Measurement of invisible length: optical precision measurement

Measurement of invisible length: optical precision measurement
In daily life, people have been used to measuring objects of different lengths with various tools, such as the 4.588-kilometer-long Nanjing Yangtze River Bridge, the 2.26-meter-high Yao Ming, and the 4.25-millimeter narrow-frame iPhone 6. These are the lengths that can be directly felt by the human eye, but how to measure the invisible lengths? How to define the thickness of 1% hair? As early as a century ago, these problems have been solved by a scientist named Albert Abraham Michelson.

In 1883, Michelson designed and invented the first Michelson interferometer in order to study the famous "ether" problem at that time, using the interference characteristics of light. Its principle is very simple. Divide a beam of light into two beams, and the two beams of light travel through the same distance before converging together. At this time, bright stripes can be seen in some places, that is, the intensity is the sum of the intensities of the two beams of light. However, dark stripes can also be observed in some places, that is, 1+1=0, without intensity. This creates a pattern of alternating stripes. When the propagation distance of one of the beams changes, the stripe pattern will change, When the difference between the propagation distances of these two beams of light reaches a wavelength of light, the pattern changes exactly once, allowing the wavelength of light to be used to measure the length of an object.

The wavelength of different colors of light is different, of which the wavelength of red light is about 625 nm, which is about one percent of the thickness of hair, while the wavelength of green light is about 520 nm, and the wavelength of blue light is about 470 nm. It is the beginning of optical precision machinery to measure the length of an object or the distance of its movement by using the wavelength of these light. Later, Michelson used the interferometer designed by himself to measure the international standard meter scale stored in Sevres, and determined that the meter scale in Paris is equal to the wavelength length of 1553163.5 cadmium red lines. This is the first time that mankind has obtained a permanent and indestructible length benchmark. Such accurate measurement makes Michelson famous around the world.

Michelson interferometer is widely used in various fields due to its simple structure and high accuracy. In 1920, Michelson, in cooperation with astronomer F. G. Piss, put a 20-foot interferometer behind a 100-inch reflecting telescope and made the first stellar interferometer to measure Orion α The amazing angular diameter of Betelgeuse is calculated to be about 3.86 × 108 kilometers, about 30000 Earth's size, has attracted the attention of the scientific community and the media. At the same time, it can be made into seismic geophones, which can detect oil and gas deposits using artificial seismic waves; Optical coherence tomography can be made to scan the morphology of human fundus, which can be used to detect glaucoma, cataract and other ophthalmic diseases; Even the smart phone screen in our hands can be used for flatness detection.

In 1907, Michelson won the Nobel Prize in Physics for "the invention of precision optical instruments and the spectroscopic and metrological research carried out by applying these instruments". His experiments were famous for their exquisite design and high accuracy. Even Einstein could not help praising him as "an artist in science"