Cornell Grating Slide

Figure 1

This is a slide with a variety of single and double slits of various widths and spacings. Also in the middle of the slide there are several diffraction gratings with different number of slits per millimeter. Use this with a laser and project the diffraction pattern onto a screen.

Figure 2: Specifications for Cornell grating components.

Materials:

  1. Cornell grating and stand [Cabinet E3]
  2. 5mW laser [Cabinet H1]
  3. Rod, base, and clamp

Demo:

The Cornell grating provides variable width single slits, double slits, and diffraction gratings with specifications shown in figure 2.

Explanation:

1) Single Slit Interference

A beam of monochromatic light (single wavelength) is diffracted when it passes through a slit with a width comparable to the wavelength of the light. Once diffracted, the light will spread out and interfere with itself, creating a new wave front with maximums and minimums in intensity. This appears as a line of bright spots when project onto a screen (figure 3).

Figure 3: Single slit interference pattern.

When calculating the location of dark spots, the following condition is used

Figure 4: Image from, labman.

w\sin{\theta} = m\lambda

where

w = slit width

\theta = angle to min

m = order number

\lambda = wavelength

For more on the nature of single slit interference, see Thin Slit Interference.

2) Double Slit Interference

Here two slits are used as a way to combine two single slit interference patterns. This results in a unique hybrid pattern with dark and bright spots following intensities defined by a single slit interference pattern (figure 4).

Figure 5: Double slit interference patterns lies within the curve for a single slit pattern. Image from, Hyper Physics.

The condition for maximums is w\sin{\theta} = m\lambda, with w being the distance between the center of both slits.

3) Diffraction Grating

Figure 6: A Helium Neon laser is diffracted by a grating. The first and second order bright spots can be seen on either side of the direct beam. Image from, Hyper Physics.

In a diffraction grating, many equally spaced parallel lines are used to diffract incoming light. The condition for maximums is the same as that for double slit interference. Because of this and the much smaller spacing between adjacent slits, the locations of maximums in a grating will be much greater than the double slit case.

 

Written by: Alek Beck