# Light: Particle or Wave?

## Does light behave like particles or waves?

Believe me or not. This question keeps most physicists baffled about the strange nature of the particles in quantum mechanics. I was 11 years old when my elder brother showed me a video of a famous experiment called the Double Slit Experiment.

That video ignited a new spark of curiosity within me. Seriously think about this, we have been to the moon and back, carried out successful missions to Mars and achieved so many advancements in science and technology. Yet, there are areas like these where we are still struggling to find the absolute answers about the very nature of the world we live in.

All right, eager to know about that video now and what it portrays. In short, it’s about an experiment conducted by Thomas Young in 1801 and is known as Young’s Double Slit Experiment.

This famous experiment is actually a simplification of a series of experiments on light conducted by Young. In a completely darkened room, Young allowed a thin beam of sunlight to pass through an aperture on his window and onto two narrow, closely spaced openings (the double slit). This sunlight then cast a shadow on the wall behind the apparatus. Young found that the light diffracted as it passed through the slits and then interfered with itself creating a series of light and dark spots. Since the sunlight consists of all colours of the rainbow, these colours were also visible in the projected spots. Young concluded that light consists of waves and not particles since only waves were known to diffract and interfere in exactly the manner that light did in his experiment.

Let Dr Quantum himself explain Young’s idea in this experiment. Watch this amazing video for an explanation with a twist at the end.

### Mathematically, we can devise equations for the positions of these interference(s):

The positions of the bright and dark fringes are,
When there is constructive interference (Bright Fringes), then
$$δ = dsinθ = mλ, m = 0, ±1, ± 2, ± 3, ...$$

$Y_{d} = (m)\frac{λL}{d}$
and

When there is destructive interference (Dark Fringes), then
$$δ = dsinθ = (m + \frac{1}{2})λ, m = 0, ±1, ± 2, ± 3, ...$$

$Y_{d} = (m + \frac{1}{2})\frac{λL}{d}$

Source: MIT course notes
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This debate has raged for generations amongst the giants of the physics community regarding the nature of light, namely whether it is a particle or an electromagnetic wave. For centuries, this mysterious and elusive phenomenon left scientists baffled because, with each experiment conducted to define its nature, it seemed to change the way it behaved.

I hope you liked the post and will continue to ponder about the nature of the light around us.