He began by listing the ‘great brains’ in physics, Newton, Young and Einstein, and how the nature of light had been a puzzle even to them. He was full of admiration of the experiments that Newton carried out with primitive facilities and how experiments led to further questions. Newton’s observation of interference patterns caused him to believe light was wave-like but other experiments suggested that light was composed of particles.
Prof. Sambles demonstrated the effect of passing white light through a crystal, some of it passing straight through while some was deflected to one side. By rotating the crystal the deflected beam was seen to rotate in step around the direct beam. A wave of light might be expected to disperse while a particle stream would remain intact, even if deflected.
Keeping to the experimental theme, soap bubbles were blown, and the audience invited to observe interference pattern colours and speculate on the nature of the bubble. It was explained that because the soap molecules are asymmetric, having hydrophillic and hydrophobic ends, they form themselves into a single molecular layer when they form a bubble, which is neither liquid nor solid but a liquid crystal.
Young’s experiments were briefly described, the fringing observed when looking edge-on at a sheet of paper and the better-known ‘double slit’ experiment, which at first seems to confirm the wave-nature of light. However repeating this experiment with single electrons, (particles?), also forms the same patterns over time, a phenomenon that quantum mechanics offers an explanation for.
Einstein’s work on the photo-electric effect was touched on as he sought to explain why the effect was dependent on the frequency of the incident light.
The lecture closed with a demonstration of polarised light and how the effect could be ‘undone’ with a simple piece of adhesive tape. Not content with that further layers of tape were added and blue, orange and green colours produced.
A lively question and answer session followed ranging from “Why do mirrors invert?” (They don’t);. “What is the ‘length of a photon’” (10-9 – 10-10 seconds) and “Can the ‘speed of light’ be exceeded?” (Yes, in a crystal, but not the group velocity). Then back to the experiments with “What happens if more tape is laid down?” (Lets see!).
This was a very entertaining lecture. Prof. Sambles clearly retains his curiosity of the natural world and made a lot of effort to pass that enthusiasm on, especially to the younger members of the audience.
I am not entirely sure that the original question was answered, modern physics having some strange concepts that don’t fit in easily with our every-day experiences, but then that is perhaps inevitable. Many an ‘expert’ has observed that the more they observe the more they realise they don’t know – as the island of knowledge expands the coastline of ignorance expands also!
Notes:
1. Interference colours on the soap bubbles were probably not as bright as would have been liked. This was probably because the room lighting wasn’t ‘broad spectrum’ but made up of several spectral lines. [I did my own experiment with my pocket ‘scope!]
2. Reference was made to Newton’s book Optiks Book 3, Part 1, Q29. [Link to whole Part] [See below for Q29]
3. One could break a laptop to get polarising film or £5.99 will buy two pieces (50 x 50 mm) [Other suppliers are available]
Newton’s Question
Qu. 29. Are not the Rays of Light very small Bodies emitted from shining Substances? For such Bodies will pass through uniform Mediums in right Lines without bending into the Shadow, which is the Nature of the Rays of Light. They will also be capable of several Properties, and be able to conserve their Properties unchanged in passing through several Mediums, which is another Condition of the Rays of Light. Pellucid Substances act upon the Rays of Light at a distance in refracting, reflecting and inflecting them, and the Rays mutually agitate the Parts of those Substances at a distance for heating them; and this Action and Re-action at a distance, very much resembles an attractive Force between Bodies. If Refraction be perform'd by Attraction of the Rays, the Sines of Incidence must be to the Sines of Re <346> fraction in a given Proportion, as we shew'd in our Principles of Philosophy: And this Rule is true by Experience. The Rays of Light in going out of Glass into a Vacuum, are bent towards the Glass; and if they fall too obliquely on the Vacuum they are bent backwards into the Glass, and totally reflected; and this Reflexion cannot be ascribed to the Resistance of an absolute Vacuum, but must be