Notes on the lecture “5 – Diffraction” slides
Slide #3
The Huygens-Fresnel principle holds for any type of wave, mechanical and
electromagnetic.
Slide #4
Images of water surface waves arriving on a single slit. The 3 images refer to 3
different wavelengths.
Slide #5
A pictorial representation of the H-F principle in the case of a plane wave arriving
on a single slit.
Slide #10
On the left a photo of a ripple-tank experiment: a plane water wave arrives on a
slit and a diffraction pattern is formed. A central brighter zone is easily
recognisable.
On the right there is a schematic representation of this experiment.
Slide #11
Any couple of very small portions (treatable as point) of the slit give rise to an
interference pattern, as if it was a Young double slit experiment
The number of these couples is infinite, being so the number of point of the slit ,
so an analytical treatment is needed .
Slide #12
The photo is from a real experiment (5 light spots are visible on both sides of the
brightest central spot, note the symmetry of the diffraction pattern). The brighter
spot is the first diffraction maximum, corresponding to m =0 in the constructive
interference formula of previous page.
The larger the angle theta (with respect to the normal to the slit plane) the dimmer
the light spot is.
Slide #13
The formula describes analytically the single slit diffraction (cfr. qualitative
comments in previous slide)
Note the difference between the central maximum and the lateral ones; the energy
collectable in the location of the principal maximum is about one order of
magnitude bigger with respect to the second maximum. The symmetry of the
diffraction pattern is explained by the square in the formula.
Slide #15
The photo shows the diffraction pattern in the case of three different colours
(small range of wavelength in the red, green and blue region of the visible light
spectrum); the central maximum and two lateral ones are recognisable.
In the case of white light (the complete spectrum of visible light) the diffraction
patterns of the diverse wavelength zones do superimpose and the result is a
blurred image.
Slide #16
A low-cost square slit experiment can be done with a piece of cloth with not too
closed texture
Looking toward a lamp light it is possible to see a diffraction pattern similar to the
one in the photo at left (less clear pattern because the laser light is very
collimated).