Education

# What is Diffraction of Light and How does it Occur?

Light waves do not always travel in straight lines when passing near a barrier, instead they bend around the barrier and spread out. When a light wave travels by a corner or through a hole or slit that is literally the same size as, or even smaller than, the wavelength of that light, diffraction occurs. This is the phenomenon to which we name as diffraction of light.

### What is light?

Light is an electromagnetic radiation that can be recognized by the human eye. Electromagnetic radiation has a very wide wavelength range, ranging from gamma rays to radio waves. The wavelengths visible to humans comprise a fairly small band within that large spectrum, ranging from roughly 700 nm for red light to around 400 nm for violet light. The spectral areas adjacent to the visible band, infrared on one end and ultraviolet on the other, are also referred to as light. The speed of light in a vacuum is a fundamental physical constant, with an acknowledged value of exactly 3 x 108  meters per second.

Properties of light include interference, reflection, refraction, diffraction and scattering. Light behaves both as wave and particle. It is called dual nature of light. From the source to the object (with which light is going to interact), light behaves as wave and when light interacts with some material then it behaves like a particle. Light always travel in a straight line.

### What is diffraction of light?

The spreading out of light waves as they pass through an aperture or around objects is known as diffraction of light. It happens when the aperture or obstruction is of the same order of magnitude as the incident wave’s wavelength. The great bulk of the wave is blocked at relatively small aperture sizes. The wave goes by or through the obstruction with little diffraction for big apertures.

Diffraction of light definition: The spreading or bending of light around an obstacle is known as diffraction. Diffraction occurs with sound waves and electromagnetic radiations like light, X-rays, gamma rays, and very small moving particles like atoms, neutrons and electrons that exhibit wavelike nature. Diffraction has the effect of preventing sharp shadows from being cast. The phenomenon is caused by interference (when waves are overlaid, they may reinforce or cancel each other out) and is most noticeable when the wavelength of the radiation is comparable to the obstacle’s linear dimensions.

The degree of bending is determined by the wavelength of light’s size (wavelength) in relation to the obstacle’s size. The bending will be nearly unnoticeable if the opening is substantially greater than the wavelength of the light. However, if both are of the same size, the amount of bending is significant and visible with the naked eye.

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Diffracted light is bent by the atmospheric particles present in the atmosphere; the most frequent atmospheric particles are small water droplets seen in clouds. Bright, dark fringes or colored bands can be created by the diffracted light. The silver lining that sometimes appears around the margins of clouds or coronas around the sun or moon is an optical illusion caused by light diffraction.

### Conditions for diffraction

In order to see the diffraction of light, it is necessary that size of the obstacle and wavelength of incoming light are of same order. Otherwise diffraction cannot occur. For example, if size of obstacle is large and wavelength of light is small then there will be no diffraction.

### Silver lining in the cloud

When sunlight collides with a cloud droplet or any atmospheric particle then light waves are altered and interact in a similar way to how water waves interact. The light will appear brighter if there is constructive interference. If destructive interference occurs, the light will get darker or completely disappear. When light is bent around particles of the same order of magnitude as the wavelength of the light, this phenomenon can occur. The diffraction of sunlight by clouds, which is frequently referred to as a silver lining in the cloud, is an excellent illustration of this property of light.

Usually we observe the pastel shades of blue, pink, purple and green in clouds that are generated when light is diffracted from water droplets in the clouds. The amount of diffraction depends on the wavelength of light. If light is of grater wavelength then you will observe colors like red and blue and if wavelength is shorter then colors from other side of light spectrum will be seen. As a light wave traveling through the atmosphere encounters a droplet of water, it is first refracted at the water and air interface, then it gets reflected as it again encounters the interface. The beam that is still traveling inside the water droplet, is once again refracted as it strikes the interface for a third time. This last interaction with the interface refracts the light back into the atmosphere, but it also diffracts a portion of the light.

When the light shines behind the expanding cumulus tower, the cloud has a silver lining. The silver lining is a bright outline along the cloud’s edge caused by light diffraction by cloud droplets at the cloud’s outer border. Around denser clouds with more droplets, silver linings can be seen more easily. Diffraction of sunlight in clouds can result in a rainbow of colors. Cloud iridescence is the name for this optical phenomena. These hues can be seen at 20 degrees of the sun and are easier to see through sunglasses.

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### Difference between diffraction and scattering

The terms diffraction and scattering are often used interchangeably and are considered to be almost synonymous but actually both are different. Diffraction is a property of light due to which it bends around any obstacle that is placed in its path. But the scattering means the change in direction of light that can be in any direction. If a light wave strikes with a particle then light will change the direction of particle and it will also deflect in some other direction but in diffraction obstacle remains same, before and after interaction of light.

Scattering is very common phenomenon but diffraction only occurs if light wavelength and obstacle size are comparable. Diffraction means the deviation in the direction of a light wave at the edge of an obstacle in its path, while scattering is defined as the process in which particles are deflected as a result of collision.