Astronomy

# The Interferometer

Young’s Double-slit Interferometer

A genius who started reading at the young age of two, Thomas Young (1773-1829), was an English polymath who did a simple but unique experiment. He proved that light travels as a wave. In 1801, Young showed that light has wave like properties. When light is spread out through two closely placed slits, it would combine to produce a pattern of light and dark fringes on a screen. The diffracted light may travel; different distance before meeting on a screen. If the crest of one set of light waves meet another crest they coincide and brightness results, where a crest meets a trough, darkness appears.

It was eventually known that if the light waves are in phase, they will double their amplitude and show light, whereas if the waves are out of phase, they would cancel each other and no light would result. It was also shown that circular waves of light starting from two points would combine to produced dark fringes at some points on a screen.

In the above picture it can be seen that if light behaves like a particle, the two slits on the screen would just produce two dark lines. If on the other hand, light is a wave (as Thomas Young showed), it would result in circular waves. The wavefronts overlap showing a patch of light (when they are in phase) and dark fringes (when they are out of phase).

Some 80 years later, the wave theory of light inspired Albert Michelson (1852-1931) to show that a single beam of light when split into two, is reflected back along the path they traveled and can be combined.

When the beams overlap, an interface pattern of light and dark fringes resulted. What is more intriuging is that the fringes would indicate the differences in the length pf the path taken by the beams.

In 1890 Michelson combined the light from a distant object, detected by two separate apertures, to produce an interference patterns. On these basis, starlight can be gathered from two small mirrors, separately placed, and the light they gather can be combined to give interference fringes. This way the benefit of a bigger telescope, equal to the distance separating the two smaller telescopes can be had.

The principle is applied at the Keck Interferometer in Hawaii, where light gathered by two 10-m telescopes, placed 85 m apart, is combined and processed by infrared cameras. The challenge was to adjust the light paths of the separate telescope to a fraction of a wavelength of light and make the beam travel longer or shorter before combining them – a task performed by advanced optics.

The advantage of the method is that it is easier to set up smaller telescopes then one big dish, which would also reduce the effect of air turbulence. The principle is also used in the Very Large Telescope, which is rated as the world’s largest and most advanced optical telescope, set up in the Atacama desert in Chile by the European Space Agency. A giant interferometer there combines the wave from four 8.2 m and four 1.8-m telescope to from a 200-m array that is theoretically sufficient to spot an astronaut on the moon.

In other applications, the principle of interferometer can be used to null or block the light from a bright star in order to see the relatively dim planet-like object orbiting it at a distance,. It is like being able to block the glaring headlight of an oncoming car to be able to see the flicker of a cycle lamp near it!

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