HI! Yesterday I did an experiment to find the width of my
hair using my green laser. The method is to hit the laser beam on the hair, the
light passing above the hair will be like going through a slit, and the light
passing below the hair will be like going though a slit. So in other words it
is a type of double slit experiment. By using the effect of wave-particle duality
of a photon, and the pattern of interference, we can calculate the distance
between the two slits, or in other words the width of my hair.

This is the setup of my so called lousy apparatus :P. I
have supported the laser on the bucket using a stack of 4 coins, to adjust the
beam to hit the hair. the hair is supported by a wire, which I have bended here
and there for support. The wall where the interference pattern forms is
6.72meters away from the hair.

This was
the bucket with the laser and hair on it. the light flare ahead is where the
beam hits the hair.
A close up of the hair and the beam hitting it.

So this
was the interference pattern produced 6.72 meters away from my hair. The number
of these fringes is 18, you can only see 15 in this pic, there are three more
to the right. you have to count till where you cant even see anything. The
length from the center fringe to the 18th fringe is 0.87meters, note all these
values down.

To do
this calculation there are two formulas which I had.

they are
formulas to find the distance between the two slits, which will be the width of
my hair.

the
formula is $d = \frac{n l \lambda}{x}$

and

$d=\frac{n*\lambda*\sqrt{x^2+l^2}}{x}$
which is the same as the upper formula, but it uses the distance from the hair
to the outer edge of the interference pattern. Im going to use both to find
more accurate results.

in these
formulas:

$\lambda$
= wavelength of laser in meters.= $532*10^{-9}m$

$n$ =
the number of fringes. = $18$

$l$ =
distance from hair to wall = $6.72m$

$x$ =
distance from center of pattern to farthest countable fringe = $0.87m$

plugging
these in the first formula we get:

$d =
\frac{n l \lambda}{x}$

$d =
\frac{18*6.72*532*10^{-9}}{0.87}$

$d =
0.000074m$

so using
this fromula I get 74microns.

Now
plugging in in the second formula we get:

$d =
\frac{n*\lambda*\sqrt{x^2+l^2}}{x}$

$d =
\frac{18*532*10^{-9}*\sqrt{(0.87)^2+(6.72)^2}}{0.87}$

$d =
0.0000746$

so using
this formula I get 74.6microns.

Now I
will take the average of these values I get 74.3 microns!!

For more
accuracy, the lasers intensity should be high, distance between the hair and
wall should be long, and the wavelength of the laser should be low.

violet
lasers are best!