What Are Interferometers And Why Are They So Important In Physics?

In 2016, the detection of gravitational waves by LIGO ( Laser Interferometer Gravitational Waves Observatory) gained abundant highlights in news. The discovery was so big that it also won it's masterminds - the 2017 Nobel Prize in Physics. But, do you what was the key instrument behind this successful endeavor? It was an interferometer. Well, it was not the first time that an interferometer made a ground breaking discovery ! Even in 1887, it was only with the help of an interferometer that Michelson & Morley brushed aside the idea of presence of Ether. So, Interferometers are indeed an important instrument in the field of Science.

What is an interferometer?

Interferometers are a type of instruments which work by superimposing two or more sources of light to create an interference pattern, which can be measured and analyzed. Hence, they have been named 'Interfere-meter' aptly. The first ever Interferometer was developed by Albert Michelson in 1880s. Though the interferometer have evolved a lot with time, still the basic idea of all interferometers derive from the Michelson interferometer.

Also Read: All Articles of Basics of Astrophysics series so far

A Basic Interferometer Design

A Michelson interferometer consists of a beamsplitter (half-silvered mirror) and two mirrors. The beam splitter is partially reflecting. When the light goes through the beam splitter, it is split into two beams with different optical paths (one going to mirror 1 and the other going to mirror 2). After being reflected back at the mirrors these beams recombine again at the beam splitter before arriving at the detector. The path difference between the two beams causes a phase difference which creates an interference fringe pattern. This interference pattern is then analysed by the detector to study the wave characteristics, material properties or the displacement of the mirrors, depending upon what measurement the interferometer was being used for.

Different Types Of Interferometers :

The Michelson interferometer- Named after  Albert Michelson, It is best known for the part it played in the famous Michelson-Morley experiment conducted in 1881. With the help of this interferometer, Michelson and his colleague Edward Morley disproved the existence of a mysterious invisible fluid called "the ether" that physicists used to believe as a component that fills the visibly empty space. The Michelson-Morley experiment was an important stepping-stone toward Albert Einstein's theory of relativity.

The Fabry-Perot interferometer - This interferometer was invented in 1897 by Charles Fabry and Alfred Perot. Also known as an etalon, it evolved from the Michelson interferometer. It makes clearer and sharper fringes that are easier to see and measure.

The Fizeau interferometer - Named after French physicist Hippolyte Fizeau, it is another variation of Michelson interferometer and is generally easier to use than a Fabry-Perot. It is widely used for making optical and engineering measurements.

The Mach-Zehnder interferometer-This interferometer was invented by Ludwig Mach and Ludwig Zehnder. Contrary to above mentioned interferometers, it uses two beam splitters instead of one and produces two output beams, which can be analyzed separately. It is widely used in the fields fluid dynamics and aerodynamics.

The Sagnac interferometer-  Named after a French physicist Georges Sagnac, this interferometer splits light into two beams that travel in opposite directions around a closed loop or ring. Hence, it is also known as the ring interferometer. It's widely used in navigational equipment, such as ring-laser gyroscopes which are the optical versions of gyroscopes that use laser beams instead of spinning wheels.

The Twyman-Green interferometer- This interferometer was developed by Frank Twyman and Arthur Green in 1916. It is basically a modified version of Michelson interferometer and is mainly used for testing optical devices.

Where Are They Used?

Interferometers are often used to make very small measurements that are not achievable in any other way. Development of lasers has come like an icing on cake in this case. The invention of lasers has enabled interferometers to make the smallest conceivable measurements, like those required by LIGO. LIGO's interferometers are designed to measure a distance 1/10,000th the width of a proton! Optical interferometers can be used as spectrometers for determining wavelengths of light and for studying fine details in the lines of a spectrum with great precision. Radio interferometers are used in astronomy for mapping celestial sources of radio waves. Acoustic, or sound, interferometers are used for measuring the speed and absorption of sound waves in liquids and gases.

Also Read: What are gravitational waves?

Hence, it's definitely not wrong to say that interferometers have really played a great role in modifying the phase of scientific research. And, I believe that will definitely be very useful in unraveling more significant scientific knowledge in near future !

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