If you have acquired some advanced level knowledge of physics, you have heard about the double slit experiment at least once. It was a famous experiment that changed thousands of years of human understanding of particles. This experiment started a revolution in particle physics and helped scientists to come to the right conclusions.
First, let's talk about the single slit experiment. A single slit is made in one wall for the single slit experiment. If light is composed of particles then the light is projected in a straight line through a slit which is projected through the slits to strike a screen. If the light is composed of particles, a pattern corresponding to the size and shape of the slit will appear on the screen. The screen pattern in the single slit experiment is a diffraction pattern where light is scattered. The smaller the size of the slit, the greater the light scattering angle.
The pattern produced by using a red laser in a single slit experiment is shown in this image. In this image the central part of the pattern is formed and if you look carefully you can see two faint side bands. You will see more bands with better and more sophisticated equipment. Diffraction patterns are believed to be caused by light waves.
In double slit experiment, two slits are made in the wall. It is theoretically believed that if particles are thrown through these slits, then the patterns of the size of the slits will be formed as shown in the figure below. And if it is a wave then different patterns will be created
In the double slit experiment, light is directed through two slits in a wall. The distance between these slits is kept approximately equal to the wavelength of the light. Now the light flows through these two slits and starts spreading. Two separate waves are generated from the two slits, which propagate upward and penetrate each other. Where the peak of the light wave is. The crests of the waves flowing from the two slits overlap and become stronger and make the trough of the wave less important where a crest meets a trough canceling each other out. So the crests of the wave combine with each other to produce a brighter light which is displayed beautifully on the screen.
You can see in the image above that a line is drawn where the crests of the waves meet. Along these lines you can see bright spots on the screen. You will not see any light traces on the screen along the area where the wave crests and troughs meet
Classical wave theory can describe most of the behavior of light waves. In the gif above you can see how the wave behaves. You can see water ripples to better understand waves. In double-slit experiments, quasi-monochromatic light from a single laser is beamed across two slits. This results in waveforms similar to the gif above where some parts have low light levels and some high light levels. The angular dispersion of light and the spot of light on the screen confirm that light behaves like a wave.
The diffraction pattern you see in this image is displayed on a double-slit experiment screen like this. If you look at the patterns, you can see that they are smaller and more decorated
As the light from the two slits again interferes, a clearer pattern with a series of alternating light and dark bands is visible on the screen. The width of the band is considered as a characteristic of the frequency of the incident light. After Thomas Young first demonstrated this phenomenon, it indicated that light consists of waves. The fact that the distribution of luminosity was caused by the alternating additive and subtractive interference of wavefronts was readily apparent from the experiments performed by Young, sufficient to prove the wave theory of light. This disproved Isaac Newton's corpuscular theory of light, which had previously been widely accepted. The later discovery of the photoelectric effect proved that light could behave as both a particle and a wave, and this was predicted by Albert Einstein.
Whether light is a particle or a wave has been debated since ancient times, and Isaac Newton clearly stated that light is composed of particles. Thomas Young did the double-slit experiment which clearly proved that it consists of waves which you have seen above.
You can also do the double slit experiment using electrons. Electron will behave like a particle and a wave just like a photon. Something travels from one place to another through a medium by waves. It is possible for an electron and all quantum particles to be both particles and waves, which puzzled scientists.
The behavior of particles in the quantum world is so strange that it may seem miraculous. Something strange seems to happen when double slit experiments are performed using electrons and making the slits very narrow.The electron behaves like a particle when it is observed and behaves like a wave when it is not observed. Isn't this strange? Or the incompleteness of quantum theory. Quantum theory is shrouded in mystery.
So Richard Feynman supposedly said
“If you think you understand quantum mechanics, you don't understand quantum mechanics.”source
In the early 19th century, Thomas Young performed the double slit experiment and clearly described the diffraction of light. This wave model was believed to be correct around 1850. In 1865, James Clerk Maxwell suggested that light could be an electromagnetic wave, which was confirmed experimentally when Heinrich Hertz discovered radio waves in 1888. In the early 20th century, Albert Einstein stated that photons are packets of energy and can travel as both particles and waves. He was awarded the Nobel Prize in 1921 for this.
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