Diffraction

What is diffraction?

#The spreading out of waves after they pass through a narrow gap or around an obstruction

What is a diffraction pattern of light

#a series of light and dark fringes on a screen

What are bright fringes?

#Areas of maximum intensity, produced by the constructive interference of each part of the wavefront as it passes through the slit

What are dark fringes?

#Areas of zero or minimum intensity, produced by the destructive interference of each part of the wavefront as it passes through the slit

What are the features of the central maximum of a diffraction pattern?

#

• much wider and brighter than the other bright fringes

• much wider than that of the double-slit diffraction pattern

What happens as the order increases?

#

• On either side of the wide central maxima are much narrower and less bright maxima

  • These get dimmer as the order increases

What are the features of the single slit intensity pattern?

#

• The central bright fringe has the greatest intensity of any fringe and is called the central maximum

• The dark fringes are regions with zero intensity

• Moving away from the central maxima either side, the intensity of each bright fringe gets less

What does a diffracted source of white light produce?

#A source of white light diffracted through a single slit will produce a diffraction pattern which is different to that produced by a double slit (or a diffraction grating)

Why is the central maximum bright white?

• The central maximum is bright white because constructive interference from all the colours happens here:

  • Much wider and brighter than the other bright fringes

  • Much wider than that of the double-slit diffraction pattern

What are all the other mxima composed of?

#A spectrum

What diffraction patterns can be observed from different wavelengths of light during white light diffraction?

• #

• The shortest wavelength (violet / blue) would appear nearest to the central maximum because it is diffracted less

• The longest wavelength (red) would appear furthest from the central maximum because it is diffracted more

How do colours appear in diffraction of white light?

#blurry

What happens as the gap of the single slit increases compared to the wavelength?

#The waves spread out less after they pass through the gap

What happens when the wavelength of light passing through the gap increases and what does this cause?

#the wave diffracts more and this causes the angle of diffraction of the waves to increase as they pass through the slit

What happens if laser is used during single slit diffraction?

#More diffraction as waves pass through single slit

Fringes would be therefore wider

What haapens when a slit is made narrower?

• #The angle of diffraction is greater

• So, the waves spread out more beyond the slit

What is a diffraction grating?

#a piece of optical equipment that also creates a diffraction pattern when it diffract

Why are diffraction gratings useful?

#They create a sharper pattern than a double slit

What does use of diffraction grating mean for fringes?

#This means their bright fringes are narrower and brighter while their dark regions are wider and darker

What is the diffraction grating?

#n(lamda) = dsin0

n = order of the maxima

d = distance between slits and the grating

0 = angle of diffraction

lambda = wavelength

How do you work out slit spacing?

#d = 1/N where:

d = slit spacing

N = Number of lines per mm

When can the maximum angle of diffraction be seen?

#The maximum angle of diffraction with which maxima can be seen is when the beam is at right angles to the diffraction grating

• This means θ = 90^o and sin θ = 1

What is the equation for the maximum order of maxima visible?

#n = d/lambda

n = order of mxima

d = slit spacing

lambda = wavelength

Add examples of diffraction grating usages