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What is Light Diffraction?

Light has many different and unique properties. A useful and important property of light is diffraction. Diffraction is the ability of light to spread and bend around objects. This is the reason when walking through a city with tall buildings, the light isn't blocked out completely.

How Does it Work?

Light travels in a wave formation. When the light hits an obstacle, it is able to move through openings. The smaller, the opening, the wider the light spreads out. Part of this is that the light interacts with the edge of objects. If the obstacle doesn't absorb all the light, some is diffracted.

An experiment was carried out to demonstrate the diffraction of light. A barrier with slits was placed in front of beams of light. The wider split allowed light with a lower wavelength to pass through without widening the beam. However, when light with a higher wavelength passed through a smaller slit, the light beam would widen past the slit. The slit's size and light's wavelength matter. If the wavelength of light is the same size or smaller than the opening, no diffraction occurs. If the light's wavelength is smaller than the opening, diffraction occurs.

Light - Diffraction and Interference

There are three major factors that affect how light is diffracted: wavelength (frequency), amplitude, and slit width.

Amplitude – for any diffraction to occur, the incident waves must have a higher amplitude than the slit width. If the wave is smaller than the slit width, no diffraction will occur.

Slit Width – Assuming an incident plane wave, decreasing the slit width will make diffraction more dramatic, and increase the angle at which the waves spread from the slit.

Wavelength – Decreasing wavelength, or increasing frequency has a similar effect as increasing the slit width. A lower wavelength decreases the diffracted angle.

Effects of Amplitude and Slit Width on Diffraction

Effects of Wavelength on Diffraction

Interference Patterns

Another important component to understand wave interactions are interference patterns. The following example displays the interference pattern from a double-slit, known as Fraunhofer Diffraction.

(Video) Interference, Reflection, and Diffraction

By Professor Dave Explains

When waves hit a boundary different things can happen. If the boundary is loose, the wave will hit the boundary and bounce back in the opposite direction, but with the same amplitude.

If the boundary is fixed, the wave will reflect back and invert the wave.

If the waves approaches a boundary that has a small gap in it, light will bend around the edges. This is a called a diffraction pattern. This light diffraction was used in quantum theory.

In the 1800s Thomas Young performed a paramount experiment. At the time, there was debate over the properties of light. Some chemists thought that light consists of particles. Others considered light to be a wave. Young set out to settle the debate.

Light Passing through a Slit

Let's imagine we had a light source that produced a single wavelength of visible light. We'll shine the light on a screen. But let's put a barricade between the light source and the screen. Let's say the barrier has a single rectangular slit in the center. What shape will appear on the screen? If light is a particle, we would expect a rectangular illumination. It would look exactly like the slit. But that's not what happens! In reality, the light gets spread out. You'll see multiple bands. Why? Light is a wave. So, it experiences diffraction. What's that? Diffraction occurs when waves bend around the edge of an object. The wave continues to propagate as it passes the slit. So, it fills the space behind the barrier. So far, it seems that light is a wave.

Young's Double Slit Experiment

So, we know what happens if we use one slit. But Thomas Young was a much more creative scientist. He wondered what would happen if his barricade had two slits. What would we expect to see if light is a particle? The screen should have two rectangular illuminations. What did Young observe? He saw bands of light. Between the bands were dark regions. What's happening? The light from the two slits was interfering. Interference occurs when waves combine. There are two kinds of interference: constructive and destructive. They differ by whether the waves are in-phase or out-of-phase. What does that mean? In-phase waves have peaks and troughs that align. So, their amplitudes are additive or constructive. Out-of-phase waves occur when peaks and troughs overlap. They cancel each other. So, we call it destructive interference. It caused the dark regions between the illuminated bands. Young thought he had settled the matter. Light was a wave!

Quantum Mechanics

Unfortunately for Thomas Young, Albert Einstein would upend his definitive conclusions. Einstein was attempting to explain the photoelectric effect. When specific wavelengths of light are shined on a metal, it produces a current. Electrons got ejected from the metal's surface. If light only behaved as a wave, the wavelength wouldn't matter. Instead, current would be proportional to intensity. Einstein proposed that light behaved as a particle. He applied Planck's quantum theory to the photons and electrons. The photon's energy was quantized and depended on its wavelength. Only particles with specific energies could remove an electron. Today, we would call it ionization energy. The electron in the outermost orbital absorbs enough energy to overcome its electrostatic attraction to the nucleus.

Einstein's solution prepared the way for the wave-particle duality. Light had both particle and wave properties.

Electrons, Protons, Nuclei, and Molecules

Einstein solved the photoelectric effect in 1905. Almost twenty years later, Louis de Broglie would change quantum mechanics. He combined Einstein's relativity equation with Planck's quantum theory. What was the result? All matter had a wavelength. The wave-particle duality is applicable to any object. We call it the de Broglie wavelength.

So, what happens if we do the double-slit experiment with electrons, protons, and neutrons? They all behave like the light. They experience constructive and destructive interference! You can even do it with atoms and molecules! In 2001, chemists achieve an interference pattern using the carbon allotrope buckminsterfullerene ($C_{60}$). In 2019, a team of physicists did a double-slit experiment using a 2000-atom molecule. The molecule had a molecular mass of over 25,000 amu! It still created the interference pattern.