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# Diffraction and Interference of Light

This lesson aligns with NGSS PS4.B

Introduction
Imagine a scenario in which there is a completely dark room with a solitary window. The window has a tiny hole in it. When light passes into the tiny hole, what happens to the light? We observe that through the tiny hole of the window, light enters but instead of just bright light, we get a pattern of light and dark bands. This phenomenon is known as a diffraction of light. Interference of light is the interaction of two waves to form the resultant wave that has a lower, higher, or same amplitude. In this article, we will learn about the diffraction of light, interference, and constructive and destructive interference of light waves.

Diffraction of Light
We have already learned that diffraction is the bending of light around the corner of an obstacle. The outcome of this encounter produces fridges of light, dark or colored bands. The diffraction of sunlight within clouds exhibits a multitude of colors.

Factors affect Diffraction
How much a wave diffracts depends on two factors: its wavelength and the size of the barrier or the opening. The greatest amount of diffraction occurs when the size of the barrier or opening is the same or smaller than the wavelength.

Considering the wavelength of light is very small—about 100 times smaller than the thickness of a human hair. Therefore, the light will diffract a lot only when it goes through a slit or some other opening that is very narrow. Light waves cannot diffract a lot around large obstacles, such as buildings. That’s why you can’t see what is around the corners. Still, light waves always diffract a little. You can notice light waves diffracting if you look at the edges of a shadow. This bending causes the edges of shadows to be blurry.

Interference of Light
Interference is like a meetup of two waves when they overlap. When waves meet, they can combine by constructive and destructive interference. The most commonly observed interference is light interference. The reason is that light waves are generated randomly by most of the sources. This means that light waves that emerge from a source do not have a constant amplitude, frequency, or phase.

A simple example of interference of light is seen in soap bubbles which reflect wide colors when illuminated by a light source.

Constructive Interference
Constructive interference occurs when the peaks of one light wave align with the peaks of another, or when the troughs align with the troughs. This overlap causes the waves to amplify each other, resulting in a brighter, more intense light. When waves combine by constructive interference, the resulting wave has a greater amplitude than the original one.

You can see this with light too. Imagine shining light of a certain color through two small holes onto a surface. The light on the surface will look like a series of bright and dark bands. The bright bands come through constructive interference to generate a light wave with greater amplitude.

Destructive Interference
On the other hand, destructive interference arises when the peaks of one light wave coincide with the troughs of another, or vice versa. In this case, the waves "cancel out" each other, leading to reduced brightness or darkness. This is like combining two ripples in a pond in such a way that they flatten each other out.

Imagine a scenario where two identical flashlights shine onto a wall, and their light waves meet. If the waves from one flashlight align with the opposite phases of the other flashlight's waves, they will effectively cancel each other's effects. As a result, the area where these waves meet will appear dimmer or even completely dark.

Summary
• Diffraction is defined as the bending of light around a corner of an obstacle.
• Interference is an interaction of two waves when they overlap. When waves meet, they can combine by constructive and destructive interference.
• Constructive interference occurs when the peaks of one light wave align with the peaks of another, or when the troughs align with the troughs.
• Destructive interference occurs when the peaks of one light wave coincide with the troughs of another, or vice versa. Consequently, the waves cancel out each other, leading to reduced brightness or darkness.

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