Physics Practicals

# Waves Module 3 - Interference

Waves Module 3 - Waves Student Guide July 4, 2023, 9:40 a.m.

# Activity 1

We will use properties of light passing through the diffraction grating to measure the wavelength of light from a laser source.

#1 Set Up

1. Take two sheets of paper and fold them along the long side. Attach the two pieces to the screen, creating a wide, narrow surface, approximately 4.25” by 22”, centered at the center of the screen. This is where you will observe the pattern that light makes after passing through the diffraction grating.

1. Set the screen on the stand. For now, you can place it at one of the ends, but there is a good chance you will need to adjust its position.

1. Set the diffraction grating on the stand. For now, you can place it in the middle, but you will definitely need to adjust it.

NOTE: be careful not to touch the surface of the diffraction grating with your fingers. When holding the grating, place your thumb and index finger on the sharp edges. Touching the surface leaves the fingerprints which can affect your experiment.

1. Place the light source (laser) on the stand, so that the light source faces the diffraction grating. Turn on the laser.

NOTE: you should never look into the source of laser light so make sure that you are either behind or to the side of the light source. If you need to be in front of it, ensure that the beam is not at your eye-level.

1. Adjust the position of the light source/ grating/ screen so that you can see at least 5 dots on the screen (the more the better).

2 When light passes through a diffraction grating, an interference pattern is observed on the screen. The angular positions of the constructive interference spots, θm are described by the equation:

mλ=dsin(θm),
where

m is the order of the interference, λ is the wavelength of the interfering wave, and d is the distance between the lines on the diffraction grating.

Calculate the line separation of the diffraction grating.

3. Fill in the table based on your diffraction pattern.

4. Plot the function sin(θm). Determine the wavelength of the laser.

5. Compare your wavelength with the manufacturer (printed on laser).

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