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How to Perform Young's Double Slit Experiment
Last Updated: August 10, 2021
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Thomas Young's double slit experiment was extremely important in the area of wave theory . His experiment proves that light exhibits wavelike properties. Monochromatic light, light consisting of one color, is split using two slits placed close together. Two coherent light waves emerge on the other side of the slits. Coherent light, meaning the waves have the same frequency and phase , will both constructively and destructively interact. This interaction causes light and dark fringes based on simple geometry. If this pattern is in fact the result of the experiment, light is proven to have wavelike properties.
The light and dark fringes are caused by the difference in phase of the light when it is incident on an object (i.e. light sensor). The light from the bottom slit has farther to travel and therefore a phase difference exists between the two rays of light. This interaction between these two waves creates a light, dark, or intermediate fringe.
An area where a light fringe occurs, the phase difference between the two coherent light waves is zero degrees. For example, there is no phase difference and the waves add together to create a fringe with maximum intensity. An area where a dark fringe occurs, the phase difference between the two coherent light waves is equal to 180 degrees. At this point, the waves add together and completely cancel each other out. Intermediate fringes occur when the phase difference is somewhere between zero and 180 degrees. In general, intensity of the fringe decreases as the phase difference between the two light waves increases from zero to 180 degrees.
- Begin with the optics bench in front of you with the 0 centimeter (0.0 in) mark on your left.
- Install the diode laser at the 0 centimeter (0.0 in) mark facing towards the right. This should require little effort.
- Mount the multi-slit apparatus to its appropriate mount. This may require some force and will make a clicking sound when properly installed. Assembly should prove to be rather simple.
- Install the newly assembled multi-slit apparatus on the optics bench between the 5 and 10 cm mark. Adjustment will be made later in the experiment to both position and slit settings.
- Remove one of the screws from the linear translator that secures the grooved bar and pull it out away from frame of the linear translator.
- Install the rotary motion sensor onto the grooved bar. This can be achieved by sliding the grooved bar into the square hole in the center of the rotary motion sensor. One important note is that the set of mounted pulleys on the rotary motion sensor should be facing upwards when installed properly.
- Replace the grooved bar to its original position and reinsert the screw.
- You should notice that one end of the connecting rod is slightly smaller in diameter and is threaded. In one hand hold the connecting rod with the threaded end up. Place the aperture mount on the rod followed by the light sensor making sure that the aperture of the light sensor is firmly against the rotating disk consisting of varying sized apertures.
- Simply screw in the connecting rod (Hand tighten only).
- Mount this newly created piece by placing the connecting rod into the last remaining hole of the rotary motion sensor, so that the light sensor apparatus is upright.
- Tighten the retention screw on the rotary motion sensor (Hand tighten only).
- Adjust the position of the nut until it enters the highest channel and the linear translator is resting on the raised surface of the optics bench.
- Slide the linear translator down the track until the light sensor is approximately 1 meter (3.3 ft) from the multi-slit apparatus. This may require moving the multi-slit apparatus closer to the diode laser.
- Select one of the double slits from the multi-slit apparatus, at this point it doesn't matter which one, by spinning the multi-slit wheel until the appropriate slit is mostly in line with the diode laser. You will be able to feel a slight popping as each slit grouping reaches this position.
- Turn the diode laser on by connecting the power supply and flipping the switch on the back of the diode laser. Note that beside the on/off switch there are two knobs. One will adjust the diode laser in the vertical direction and the other in the horizontal direction.
- At the other end of the optics bench adjust the aperture setting of the light sensor mount until slit #3 is selected.
- With the laser on, adjust both the horizontal and vertical knobs until you have created an interference and diffraction pattern (looks like a line of light) that shines through the narrow slit of the light sensor apparatus that you just adjusted, insuring that that the pattern is also perpendicular to slit #3.
- Setup your computer, including a graph on Data Studio plotting intensity vs. distance.
- Select a set of appropriate double slits from the multi-slit apparatus and spin them into position. One thing to keep in mind when choosing is that the smaller the slit size the harder it will be to detect maximums of higher m values.
- Turn off the lights and adjust the computer screen in order to reduce ambient light.
- Turn on the diode laser.
- Move the rotary motion sensor to one side of the linear translator.
- Click 'Start' on Data Studio to begin collecting data.
- Slowly and smoothly move the rotary motion sensor to the other side of the linear translator.
- Click 'Stop' to finish collecting data.
- Slit Distance = d (Values may be found on the multi-slit apparatus)
- Slit Size = a (Values may be found on the multi-slit apparatus)
- Distance from multi-slit apparatus to the light sensor = l
- Distances from central maximum to other maximums (see graph) = x
- Wavelength = λ
Expert Q&A
- Move the rotary motion sensor at a slow and constant speed. Thanks Helpful 0 Not Helpful 0
- Make the room as dark as possible. This can not be stressed enough and also includes the computer screen. Thanks Helpful 0 Not Helpful 1
- Start with larger double slit settings and work your way down. Thanks Helpful 0 Not Helpful 1
- Do not look directly into the diode laser. It causes permanent damage. Thanks Helpful 4 Not Helpful 1
Things You'll Need
- Optics bench
- diode laser & power cord
- light sensor
- light sensor aperture mounting apparatus with connecting rod
- linear translator
- multi-slit apparatus & apparatus mount
- Data Studios
- rotary motion sensor
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