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Thomas Young invented the modern double-slit experiment with his interference experiment, commonly known as Young’s double-slit interferometer. This experiment helped the wave theory of light gain widespread adoption. Young’s double-slit experiment includes two coherent light sources separated by a small distance, typically in the order of the wavelength of the spectrum of light in use.
Light Interference
In the case of two light waves moving in the same direction at the same frequency and with zero or constant phase difference, their intensity is redistributed, becoming greatest at some spots and minimum at others. This phenomenon is referred to as light Interference.
Young’s Double Slit Experiment (YDSE)
In 1801, Thomas Young demonstrated the wave theory of light for the first time with a double-slit interference experiment. Young created a pinhole in the wall called S1 and S2 and allowed sunlight to pass through it. At first, S1 and S2 are at a reasonable distance from each other on the second screen.
Thomas Young demonstrated how light rays superimpose to form the patterns of light and dark bands.
On the screen, he noticed a few bands of different colours that were bright and dark. To boost the brightness of the bands, pinholes S1 and S2 are substituted with narrow slits, and the sunlight is replaced with a monochromatic source. Finally, the interference pattern, composed of evenly-spaced bright and dark fringes, is obtained.
When conducting Young’s double-slit experiment, two coherent sources of light must be situated at a distance that is larger than the wavelength of light. Young’s double-slit experiment clarified the light wave theory.
A single light source was diffracted into two slits and employed as a coherent source in the original Young experiment. Modern experiments frequently use lasers as coherent sources.
Making Assumptions
Three assumptions are made to derive the mathematics for Young’s Double-slit Experiment.
- The light source involved is coherent, meaning that its wavelength and phase difference are constant.
- The spacing between the two slits is significantly less than the pinhole and the screen.
- The slit distance is considerable compared with the wavelength of light.
Setup for Experimentation
It allows lighting from coherent sources to travel through two slits and fall on a screen. As a result, bright and dark fringes are visible on the screen. This is due to light interference, which reflects light’s wave nature.
It is possible to describe the patterns based on constructive and destructive interference. However, this would entail the existence of waves. Later, Einstein and Plank’s work established the dual nature of light.
Description of the Experiment
As sunlight enters through the pinhole, a spherical wave forms. According to Huygen’s wave theory, these waves’ radii grow as they move away.
S1 and S2 produce spherical waves that move away from each other as they go to the second screen; thus, these waves superimpose. At the point where the wave crest (or trough) meets the wave crest (or trough) of another, the resultant amplitude (maximum intensity, as I = A2) is maximum, and at the point where the wave crest (or trough) of one wave meets the wave trough of another, the corresponding intensity is minimum. This creates a lot of dark and light bands on the screen.
Mathematical Explanations
These equations are derived using the theory of wave superposition.
The Location Of The Bright Fringes
Bright fringes occur during constructive superposition. The path difference Δz for bright spot is,
Path difference, Δz = nλ (n = 0, ±1, ±2, . . . .)
Here, n is a number.
The distance between the centre and the nth bright fringe is
xn = nλD/d. Here, D is the distance between slit and screen, d is the distance between slits and λ is the wavelength of the light used.
It is used to calculate the distance between centre and nth bright fringe.
Similarly, the distance between the centre and the (n-1)th bright fringe is
x (n-1)= (n -1)λD/d
Where n = 0, a bright fringe is seen in the screen’s centre. This area is referred to as the Central Maxima.
The Position of the Dark Fringes
Whenever there is destructive interference, the dark fringes appear. This can occur only if the following conditions for the path difference exist:
Path difference, Δz = (2n + 1) (λ/2) (n = 0, ±1, ±2, . . . .)
That is, x = (2n +1)λD/2d
The term minima refers to the dark fringes. The first minima is located immediately adjacent to the centre maxima.
The Fringe Width
The fringe width is the distance between two consecutive bright (or dark) fringes.
β = λD/d
The fringe width is dependent on the distance between the slit and the screen, whereas the angular fringe width is independent.
θ= λ/d Here θ gives the smallest possible angle between two point sources.
= β/D
Angular width doesn’t depend on the number “n”. So all fringes have the same angular width.
Thus, regardless of the interference, the overall light intensity remains constant. Energy is moved from destructive interference to constructive interference in the interference pattern. There is no energy created or destroyed. Thus, the law of energy conservation is observed in the process of light interference.
Conclusion
The term “double-slit experiment” relates to an observational study in which light is directed to diffract through slits, resulting in infringes or wave-like interference structures on the opposing screen. Electrons and other quantum particles were identified in the double-slit experiment either as particles or as probability waves. By studying Young’s double-slit experiment, one can better understand the complexity of light.
