through two slits about the same size as the wavelength of the laser light so the light diffracts
each slit acts as a coherent point source making a pattern of light and dark fringes
how are the bright spots formed in youngs double slits
constructive interference takes place as they are in phase which also mean the path difference in (n)lambda
What is the equation associated with youngs double slits
Fringe spacing = (wavelength x distance between screen and slits)/slit separation
What happens if you use a white light instead of a laser in youngs double slit experiment
White light gives a wider maxima and a less intense diffraction pattern with a central white fringe with alternation bright fringes which are spectra
Which wavelength diffracts the least and the most
Least - violet
Most - red
What are the safety precautions needed while using a laser
Wear laser safety goggles
Don’t shine laser at reflective surfaces
Display a warning sign
Never shine the laser at someone
How was the evidence of the wave nature of light provided by youngs double slit experiment
Because diffraction and interference are wave properties and so proved that EM radiation must act as a wave
What was the other theory of what light was made out of apart from waves
Formed from tiny Particles
What is 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 or close to the same size as the wavelength
What happened if the gap is smaller than the wavelength or larger than the wavelength
Smaller - most waves are reflected
Bigger - less noticeable diffraction as when a wave meets an obstacle you get diffraction around the edges the wider the obstacle compared to the wavelength the less diffraction occurs
What type of light can be diffracted through a single slit onto a screen
Monochromatic light
When a monochromatic light is shined through a single slit onto a screen what interference pattern forms
The pattern is made up of light and dark fringes it has a bright central fringe which is double the width of all the other fringes with alternating dark and bright fringes on either side the intensity of the fringes decreases from the central fringe
What are the bright and dark fringes caused by
Bright - caused by constructive interference where the waves meet in phase
Dark - caused by destructive interference where the waves meet out if phase
How does the diffraction pattern look different when using white light instead of a monochromatic light
As white light is made up of all colours the colours have different wavelengths so diffract at different amounts so you get a spectrum of the colours instead of one colour from a single wavelength
how can you change the width of the central maximum
you can change slit width and wavelength
what does increasing the slit width do to the amount of diffraction and what happens to the central maximum
it decreases the amount of diffraction so the central maximum becomes narrower and its intensity increases
what does increasing the wavelength do to the amount of diffraction and happens to the central maximum
it increases the amount of diffraction and as the slit is closer in size to the lights wavelength the central maximum becomes wider and its intensity decreases
what is a diffraction grating
is a slide containing many equally spaced slits very close together
when a monochromatic light passes through a diffraction grating what happens to the interference pattern and why does that happen
it becomes more sharper and brighter than it would be if passed through a youngs double slit this is because there are many more light rays reinforcing the pattern
what is the ray passing through the centre of the diffraction grating called
0th order
what is the formula associated with diffraction gratings
distance between slits x sin(angle) = order x wavelength
when wavelengths increases what happens to the distance between the orders
the distance between the orders will increase because the angle is larger due to an increase in diffraction as the slit spacing is closer to the size of the wavelength
what is the maximum value of sin(angle)
1 so 90 degrees
what is the path difference between each order
n lambda when n is a whole number
what do you get when you split up light from a star using a diffraction grating and what does it show
a line absorption spectra which can be used to show which elements are present in a star
wat is a refractive index
is a property of a material which measures how much it slows down light passing through it
how do you calculate refractive index
refractive index = speed of light in a vacuum / the speed of light in a substance
a material with a higher refractive index can also be known as what
being more optically dense
What happens when a wave enters a different medium
It refracts causing it to change direction either towards or away depending on the materials refractive index
What causes the direction of the wave to change in a different medium
The speed of the wave changes causing it to change direction
If the object is more optically dense does the ray refract towards or away from the normal
Towards as it slows down
As the angle of incidence increases what other angle increases
The angle of refraction
When does the angle of incidence reach the critical angle
When the angle of refraction is exactly 90 degrees and the light is refracted along the boundary
When can total internal reflection occur
When the angle of incidence is greater than the critical angle
When the incident refractive index is greater than the refractive index of the material at the boundary
What is useful application of total internal reflection
Optical fibres
What are optical fibres
They are flexible, thin tubes of plastic or glass which carry information in the form of light signals
What is the structure of an optical fibre
They have an optically dense core surrounded by cladding
What does the cladding do to the optical fibre
It has a lower optical density allowing TIR to occur and also protects the core from damage and prevent signal degradation through light escaping the core which can cause information to be lost
What are the two things that cause signal degradation
Absorption and dispersion
How does absorption cause signal degradation
A part of the signals energy is absorbed by the fibre reducing the amplitude of the signal which could lead to loss of information
How does dispersion cause signal degradation
It cause pulse broadening which is where the received signal is broader than the original transmitted signal and these broadened signals can overlap causing loss of information
What are the two types of dispersion
Modal and material
What is modal dispersion
Caused by light rays entering the fibre at different angles therefore they take different paths along the fibre this leads to the rays taking a different amount of time to travel along the fibre causing pulse broadening
How can modal dispersion be reduced
By making the core very narrow therefore making the possible difference is path length smaller
What is material dispersion
Caused by light consisting of different wavelengths meaning rays will travel at different speeds along the fibre which leads to pulse broadening
How can material dispersion be prevented
By using a monochromatic light
What can both absorption and dispersion be reduced by
Using an optical fibre repeater which regenerates the signal during it travel to its destination