caused by the Power of the Glass attracting the Rays at their going out of it into the Vacuum, and bringing them back. For if the farther Surface of the Glass be moisten'd with Water or clear Oil, or liquid and clear Honey; the Rays which would otherwise be reflected, will go into the Water, Oil, or Honey, and therefore are not reflected before they arrive at the farther Surface of the Glass, and begin to go out of it. If they go out of it into the Water, Oil or Honey, they go on, because the Attraction of the Glass is almost balanced and render'd ineffectual by the contrary Attraction of the Liquor. But if they go out of it into a Vacuum which has no Attraction to balance that of the Glass, the Attraction of the Glass either bends and refracts them, or brings them back and reflects them. And this is still more evident by laying together two Prisms of Glass, or two Object-glasses of very long Telescopes, the one plane the other a little convex, and so compressing them that they do not fully touch, nor are too far asunder. For the Light which falls upon the farther Surface of the first Glass where the Interval between the Glasses is not above the ten <347> hundred thousandth part of an Inch, will go through that Surface, and through the Air or Vacuum between the Glasses, and enter into the second Glass, as was explain'd in the first, fourth and eighth Observations of the first Part of the second Book. But if the second Glass be taken away, the Light which goes out of the second Surface of the first Glass into the Air or Vacuum, will not go on forwards, but turns back into the first Glass, and is reflected; and therefore it is drawn back by the Power of the first Glass, there being nothing else to turn it back. Nothing more is requisite for producing all the variety of Colours and degrees of Refrangibility, than that the Rays of Light be Bodies of different Sizes, the least of which may make violet the weakest and darkest of the Colours, and be more easily diverted by refracting Surfaces from the right Course; and the rest as they are bigger and bigger, may make the stronger and more lucid Colours, blue, green, yellow and red, and be more and more difficultly diverted. Nothing more is requisite for putting the Rays of Light into Fits of easy Reflexion and easy Transmission, than that they be small Bodies which by their attractive Powers, or some other Force, stir up Vibrations in what they act upon, which Vibrations being swifter than the Rays, overtake them successively, and agitate them so as by turns to increase and decrease their Velocities, and thereby put them into those Fits. And lastly, the unusual Refraction of Island Crystal looks very much as if it were perform'd by some kind of attractive vir <348> tue lodged in certain Sides both of the Rays, and of the Particles of the Crystal. For were it not for some kind of Disposition or Virtue lodged in some Sides of the Particles of the Crystal, and not in their other Sides, and which inclines and bends the Rays towards the Coast of unusual Refraction, the Rays which fall perpendicularly on the Crystal, would not be refracted towards that Coast rather than towards any other Coast, both at their Incidence and at their Emergence, so as to emerge perpendicularly by a contrary Situation of the Coast of unusual Refraction at the second Surface; the Crystal acting upon the Rays after they have pass'd through it, and are emerging into the Air; or, if you please, into a Vacuum. And since the Crystal by this Disposition or Virtue does not act upon the Rays, unless when one of their Sides of unusual Refraction looks towards that Coast, this argues a Virtue or Disposition in those Sides of the Rays, which answers to and sympathizes with that Virtue or Disposition of the Crystal, as the Poles of two Magnets answer to one another. And as Magnetism may be intended and remitted, and is found only in the Magnet and in Iron: So this Virtue of refracting the perpendicular Rays is greater in Island Crystal, less in Crystal of the Rock, and is not yet found in other Bodies. I do not say that this Virtue is magnetical: It seems to be of another kind. I only say, that what ever it be, it's difficult to conceive how the Rays of Light, unless they be Bodies, can have a permanent Virtue in two of their Sides <349> which is not in their other Sides, and this without any regard to their Position to the Space or Medium through which they pass.
What I mean in this Question by a Vacuum, and by the Attractions of the Rays of Light towards Glass or Crystal, may be understood by what was said in the 18th, 19th and 20th Questions.
