Measuring the Width of Microscopic Strands Using Light Diffraction

In this short and simple experiment you will be able to find the width of microscopic strands (such as hair, fishing line and even a spider web) using light diffraction.

The estimated amount of time to complete this project is about 1 hour, give or take how quick you can work with tools.

To carry out this experiment you will need:

-Laser Pointer (With known wavelength)

-Meter Stick

-Tape Measure (meters)

-Strands to Measure (Hair and/or Fishing Line)

-Saw

-Wood (To build the frame)

-Wood Glue and/or Nails, Clamps

-Two Binder Clips

-Tape

Now, you'll want to construct your frame for the laser. This frame is to hold the laser still and centered while the process is carried out.

Using your saw, cut two 7 inch by 7 inch pieces and one 7 inch by 9 inch piece out of the wood. Then cut four 1 inch by 1 inch cubes out of each corner of the 7 by 7 inch pieces.

Cut 2, 1 inch by 9 inch strips and attach them as shown in the pictures above. These strips will be where the strand is stretched across.

Press the pieces together using wood glue and clamps. Nails can be added for extra strength, although, they are not needed.

A separate block is needed to hold the back end of the laser up and perfectly horizontal.

Drilling the Hole for the Laser

In the center of the 7 by 9 inch piece, drill a hole at the center point of the piece. the hole should be about 1/2 inch wide, giving the best fit for the laser pointer.

Using the binder clips and tape, attach your strand of material to the 9 inch strips of wood. Place the apparatus at a distance where the diffraction pattern seems most clear on a flat, wide surface. Tape the button of the laser down. Place the laser inside of the drilled hole and rest the other half of the laser on the separate block. Align the laser up with the strand by moving the separate block of wood. Once the two are aligned, a speckled pattern should form on the far surface (as seen in pictures).

Using your meter stick, measure either the distances between the center of the “dark spots” or measure the distances between the center of the “bright spots” that occur within the speckled pattern.

*Be cautious of the units you’re measuring in. if you use centimeters, convert it to meters and use scientific notation

You will then need to measure the distance between the strand and the pattern displayed on your screen or surface with your tape measure in meters.

Using the above equation will help solve for the width of your strand.

y = the width of the strand (what we're solving for)

m = We use the letter m to represent the ORDER of the fringe (whether you measure the dark fringes or the bright fringes) from the bright central.

Lambda (Upside down "y".. Instructables doesn't do Greek letters) = the wavelength of the laser pointer. This is why you should choose a pointer with a known wavelength

D = the distance from the screen to the strand

d = the distance between fringes (Measure Dark OR Bright spots)

*We used an order (m) of 1 for our measurements

**Be sure to convert all of your measurements to meters and use scientific notation.

In the End

Attached is a picture of our calculations. As you can see, the widths of the strands of hairs is a lot smaller than anticipated. This same experiment can be done with other small strands such as fishing line, a spider web, and even something as thin as a piece of paper. Some improvements we could make is how our laser is held in our apparatus and how the strands are held while having the laser shine on them.