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Slit Diffraction Experiment

SKU: E01

Slit Diffraction Experiment

1. Online Guidance

In 1800, Thomas Young argued, in a paper submitted to the Royal Society, in favor of the wave nature of light. His ideas were met with skepticism as they (ostensibly) contradicted Newton’s contemporarily accepted corpuscular theory of light. Young proceeded to submit two more papers in 1801 and 1803 on the subject of the wave nature of light, the latter of which described the proposed double slit experiment. In his demonstration to the Royal Society, Young redirected a beam of sunlight through a pinhole and placed a thin card to act as the double slit. This demonstration, in 1803, was the first undeniable evidence that light behaves like a wave. Modern recreations of Young’s experiment use LASERs as a source of coherent light, in place of the pinhole.


Objective

In this experiment, you will explore one of the most fascinating aspects of light – diffraction, as well as one of the most famous experiments in the history of physics – the double-slit experiment.


Figure 1 The variety of slits in the kit. (Top row) Single slits of varying widths. (Middle row) Double slits of varying separation. (Bottom row) From left to right: Lines, Grid, and a single barrier (negative slit).

Description

The working principle in this experiment is the ability of light to interfere with itself. The setup for this experiment is a simple one. A LASER beam is blocked by a plate with some shape of opening on it. The beam diffracts through the opening, bending, and spreading, ultimately interfering with itself. This interference pattern can then be projected onto a screen. Information about the dimension of the opening – or, conversely, the wavelength – can be determined by measuring the interference pattern.


Figure 2 A geometric approach to the double slit experiment.

The Huygens Principle tells us that each point on a wavefront can be treated as a source. Therefore, when the opening is two slits, we can make geometric calculations to determine the optical path difference between the two slits as if they were point sources. It can be shown that the path difference depends on the angle  as follows:

Where y is the height of P on the screen. This approximation is valid when the screen is very far away – that is, L>>y. If the path difference is integer multiples of the wavelength, λ, then there is constructive interference, and we observe a peak of intensity.

Then, the difference between successive peaks is given by:

This equation is central to the double slit experiment, and an analog can be proven for the case of a single slit, where the slit width, b, replaces the slit separation d.

The Fraunhofer integral is the proper way to make calculations regarding such scenarios and the expression is as follows:

Where A(x,y) is the aperture opening function and z is the distance to the screen. The rest of the parameters/variables can be seen in this accompanying figure:


Figure 3 A graphical explanation of quantities in the Fraunhofer integral.

An analytic treatment using the Fraunhofer integral yields the following expressions for the intensities.For a single slit of width b:

While for the double slit, the expression is:


Figure 4 A K-Optics setup for single slit diffraction.

2. Examples of Use

Since the separation of intensity peaks is linear with the wavelength, it can be concluded that shorter wavelengths diffract less than longer wavelengths, but most importantly that diffraction gratings can separate light into component colors!

This fact is widely used in astronomy. Many times, astronomers require knowledge of the spectral distribution of light – this is how galaxies can be distinguished from regular stars. In order to satisfy this need, many telescopes employ a diffraction grating through which starlight (or any other kind of light) is passed to be broken down to constituent colors.

Anyone can witness the same effect when looking at light reflected from a CD. The microscopic structure of the CD forms a type of grating which separates the colors of white light.


Figure 5 A diffraction grating separates sunlight into constituent colors.

3. K-Optics Kit

The K-Optics slit diffraction experiment is a low-cost solution for a central experiment in optics, allowing experiments to incorporate first-hand experience for the student. Additionally, this kit comes with nine different gratings with a variety of measurements that can be performed. The kit employs a variety of K-Optics parts, each designed carefully and purposefully to seamlessly integrate into the K-Optics ecosystem.

4. Animation

The following animation describes the setup assembly for Optic Table (1x1)

Slit Diffraction Experiment

5. List of Elements