Diffraction Gratings

Diffraction Gratings

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A diffraction grating is a fancy plate that does something cool with light. You know how white light has different colors like red, orange, yellow, and so on? A diffraction grating splits that light up so you can see all the colors separately. The easiest kind of grating has a bunch of evenly-spaced slits. Refraction gratings work by bending the light waves around the edges of the slits, which makes them overlap and create new patterns of light. Sometimes the patterns add up and get bigger, and sometimes they cancel out and get smaller. This creates a really cool design called an interference pattern, which you can see in the picture below. When you shine a beam of light through a diffraction grating, it makes a pattern of bright and dim spots.

Keywords: diffraction gratings, optical plate, white light, wavelength, refraction gratings, light beam, interference pattern, parallel beam of light.

Diffraction interference pattern

A diffraction grating pattern is an interference pattern consisting of maximums and minimums when light is diffracted. It is created when white light is incident on a parallel grating plate with several or even hundreds of evenly-spaced identical slits. The incident light is diffracted, creating spherical waves around the openings which then interfere with one another, forming a pattern of maximums and minimums. The groove pattern, or the spacing between grooves (d), determines the angles at which different orders are diffracted. The grating may also be designed to vary across the grating for different levels of diffraction across the part.

Diffraction grating pattern
Diffraction grating pattern

When a light beam hits a diffraction grating plate, it splits into seven different colors based on their wavelengths. The dots on the back screen represent the maximums, while the empty space between them is called the minimum. The maximum that is parallel to the light beam is called the zero-order maximum, while the dots on the sides are the first and second order maximums. These points are caused by the interference of many different rays of light. The angles at which the maximums occur are called fringes, and can be calculated using the grating equation. The equation shows that the separation angle is proportional to the wavelength, so the longer the wavelength, the greater the angle. The spacing between the slits, or d, affects the angle of diffraction. A larger number of slits per meter means a bigger angle of diffraction.

The white light spot is in the middle where the angle is zero, while the blue light is closest to the white spot in the first-order maximum points. Red light has the greatest angle, making it the light furthest away from the zero-order. This pattern repeats for each order point.


Diffraction grating diagram
Diffraction grating diagram

Angular separation

The grating equation can be used to calculate the angular separation between each maximum, denoted by θ1. By solving the equation for θ, we can find the angle between a specific order of maximum and the zero-order. To find the angle for a specific order of maximum, we substitute the order number, n, into the equation. For example, if we want to find the angle between the zero-order and the second-order maximum, we would substitute n = 2 into the equation. The resulting angle would be θ2. The equation is shown below:

d(sinθ - sinθ₀) = nλ

In this equation, d is the spacing between the slits, θ is the angle of diffraction, θ₀ is the angle of incidence, n is the order of maximum, and λ is the wavelength of light. By rearranging this equation, we can solve for θ:

θ = sin⁻¹ [(nλ/d) + sinθ₀]

Using this equation, we can calculate the angle between each maximum and the zero-order. This allows us to determine the position of each maximum on the screen.

The maximum angle required for orders of maxima to be created is when the beam is at a right angle to the diffraction grating. Hence θ = 90o and sin(θ) = 1.


Separation angle diagram
Separation angle diagram


Αn experiment was conducted using a diffraction grating with an opening of 1.9 μm. The wavelength of the light beam is 570 nm. Find the angle x between the two second-order lines. Use the equation solved for θ and substitute the given values. Use n = 2 as second-order maximum angle required. However, the diffraction grating equation gives the separation angle, which is the angle between the central zero maximum. But the question requires the angle between the two angles as seen in the diagram below. Hence angle θ is doubled to find angle x.

Finding the angle between two second-order lines
Finding the angle between two second-order lines

A light with a wavelength of 480 μm passes through a diffraction grating. The separation angle is 40.85° and the diffraction creates the first-order maximum. Find the opening of the slits. Solution Use the diffraction grating equation but rearrange for d. Substitute the given values.

What is the diffraction grating experiment?

The aim of the experiment is to calculate the wavelength of light.


Diffraction grating Laser beam Ruler Binder clips Tape Colour filter

To conduct the experiment, position a white light source opposite a diffraction grating with a wall behind it serving as a projection screen. Secure the light source and the diffraction grating to their respective positions using tape and binder-type clips. If necessary, position a piece of colored plastic or a color filter between the source and the diffraction grating. Direct the white light beams through the diffraction grating and observe the pattern projected on the wall. Adjust the angle between the beam of light and the glass as needed to obtain the desired diffraction grating pattern. Identify the zero-order beam and the diffracted beams based on the intensity of the spots shown on the wall. Measure the distance between the glasses and the white spot on the screen using a ruler. Repeat the experiment with several laser pointers and measure the distance between the straight unbent beam and the diffracted beams, or h, for each different light beam. Calculate the wavelength for each laser and compare it to the manufacturer's wavelength for the laser used. If desired, insert a piece of colored cellophane plastic or filter between the white light beam and the diffraction grating and record any observations.

To calculate the wavelength, we can rearrange the equation to solve for λ. Using trigonometry, we can find the angle θ, and the distance D can be used to calculate the separation angle. These observations help us to understand how the diffraction grating is affecting the light passing through it, and allows us to make accurate measurements and comparisons of the different wavelengths of light.

Experimental pattern diagram
Experimental pattern diagram

The filter or coloured plastic filters out colours from the spectrum and only allows one wavelength of light to pass through, hence only colour appears.

Errors and uncertainties

To obtain more accurate measurements of h, multiple measurements should be taken and averaged. A scale can be used to record h and minimize uncertainty. It is also recommended to conduct the experiment in a darkened room to make the fringes and measurements clearer. Additionally, using a grating with a large number of slits will result in larger magnitudes of h, which can help to minimize uncertainty. This will result in more accurate measurements and a better understanding of the behavior of the diffraction grating. By taking these steps, we can ensure that our results are as accurate and reliable as possible, and that we can draw meaningful conclusions about the properties of light and the behavior of diffraction gratings.


Diffraction gratings have a wide range of applications in various optical devices, including spectrometers, lasers, CD and DVDs, monochromators, and optical pulse compression devices. By using a diffraction grating, white light can be divided or dispersed into its component colors, each with its own unique wavelength. This results in a diffraction grating pattern consisting of maximums and minimums when light is diffracted. The angular separation between the unbent and bent light beams is known as the separation angle. Understanding the behavior of diffraction gratings and their applications is essential for the development of many optical devices and technologies, and can lead to advances in fields such as medicine, telecommunications, and astronomy.

Diffraction Gratings

How does a diffraction grating work?

By refraction of light around openings. This forces the waves to interfere with one another either constructively or destructively, creating an interference pattern. 

How do you make a spectroscope?

By using diffraction gratings which split light into its components, allowing accurate wavelength measurement of spectrum-emitting substances. 

What is diffraction grating used for? 

A diffraction grating is used for optical devices such as CDs, DVDs, monochromators, lasers, spectrometers, etc.

How does diffraction grating separate colours?

The light passes through several slits in the grating and is separated into different colours based on speed and angles of diffraction.

What is a diffraction grating?

It is an optical plate that divides white light into an interference pattern composed of all colours of the light spectrum, in a dispersed manner.

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