Optics: Fresnel diffraction - Circular Apertures -Demonstrations in Lasers and Optics Video Lecture - Electronics and Communication Engineering (ECE)

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now we're ready to look at Fornell
diffraction from circular apertures the
set up is again the same as before but
let me go over it again here we have a
helium-neon laser beam from the laser
gets reflected by this mirror then gets
reflected again by this mirror into this
lens this is a short focal length lens
which is focused into a pinhole an
adjustable pinhole and the light from
the pinhole is then is a spherical wave
you can see it here on the card this is
vertical wave and then we let it hit the
screen now we're ready to look at
Fornell diffraction from apertures we
have two apertures for you and I'm going
to place the first one in the in the
beam so now if we can look at the screen
while I'm adjusting
the aperture now here we are it's a
fixed aperture it's a thousand microns
in diameter and as we can see we see the
same kind of fernell diffraction pattern
as we saw with the with the slit look we
see lots of fringes they get finer and
finer as you approach the center and
also the contrast is less and less as
you approach the center but what I'm
going to do now instead of keeping the
distance fixed what I'm going to do is
vary the distance between the aperture
and the the and the light or the
aperture from the pinhole and I would
like you to watch what what happens so
now I'm going to move the pin hole at
the aperture very close to the to the
pinhole and then I move away and then
you can see first you see lots of
fringes and as I move away you see fewer
and fewer fringes and if you look in the
centre hope you can see that is a white
spot in the centre now we have a dark
spot in the centre and then now we're
getting fewer and fewer fringes until we
have only only two or three now let me
go back towards the pinhole or towards
the lens see the increase in the number
of fringes now this is again this is
very interesting and I'm going to leave
it to you to figure out here we are very
close to the lens and then we move away
and the lens all right now what I'm
going to do I'm going to move to the
second the second aperture which is 400
microns in in diameter and here we are
let me let me again pick it up there we
are now this looks diffraction pattern
looks slightly different and again I
want you to to explain what's going on
here again now I'm going to move the
this new aperture 400 micron diameter
aperture as close as possible to the
lens you can see one ring inside and
then now you see the bright dot in the
middle then the bright dot becomes a
dark spot in the middle and now we get
the white dot in the brighter dot in the
in the middle and here it's almost
beginning to look like from half a
diffraction so here let me move towards
the lens towards the focus of the lens I
mean and see how the that changes and as
we move away that changes again
so let me hold it over here let you look
at it and I hope you'll be able to
figure all this out remember the the
diameter of this aperture is 400 microns
and in the previous one it was a
thousand thousand microns and again the
light is a is a say it was being focused
by Y lens onto a small pinhole and then
we had a diverging beam that is
impinging on on these two on these two
apertures so in summary we've seen a
variety of diffraction patterns we've
seen one dimensional front Hoffer
diffraction pattern and also we've seen
two dimensional front half a diffraction
patterns and finally we looked at
fernell diffraction associated with with
a slit or an adjustable slit and also
with with apertures and then we saw how
the diffraction pattern changes as we
move the aperture closer and closer to
the focus of the lens