Interference and Fringes: Young’s Double Slit Experiment

The Young’s Double Slit Experiment experimental setup revealed a wave-like behavior of light waves together with other wave patterns. Young's double slit experiment provides important evidence for wave-like behavior of light since it proves the wave theory of light propagation. The experiment led to the development of wave optics by revealing essential findings that contributed to the understanding of quantum mechanics' especially wave-particle duality. 

The observed interference pattern provides direct evidence for the wave nature that is essential for understanding diffraction and interference phenomena. The experimental evidence cannot be properly explained  with just a photonic (particle) model of light hence wave-based models are essential for understanding light behavior.

Explanation of Young’s Double Slit Experiment

• Light Source: A coherent light source such as laser that produces light waves of a single wavelength (monochromatic light).
• Barrier along with Two Slits: A thin barrier is used containing two very narrow parallel slits placed in the path of the light beam. The distance between neighboring slits remains within micrometres to millimetres in size.
• Screen: A screen is placed at a distance from the double slit which is used to observe light waves that emerge from the slits.

Explanation: The Young’s double slits experiment causes the coherent light to spread out when it travels through the slits. The diffracted waves from each slit interfere with each other after passing through the slits. An interference pattern appears on the screen after the interference. Alternating bands of light and dark fringes appear on the screen after interference because the light waves either reinforce with constructive interference or cancel each other with destructive interference.

What do you mean by interference?

Light interference is the phenomenon where light energy is altered due to the overlapping of light waves from two coherent sources. The interaction of these waves leads to a redistribution of energy.

What do you mean by Constructive Interference and Destructive Interference?

Constructive Interference:

Constructive interference takes place when the crest of one wave matches with the crest of another wave or when the trough of one wave matches with the trough of another wave. The wave alignment produces waves with combined amplitudes and maximum intensity levels.

Destructive Interference: 

When the crest of one wave aligns with the trough of another wave, or when the trough of one wave aligns with the crest of another, their amplitudes cancel each other out, leading to a reduced intensity.

What are coherent sources and incoherent sources?

Coherent sources: When two light sources produce continuous waves at identical frequencies or wavelengths while maintaining zero or constant phase difference between waves. A single source provides these light emissions.

Incoherent sources:  Two light sources that release waves with varying and unsteady phase differences are called incoherent sources.

Lets see an important question for NEET 2025:

Question:

In Young’s double slit experiment, the intensity at the central maximum is I₀​ The Intensity at a distance β/4  from the central maximum is (β is fringe width)

1. I0​
2. I0​/2
3. I0​/√2
4. I0​/4

The correct answer is: B

Conclusion

Young’s Double Slit Experiment is a profound demonstration of the wave-particle duality of light and a key experiment in the development of quantum mechanics. It has shaped our understanding of the nature of light and matter and continues to challenge our conceptions of reality. By showing that light and other particles can exhibit both wave-like and particle-like properties depending on the observer’s perspective, it has had a lasting impact on the field of physics.

While the experiment itself is simple, its implications are far-reaching, leading to the development of quantum mechanics and inspiring modern innovations in fields like quantum computing and telecommunications. Young’s Double Slit Experiment remains a critical touchstone for exploring the mysteries of the universe at the most fundamental level.

Frequently Asked Questions

1. What is the purpose of Young’s Double Slit Experiment?
The purpose of Young's Double Slit Experiment was to demonstrate the wave nature of light. It showed that light can produce interference patterns, a behavior characteristic of waves, rather than behaving like particles as was traditionally believed.

2. Why does the interference pattern appear even when photons are sent one at a time?
The interference pattern appears because each photon acts like both a wave and a particle. When photons pass through the slits, they interfere with themselves, creating a wave-like interference pattern even when they are sent individually.

3. What is the observer effect in the context of the double-slit experiment?
The observer effect refers to the phenomenon where the act of measuring or observing which slit the photon passes through collapses the wave-like behavior, causing the photon to behave as a particle and eliminating the interference pattern. This suggests that observation influences the behavior of quantum particles.