# Use a diffraction grating to measure two wavelengths

Your job is to use a diffraction grating to measure two wavelengths:

• the wavelength of a red laser (again)
• the wavelength of a yellowish lamp

The first involves the same procedure you've used in the recent past: shine laser light through a device and measure the separation of bright or dark spots on a distant screen:

Set up an optics bench and laser as shown above. Instead of shining the laser through a wheel, shine it through a diffraction grating which sits on an optics component carrier. Place a piece of paper on the wall, and arrange things so that you can mark on a piece of paper the central bright spot and one spot to the side. The distance L will probably be smaller than you've used in the past -- perhaps 50 or 60 cm.

1. Measure the separation between the central spot and a single spot to the side.
2. Make a table of your measurements, which will be the distances L and y. Each value must have units and an estimate of uncertainty.

3. Now, compute the wavelength of the light. Just use a simple formula to find the wavelength. You must also propagate the uncertainties in your measured distances to determine the uncertainty in this wavelength.

If all went well, your value for the wavelength of laser light should be similar to, but more precise than, your earlier measurements.

#### The wavelength of a yellowish lamp

Remove the diffraction grating from the optical bench and hold it carefully in your hand, a few inches in front of one of your eyes. Look through it at the spot formed by the laser on the wall.

1. What do you see?
2. Now, hold the grating in front of your eye as you look at the lamp sitting on the desk at the front of the room.

3. What do you see now?
4. How can you use this technique to measure the wavelength of light emitted by the lamp? Draw a picture to explain.
5. Measure the wavelength of the yellowish light emitted by the lamp. Show all your measurements, including uncertainties. Derive the uncertainty in wavelength of the light.
6. What is the actual wavelength of this light? Is your value consistent with the actual value, within the uncertainty?

Hint: Sample Problem 36-6