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Level 191

Diffraction


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What is diffraction?
The spreading out of waves after passing through a gap or round an obstacle
When is the most significant diffraction?
When the size of the gap is similar to the wavelength
Produces a fringe pattern
What does diffraction from a single slit produce?
W = (2D x wavelength)/a
Diffraction from a single slit equation:
Light is diffracted by each slit.
What happens when light passes through a diffraction grating?
dsinθ = nλ
The diffraction grating equation:
Why is the diffraction grating so good?
because there are so many slits there will always be a wave in anti phase with another. This means if the path difference is not a whole number, no light will be seen.
What is interference?
When two waves of the same type (eg. both light) occupy the same space
What does wave superposition result in?
The formation of an interference pattern made up of regions of reinforcement and cancellation.
What does coherent mean?
Waves have the same frequency and therefore wavelength, and a constant phase difference
What is path difference?
The difference in distance travelled by two waves, often measured in wavelength rather than meters
What happens in an interference pattern when the path difference is nλ?
At a maximum (when the path difference is nλ), the waves will add constructively to produce a bright fringe.This is because the light from one slit reinforces light from the other. The light is in phase
What happens in an interference pattern when the path difference is (n+0.5)λ?
At a minimum (when path difference is not a whole number of λ), a peak will meet a trough causing destructive interference
What happens in an interference pattern when two meeting waves have travelled the same distance?
Constructive interference as there is no path difference, a peak will meet a peak or a trough will meet a trough
What does light source need to be for Young's double slit experiment and how is this achieved?
Needs to be monochromatic (one colour/frequency). This can be achieved using a colour filter with white light or using a monochromatic light source such as a laser
Why is a single slit used in Young's double slit experiment?
Used to obtain a coherent light source. It is not needed if a laser is used
Describe the double slits.
To produce two sources of light.
When is a bright fringe formed in Young's double slit experiment?
A bright fringe is formed where light from one slit reinforces the light from the other, so when the light from both slits are in phase. They will have a path difference equal to a whole number of wavelengths
w = (λD )/s
Equation for Young's double slit experiment:
What does monochromatic mean?
Light waves of a single wavelength and so a single colour
What does collimated mean?
Light waves that are parallel and therefore will spread minimally as they propagate
What does polarised mean?
Light waves that oscillates in one direction only
When is a dark fringe formed in Young's double slit experiment?
A dark fringe is formed due to the cancellation of the light, when the light from the slits are out of phase. They will have a path difference equal to (n+0.5)λ
Appearance of diffraction pattern from single slit
Central maxima contains most of the diffracted light and is double the width of of subsequent maxima, intensity decreases rapidly away from central maxima
Why does a single slit produce an diffraction pattern?
as light passes through the slit it diffracts at either edge of the slit, the waves interfere producing maxima and minima
Equation for single slit diffraction
mλ = asinθ (where m is an integer and a is slit separation)
When would maximum diffraction occur for a single slit?
when a=λ as sinθ =1, so θ= 90degrees i.e. the central maximum would spread across the whole screen
What would happen to single slit diffraction pattern if slit width, a, is decreased?
the width of the central maxima becomes greater, fringes becomes broader, and intensity becomes less due to the greater amount of diffraction
maxima are brighter, sharper and more widely spaced so angles are easier to measure
Why is a diffraction grating more accurate than double slits to measure the wavelength of light?
Why does a diffraction grating produce an interference pattern?
light waves diffract as they travel through the slits, narrow slits (when slit width is approximately equal to wavelength) give a wide diffraction, when the path difference between waves is an integer number of waveleng…
Equation for diffraction grating
nλ = dsinθ (where n is an integer and d is the slit width)
Derivation of dsinθ = nλ,
path difference between light (travelling in same direction) from adjacent slits is dsinθ (where d is the slit separation) and constructive interference occurs when the path difference is nλ (where n is an integer equal to the order number)
Formula for the maximum number of orders
n= d/λ (since max angle of diffraction is 90degrees and sin90=1)
d= 1/N
Formula to give slit separation, d, from number of slits per metre, N
Why may some orders not be visible?
A diffraction grating produces an interference pattern with a single slit diffraction pattern superimposed upon it. For a single slit the position of the first minimum is given by sinθ…
orders would become more spread out
What would happen to the appearance of orders if the slit separation, d, was made narrower?
orders would become less spread out
What would happen to the appearance of orders if wavelength of incident light decreases?
Applications of diffraction grating
spectral analysis of light from the stars - missing wavelengths in spectrum show which wavelengths have been absorbed by gases in the star, so the gases present in the star's atmosphere can be identified
Appearance of orders using white light through a diffraction grating
central white zero order maxima, subsequent orders are composed of a spectrum of colours, with wavelength increasing further away from the central maxima i.e violet light closest, red light furthest away
Artificial Light
made when matter gives off energy in the form of light rays (1.by heating something up, 2)by running an electric current through a solid)
Electromagnetic Spectrum
a group of different types of waves (radio waves, microwaves, infrared waves, visible light, ultraviolet light, x-rays, gamma rays)
reflection
a wave strikes a boundary and bounces back
refraction
a wave is bent due to a change in speed
prism
a piece of glass that separates white light into ROY G BIV
visible
can be seen
Depth
The ability to see distance (or how far an object is from us)
retina
innermost layer that perceives and transmits light to the optic nerve
Lenses
bends light to focus
300 million meters/second
How fast can light travel?
light
Example of an electromagnetic wave
Diffraction
the bending and spreading of waves, usually around an obstacle
Density
mmP/RT
Angle of Refraction
The amount the light bends