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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