Hair Thickness

How thick do you think your hair is? Use any measurement scale or comparison that makes sense to you (example 4 papers thick).

Do you think everyone in the class has hair of the same thickness?

(If you have done the “Spots, Links, and Lasers” experiment, think about this question.) When you shine a laser on a human hair, what do you think the resulting light pattern will look like? Draw it here.

Cut out the center of a 3 in. x 5 in. index card.

Have everyone in the group donate a hair. Ask nicely as this may hurt a bit!

Tape the hair in the middle of the index card. (Fig. 1)

Attach binder clips to sides of card so that it will stand up. (Fig. 2)

Place the laser pointer on top of a book so that the laser beam will shine directly on the hair and tape it down securely. (Fig. 3)

Put this set-up 4 meters from a wall and tape the large piece of paper to the wall so that the laser beam will shine on it.

Turn off the lights in the room and turn on the laser pointer.

Make sure the laser is shining directly on the hair.

You should see a pattern of bright spots and dark spots on the wall. Draw this pattern on the paper and then turn off the laser.

Repeat this for the hairs of all members in the group. You can mount them all on the same card and just move the laser.

What were your results?

• How does the pattern compare to what you predicted?

The spacing of the bright and dark spots depends on the thickness of the obstacle the light has to go around, in this case the hair. The mathematical way of saying this is: thickness=(wavelength of laser) * (Distance from hair to wall)/ (average spacing between bright spots). So now you need to find all those values. You also need to make sure all your measurements are in meters. The distance from the hair to the wall is 4 meters

Record your calculations for each person in your group.

How thick is your hair?

How can a laser be used to measure the thickness of a hair?