The Right Hon. Lord Rayleigh lately delivered a lecture at the Royal
Institution upon "The Colors of Thin Plates," a term which he explained was
applied to thin films of substances, such as oily films on the surface of
water or the equally familiar soap bubble. Although the reflection of
colors from the surface of a soap bubble is probably the most noticeable,
yet the "plate" which lends itself most readily for experiment is a film of
air confined between two sheets of glass. If a ray of white light be
reflected from the surface of the film upon a screen, the so-called
Newton's rings, a series of colored concentric rings, are obtained. If,
instead of reflected light, the ray of light transmitted through the film
of air be allowed to fall upon the screen, the same phenomenon is
observable, but the effect is very considerably minimized, owing to the
great preponderance of white light, which overlies as it were the colored
rings. Even in the first instance, as the lecturer was able to show later
on, the colors are not nearly so intense as they may be obtained, owing to
some white light being reflected from the surfaces of the two sheets of
glass. With regard to the appearance of the phenomenon, it is observed that
the part which corresponds to the thinnest part of the film is considerably
darker than the rest of the spectrum; around this is a bright ring of
white, succeeded by constantly increasing concentric rings of different
colors apparently repeating themselves. Lord Rayleigh also obtained the
same results with a film of a solution of soap and glycerine, but in this
case the dark portion was observed at the top of the spectrum, the other
colors arranging themselves in order in the soap film thinned by the force
of gravitation, thus showing that the colors vary according to the
thickness of the film. Another form of the experiment called forth a
considerable amount of applause from the audience. Lord Rayleigh caused a
gentle stream of air to play obliquely upon a soap film, so that the part
struck was moved forward and the whole film rotated. Then with the
alteration of the force of the current of air, which of course regulated
the centrifugal force, alternating thicknesses of film were obtained,
causing a varying display of beautiful colors and combinations of colors.
This last experiment also tended to prove that the bands of color are not
arranged in a certain order, but vary according to the thickness of the
film, a conclusion arrived at by Brewster, who observed that if a film
reflecting certain colors be carefully inverted so as not to disturb the
gravity, the colors reflected are also inverted. Lord Rayleigh explained
the phenomenon by referring to Young's wave theory of light. He regarded
the film as having two surfaces from which light is reflected, an anterior
exterior surface and a posterior interior surface. If a ray of light be
thrown upon the film, a part of the light is reflected from the first
surface, but the greater part is transmitted, and some of this is reflected
from the second surface, passes back through the film, and is combined with
the light reflected from the first surface. If then the light reflected
from the second surface be in the same state of vibration as that reflected
from the first surface, the effect of their combination will be to increase
the amount of light reflected from the first surface, but if otherwise, the
effect will be a partial neutralization of the light reflected from the
first surface. That is to say, if the retardation of the light which is
reflected from the second surface, owing to its twice traversing the
thickness of the film, be equivalent to a wave length of the vibration of
the light, it will increase the intensity of the light reflected from the
first surface. If, however, the retardation be only equivalent to half a
wave length, the intensity of the light will be decreased. Thus, then, with
a ray of monochromatic light it will be seen that the effect of difference
in the thickness of the film will be to alter the intensity of the
reflected ray, but with a white light composed of several colors the result
will be more complicated. As each color has a different wave length in
vibration, it will be seen that each color will act independently of the
others, and a certain thickness of film which, upon the combination of the
two reflected rays, will cause one particular color to be intensified, will
at the same time cause the other colors to be more or less obscured.
Thus as the thickness of the film is altered different colors preponderate,
causing the appearance of rings or bands, according to the nature of the
experiment. The dark appearance on the screen corresponding to the thinnest
part of the film is probably due to refraction of the ray of light
reflected from the second surface, consequent in its passing from a rare
into a denser medium, and again from the denser medium into the rare, which
refraction Lord Rayleigh considers to effect a retardation equivalent to
half a wave length. Lord Rayleigh supported this theory of the formation of
Newton's rings by several interesting experiments. A beam of light was
intercepted by two of Nicol's prisms, one of which acted as a polarizer and
the other as an analyzer of the light, so that no light was able to pass
through both on to the screen. Between the two prisms a double refractive
lens was now placed, in this case a double concave lens of selenite, when
the same series of concentric rings observed with the film of air was
obtained on the screen, only much more intense, while a wedge of selenite
gave the bands of color in the same order as with the soap bubble.
But perhaps the most striking proof of the dependence of the colors upon
the thickness of the film was shown by the reflection of a beam of light
from a piece of mica composed of twenty-four very attenuated plates
overlapping each other. With each layer a marked gradation in color was
visible.
The remainder of the lecture was devoted to an explanation of the
determination of the chromatic relations of the colors of the spectrum.
Lord Rayleigh at this point made a rather startling statement that any
color can be produced by two other colors. As an example of such a
formation, a ray of white light was passed separately through a solution of
yellow chromate of potash and an alkaline litmus solution, throwing
respectively a yellow and violet-blue color upon the screen. When the ray
was made to pass through the two solutions successively, an orange-yellow
color was obtained upon the screen, which color Lord Rayleigh asserted to
be made up of red and green rays. To prove this, the ray of white light was
decomposed by means of a prism, and the decomposed rays passed through the
two solutions. The one solution was found to exclude all the yellow and
orange rays from the spectrum, while the other excluded all the blue and
violet rays, so that when the ray had passed through both solutions, only
the red and green rays were left. If, instead of allowing the decomposed
ray of light to pass through a slit, and thus obtain definite bands in the
spectrum, the ray was passed through a circular hole, the red and green
colors overlapped each other on the screen, forming by their combination
the identical orange-yellow color obtained with the primary white light. It
was then stated that if three definite positions be taken in a spectrum in
the red, green, and violet bands respectively, and these positions be
represented by the corners of an equilateral triangle (Clerk Maxwell's
triangle), it has been mathematically determined in what position within
this triangle the colors of Newton's rings would fall. Lord Rayleigh, by
means of a diagram and the selenite wedge, showed that the relations to the
three standard colors in practice were identical with the position assigned
them by theory.
In conclusion, the lecturer showed a piece of glass, the surface of which
had been decomposed, a ray of light transmitted through which showed upon
the screen patches of very pure color. These he considered to be due to the
glass consisting of a number of thin plates, some of which had been removed
by the decomposition.
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