Forgotten milestones in the history of optics. Greg Gbur Department of Physics and Optical Science UNC Charlotte. Introduction. History is important! A proper study of historical experiments can give crucial context, and understanding
Department of Physics and Optical Science
Aristotle (384-322 B.C.E.)
Euclid (c. 300 B.C.E.)
Ptolemy (90-168 C.E.)
Adapted from Bradley Steffens, Ibn al-Haytham, First Scientist (Morgan Reynolds, Greensboro, NC, 2007), as is much of the discussion of al-Haytham. A great HS-level introduction to Ibn al-Haytham, and the only popular biography I know of.
(from Iraqi 10 dinar note)
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Using geometrical optics, we can demonstrate that light passing through a small pinhole into a darkened room forms a “reversed” image of the object:
Naturalists prior to al-Haytham had observed this type of effect via, for instance, sunlight traveling through gaps in the leaves, but none apparently had studied the phenomena systematically
Ibn al-Haytham used multiple light sources to demonstrate that light followed straight line paths through the holes:
By screening one light source or another, was able to demonstrate that the “image” was inverted on passing through the hole!
Essentially the Doppler effect!
The combination of the finite speed of light and the motion of the earth leads to stellar aberration, a phenomenon in which starlight appears to come from different directions at different times of year (first observed in 1725 by James Bradley):
According to Newtonian theory of refraction, light particles refract because they speed up in matter; i.e., speed c becomes speed nc:
To reproduce Snell’s law (with n2 = n), must have:
Arago realized that light traveling at different initial speeds should be refracted at different angles:
same direction of incidence
refraction of light 1
refraction of light 2
refraction angles different!
Rays produced when high-energy electrons collide with an anticathode in a “cathode ray tube”; this one is a “Cossor tube”
Physical origin of X-rays was not immediately clear. Were they a new form of particle? A new form of wave? Or another manifestation of electromagnetic waves?
Three properties of X-rays seemed very unlike light and other E/M radiation:
Polarization would be a good indication of the electromagnetic nature of X-rays; however, ordinary methods of polarizing light do not work for X-rays: they shoot right through polarizers, and because they don’t specularly reflect Brewster’s angle doesn’t work.
Researchers knew that X-rays passing through gas scatter and produce “secondary” X-rays:
Professor Wilberforce suggested to Barkla that one could use the secondary radiation as a polarized source, and scattering the secondary radiation, produce a tertiary beam of radiation, which should have a dipole behavior:
Unfortunately, secondary radiation is weak: tertiary radiation is negligible!
Barkla realized, however, that polarized X-rays must be produced right at the anticathode:
An appropriately-collimated beam of radiation from the anticathode could be scattered from a gas, and the secondary radiation would have polarization properties!
Rotation of the bulb should result in the secondary radiation appearing in the vertical position, for horizontal rays, or the horizontal position, for vertical rays
“As the bulb was rotated round the axis of the primary beam there was, of course, no change in the intensity of primary radiation in that direction. There was, however, a considerable change in the intensity of secondary radiation in both the horizontal and vertical directions, one reaching a maximum when the other attained a minimum. By turning the bulb through a right angle the electroscope which had previously indicated a maximum of intensity indicated a minimum, and vice versa. The position of the bulb when the vertical secondary beam attained a maximum of intensity and the horizontal secondary beam a minimum was that in which the kathode stream was horizontal, the maximum and minimum being reversed when the kathode stream was vertical. By turning the bulb through another right angle, so that the kathode stream was again horizontal but in the opposite direction to that in the other horizontal position, the maximum and minimum were attained as before.”
In his 1930 text Principles of Quantum Mechanics, the brilliant scientist Paul Dirac made the following statement:
“Some time before the discovery of quantum mechanics people realized that the connexion between light waves and photons must be of a statistical character. What they did not clearly realize, however, was that the wave function gives information about the probability of one photon being in a particular place and not the probable number of photons in that place. The importance of the distinction can be made clear in the following way. Suppose we have a beam of light consisting of a large number of photons split up into two components of equal intensity. On the assumption that the intensity of a beam is connected with the probable number of photons in it, we should have half the total number of photons going into each component. If the two components are now made to interfere, we should require a photon in one component to be able to interfere with one in the other. Sometimes these two photons would have to annihilate one another and other times they would have to produce four photons. This would contradict the conservation of energy. The new theory, which connects the wave function with probabilities for one photon, gets over the difficulty by making each photon go partly into each of the two components. Each photon then interferes only with itself. Interference between two different photons never occurs.”
Classically, two quasi-monochromatic waves will stay in phase for a finite period of time; during that time, it should be possible to see an interference pattern between them:
“Two light beams from two independent ruby masers are aligned with the help of two adjustable 45º mirrors and superposed on the photocathode of an electronically gated image tube. The tube is magnetically focused and the image produced on the output fluorescent screen is photographed.”
Fringes were observed, as can be seen both in the photocathode image (left) and the microphotometer tracing on the right:
So it would seem that different photons can interfere with one another, violating Dirac’s original statement… or can they?
Recent discoveries have shifted the focus of optics from, “What is the behavior of light?” to, “How can we make light behave how we want it to?”
Pretty much any historical paper, from 1600s through the 1930s, can be found freely available on Google books and through other sources!