Scientists use many sophisticated instruments
to measure the dimensions of objects that are too small to see.
But one of their simplest tools
are x-rays.
Because x-rays have wavelengths similar to the width of single atoms in a
protein molecule.
They can be used to illuminate the protein
and generate a diffraction pattern, a protein sample or deflect or bend an
x-ray beam around itself to create a projected pattern
that reveals it's inner structure and dimensions. Scientists can then calculate
the precise size
and characteristics of the protein by measuring its image.
This experiments will demonstrate
the diffraction technique,
typically used with an x-ray beam to measure tiny objects down to the
nanoscale. We use a simple laser beam to measure the diameter of a human hair.
The equipment required for Part A:
an industrial laser,
a slide holder with a single strand of human hair,
a whiteboard or a sheet of white cardboard,
a tape measure
and a ruler,
sticky tape.
The equipment required for Part B:
a laser pen,
two retort stands with boss heads and clamps, a single strand of human hair, a
whiteboard or a sheet of white cardboard, a tape measure and a ruler,
sticky tape.
The experiment is in two parts.
We use one technique,
but two different sets of equipment to project a red laser beam passed a
single strand of human hair
suspended in a slide frame.
You will see the hair diffracting the light in a scattered pattern on an adjacent
screen.
We will then use a measuring tape and a ruler
to calculate the diameter of the hair.
We start with our red laser source, using an industrial laser.
We will also need a human hair to suspend between the clamps in the slide frame,
using household sticky tape.
We've set up a whiteboard directly in front of the laser source
at least 1.2 metres
from the position of the suspended hair to receive the diffracted image.
When we're ready
we dim the lights and turn on the laser source.
Note this bright spot on the hair.
This confirms the optimum position for making our measurements.
You'll see the result as the hair scatters the laser beam around itself,
a distinct diffraction pattern projected onto the adjacent screen.
That's how we do it.
So, where do we go from here to find out the diameter of the hair?
It's really just simple physics.
First, we measure the distance from the suspended hair to the white board in
centimeters. Then we measure the distance between two adjacent bright
spots.
That is that two bright spots here,
also measured in centimeters.
Here is the center.
Once we have taken some measurements the formula allows us to calculate the
diameter.
D is the distance between two bright parts in the diffraction pattern,
L is the distance from the hair to the screen,
andLlander is the wave length of the laser beam.
All units
must be converted to metres.
Let's put some values into the formula.
If D is 2 centimeters,
and L is 120, centimeters
and Lander is 633 nanometres,
the diameter of a human hair is approximately
40 micrometers.
We could do this calculation just as effectively
using a laser pen.
We lock down the clamp holding the pen to ensure a continuous light source.
We then follow the same procedures we used with the industrial unit.
We suspend the hair in the same way, ensure the distance between the hair
and the diffraction surface
is at least 1.2 metres
and calculate the diameter of the hair.
We have just demonstrated how to measure the diameter of a human hair by using a
simple diffraction pattern.
Although our object is a human hair that is visible,
the experiment illustrates some of the principles used to measure objects on
the nanoscale.