Do you have thick or thin hair? Let’s find out using a laser to measure the width of your hair and a little knowledge about diffraction properties of light. (Since were using lasers, make sure you’re not pointing a laser at anyone, any animal, or at a reflective surface.)

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Light is also called “electromagnetic radiation”, and it can move through space as a wave, which makes it possible for light to interact in surprising ways through interference and diffraction. This is especially amazing to watch when we use a concentrated beam of light, like a laser.

If we shine a flashlight on the wall, you’ll see the flashlight doesn’t light up the wall evenly. In fact, you’ll probably see lots of light with a scattering of dark spots, showing some parts of the wall more illuminated than the rest. What happens if you shine a laser on the wall? You’ll see a single dot on the wall.

In this experiment, we used a laser to discover how interference and diffraction work. We can use diffraction to accurately measure very small objects, like the spacing between tracks on a CD, the size of bacteria, and also the thickness of human hair.

Here’s what you need:

• a strand of hair
• laser pointer
• tape
• calculator
• ruler
• paper
• clothespin

WARNING! The beam of laser pointers is so concentrated that it can cause real damage to your retina if you look into the beam either directly or by reflection from a shiny object. Do NOT shine them at others or yourself.

Download Student Worksheet & Exercises

1. Tape the hair across the open end of the laser pointer (the side where the beam emits from)
2. Measure 1 meter (3.28 feet) from the wall and put your laser right at the 1 meter mark.
3. Clip the clothespin onto the laser so that it keeps the laser on.
4. Where the mark shows up on the wall, tape a sheet of paper.
5. Mark on the sheet of paper the distance between the first two black lines on either side of the center of the beam.
6. Use your ruler to measure (in centimeters) to measure the distance between the two marks you made on the paper. Convert your number from centimeters to meters (For me, 8 cm = 0.08 meters.)
7. Read the wavelength from your laser and write it down. It will be in “nm” for nanometers. My laser was 650 nm, which means 0.000 000 650 meters.
8. Calculate the hair width by multiplying the laser wavelength by the distance to the wall (1 meter), and divide that number by the distance between the dark lines. Multiply your answer by 2 to get your final answer. Here’s the equation:

Hair width = [(Laser Wavelength) x (Distance to Wall)]  / [ (Distance between dark lines) x 0.5 ]

In the video:

• wavelength was 650 nm = 0.000 000 650 meters
• distance from the wall was 1 meter
• the distance between the dark lines was 8 cm = 0.08 m

Using a calculator, this gives a hair width of 0.000 0162 5meters, or 16.25 micrometers (or 0.000 629 921 26 inches). Now you try!

What’s Going On?

The image here shows how two different waves of light interact with each other. When a single light wave hits a wall, it shows up as a bright spot (you wouldn’t see a “wave”, because we’re talking about light).

When both waves hit the wall, if they are “in phase”, they add together (called constructive interference), and you see an even brighter spot on the wall.

If the waves are “out of phase”, then they subtract from each other (called “destructive interference”) and you’d see a dark spot. In advanced labs, like in college, you’ll learn how to create a phase shift between two waves by adding extra travel length to one of the waves along its path.

So why are there dark lines along the light line when you shine your laser on the hair in this experiment? It has to do with something called “interference”.

One kind of interference happens when light goes through a small and narrow opening, called a slit. When light travels through a single slit, it can interfere with itself. This is called diffraction.

When light travels through one of two slits, it can interfere with light traveling through the other slit, a lot like how water ripples can interfere with each other as they travel over the surface of water.

If you’re wondering where the slit is in this experiment, you’re right! There’s no narrow opening that light it traveling through. in fact, light appears to be traveling around something, doesn’t it? Light from the laser must travel around the hair to get to the wall. The way that light does this has to do with Babinet’s Principle, which relates the opposite of a slit (a small object the size of a slit) to the slit itself.

It turns out amazingly enough that when light hits a small solid object, like a piece of hair, it creates the same interference pattern as if the hair were replaced with a hole of the same size. This idea is called Babinet’s Principle.

By measuring the diffraction pattern on the wall, we can measure the width of a small object that the light had to travel around by measuring the dark lanes in the spot on the wall. In our lab, the small object is a piece of your hair!

Questions to Ask:

1. What would happen to the diffraction pattern if the hair width was smaller?
2. Using this experiment, how can you tell if the hair is round or oval?
3. If we redid these experiments with a different color laser instead of red, what changes would you have needed to make?
4. How can you modify this experiment to measure the width of a track on a CD? Does the track width change as a function of location on the CD? If so, is it larger or smaller near the outside?

Exercises 

1.  Which light source gave the most interesting results?
2. What happens when you aim a laser beam through the diffraction grating?
3. How is a CD different and the same as a diffraction grating?
4. Why does the feather work?

Click here to go to next lesson on Introduction to Lasers

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