Objectives

1. Measure the location of bright fringes in the pattern produced by shining a laser at a screen with two closely spaced slits.
2. Calculate the separation between the slits based on the fringe locations.

Resources

• Optical bench
• Green diode laser
• mounted multiple-slit diffraction mask
• mounted screen
• ruler
• camera or phone

Background

When waves of any type pass through each other, their amplitudes add together at points where they overlap. They can cancel each other out or enhance each other at different points, depending on their relative phase and amplitude at a given position. Shining laser light at a pair of narrow slits separated by a distance comparable to the wavelength of the light allows the interference of light to be easily observed. The light from each slit acts as two separate sources of monochromatic light that can interfere with each other and produce regular patterns.

If light of wavelength \lambda is shone at a pair of slits separated by a distance d, the light will produce bright fringes at locations given by d\sin\theta=m\lambda where m=0,1,2,\cdots denotes the m^{th} dark fringe of the interference pattern.

Setup

Set up the laser, slit mask, and screen on the optical bench as shown in the image above. Move the slit mask and screen as far apart as possible and record the distance. Adjust the slit mask so that the laser is hitting the pair of slits with a separation (d) of 0.25mm and width (a) of 0.04mm.

Suggested Method

Take a picture of the interference pattern on the screen and open it in an image editing program such as Microsoft Paint that will give you x,y pixel positions in the image. Measure the actual distance between the two most widely-spaced fringes you can see in the interference pattern, and compare this to their pixel spacing in the image to compute a pixels-to-meters scale. Use this scale to accurately measure the distance of as many bright fringes as you can from the brightest central fringe in the image.