Double Slit Experiment

The double slit, dual slit or two slit experiment shows that light and matter display the properties of both waves as well as particles. It also testifies the probability of the phenomena of quantum mechanics. Originally performed by the English scientist Thomas Young in 1801, to demonstrate the wave characteristics of light, it is largely used in modern times to explain important optical phenomena.

Double Slit Experiment
Double Slit Experiment

Thomas Young’s Double-Slit Experiment

Young presented a paper to the Royal Society in 1799, according to which, light exhibits wave motion. It was a contradiction to the prevalent corpuscular theory of light supported by Newton and other famous scientists like Laplace and Biot. It propounds that a luminous body emits light in the form of tiny particles. Nonetheless, Young took his ideas forward by devising an experiment which later became a milestone in the study of Optics.

In this famous experiment, Young, in his quest for two coherent sources of light, came up with something unique. Light from coherent or correlated sources have a fixed phase difference and the same frequency.

He used sunlight entering a room through a pinhole in a window shutter as his light source. A mirror directed the beam horizontally across the room. He placed a small paper card in the path of the light to split it into two parts which were then allowed to be projected on a screen placed some distance away.

Alternate dark and light bands were produced on the screen when viewed through a telescope. Better known as interference fringes, they are formed due to the interference of light.

The Double Slit Experiment Interference Pattern Explanation

The explanation of the experiment requires the knowledge of interference. In Physics, interference is the superposition of two waves to form a resultant wave of either higher or lower amplitude. The said waves can be those of light, sound, water or any matter. Whatever the source, the two waves need to be coherent.

The Double Slit Wave Experiment
The Double Slit Wave Experiment

The wave property of light causes the light rays passing through the dual slits to interfere with each other. Any waveform consists of crests and troughs. Constructive interference occurs when there is an overlapping of two crests or two troughs. This gives rise to an amplification of light waves resulting in the bright bands on the screen. On the other hand, when two light waves interfere crest-to-trough, destructive interference takes place forming the dark areas.

Had light rays behaved as particles, interference would not have been possible.

Double Slit Experiment Theory and Equations

Double Slit Experiment Diagram
Double Slit Experiment Diagram

Refer to the figure above. Here d is the distance separating the two slits and L denotes the distance between the slits and the screen. If d is much smaller than L, then the difference in the paths travelled by the two rays, r1-r2, before striking the screen is given approximately as,


Θ is the angle that the rays make relative to a perpendicular straight line joining the screen to the slits.

If the rays are in phase while passing through the slits, then the condition for constructive interference is given by the equation,


where m=±1,±2,±3,……

On the other hand, if the rays are out of phase with each other, then the below equation gives the condition for destructive interference.


where m=±1,±2,±3,……

Let y be the distance between an interference fringe and the point on the screen opposite to the middle point of the line joining the slits. If y is much smaller than L, then below approximate formula can be used.


Hence, the equations specifying the y coordinates of the bright and dark spots are given respectively by,



The distance between consecutive dark spots is given by,


The above equation gives a method of indirectly measuring the wavelength of the light used, for known values of L, d and Δy.

Dual Slit Experiment Video

Electron Double Slit Experiment

Though Young’s experiment demonstrated only the wave nature of light, any matter shows a similar interference pattern on passing through a double slit. Feynman outlined a thought experiment in which he asked the readers of volume three of The Feynman Lectures of Physics (published in1965) to bombard electrons through a pair of slits onto a screen. On doing so, a diffraction pattern will be observed on the screen.

Double Slit Diffraction

Diffraction is a process by which a system of waves is spread out on passing through a narrow aperture or across an edge. It is usually accompanied by the interference of waves according to the Huygens-Fresnel Principle.

Single Slit Experiment

Now, if a single slit is used instead, then the diffraction pattern would not appear. Only a single bright band, an image of the single slit, will be visible on the screen. Comparing it with the double slit experiment observation proves the wave nature of electrons.

Scientists already knew of this behavior due to the experiments of Claus Jönsson in1961. But while Jönsson came to the conclusion that an electron beam can behave like a wave, Feynman propounded that even if it is a single electron it will behave similarly. His hypothesis was proved to be correct by an experiment by Italian Physicist Giulio Pozzi and his associates in 1974.

Single Electron Theory of Two Slit Experiment

On passing through a double slit, a single photon or electron interferes with itself. Though this may sound strange as it is impossible for an electron to know the presence of both the slits and pass through them simultaneously, your doubts will be cleared if you consider the concepts of quantum physics. The position of an electron cannot be determined until it is observed. There is always a certain amount of fuzziness associated with it. This uncertainty of finding any subatomic particle in a certain position allows a single electron to interfere with its own wave.

The Observer Effect in Double Slit Experiment

The particulate nature of electrons or any subatomic particle for that matter cannot be ruled out completely. One electron does form a visible dot on a CRT screen testifying their existence as particles. Hence, there is a certain dual nature of this type of matter. At times, they act like particles, at times as a wave. It all depends on which type of instrument you are using to measure or observe it. That is why the double slit experiment is said to exhibit an observer effect. This effect refers to the changes that an observation will create in a phenomenon. It mainly occurs due to an alteration of the state of the measured physical quantity by the instruments themselves.

The double slit experiment today is performed with a laser beam as a light source to increase the accuracy of the experiment. If done the right way, the wave-particle duality of light and matter is perhaps best demonstrated by this classic experiment.



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