Spots, Lines and Lasers

Optional: Look at the pieces of cloth under the microscope, magnifying glass, or just hold up to the light. Draw the patterns you see in your notebook.

Predict: What type of pattern do you think you will see when you shine the laser through a piece of cloth?

Assemble the Diffraction Grating Card as instructed on the card.

Attach binder clips so that the card can stand unsupported.

Place the card several meters from the wall. Measure this distance.

Tape a piece of paper to the wall so that the pattern created by shining the laser through the cloth falls on the sheet of paper.

Shine the laser through the white cloth. You should see a pattern on the sheet of paper.

Use a pencil to record the pattern you see onto the screen. Make sure you label the paper so you know which cloth strip was used.

Repeat this for the 3 remaining pieces of cloth.

What were your results?

1. How does the pattern you saw compare to the pattern you predicted you would see?
2. What do you think is happening to form the pattern? Can you explain using the ideas of interference?
3. As the number of threads per centimeter increased, what happened to the pattern?

You can use the pattern you see to find the wavelength of your laser pointer. The bright spots correspond to the places where light waves are all going up and down together, where they are "in phase" and the dark spots are when the light waves are out of phase. By knowing when that happens, you can learn about the material that created the pattern. If you think about it, you have lots of data to work with here. The more data you have the more accurately you can find an answer. You are going to find the wavelength of your laser using all 4 pieces of cloth and then average them together to get the best answer.

Look at the pattern you copied for the first piece of cloth. Measure the distance between the bright spots and come up with an average distance.

Next you want to figure out how far apart the threads are in the cloth. The number of thread per centimeter is written on the card. From that, you need to find the distance, in centimeters between each thread.

Now you need one more piece of data, the distance from the cloth to the wall. You should have measured that while doing the experiment. Now to put it all together.

The wavelength of your laser can be found like this: Wavelength=Wd/L. So what do all the letters mean? W=the distance between the bright spots.This should be the average distance that you found in step 4. L is the distance to the wall and d is the distance between the threads. Looking at this formula, can you see what the pattern changed the way it did as the number of threads per centimeter changed?

You should now do this for the other three pieces of cloth. Now average all those wavelengths together to come up with your answer. The more data you get, the more confident you can be in your final answer. This is what all good scientists do!

All of your answers should be in meters. You might have to convert your answers to do this. Now you should convert to nanometers. There are 100 centimeters in a meter and 1,000,000,000 nanometers (nm) in a meter. That’s a lot!

1. What is the wavelength of your laser? (Circle one)
   1. 1. About 200 nm
   2. 2. About 3,000 nm
   3. 3. About 600 nm
   4. 4. About 2 nm.

Compare your answers to the answers of other groups in your class. If they are the same, high five. If they are different, look at each other’s work to see where you differ.

Based on what you just did, come up with an equation for calculating the wavelength of a laser.

Why do you think what we are learning is important?