Essay Newton

Introduction

While reading Newton's Philosophy of Nature, I was particularly struck by the essay "The New Theory About Light and Colors" on p. 68.

I procured me a triangular glass prism, to try therewith the celebrated phenomena of colors.

- p. 68

Summary

A brief summary of the essay:

• Initial experiments
• Quantitative approach
• Experimentum Crucis
• Aside on telescopes
• Deductions
• Further experiments

Initial experiments with the prism:

• Newton procured himself a prism
• He procured a dark room, made a small hole for light to come through, placed prism in front
• Observed that the projection on the opposite wall was oblong, not circular; L = 5 W
• Tested effect of variables: thickness of prism, brightness of light, prism inside versus outside the window
• Observed: these had no effect on the shape of the projection - still oblong, still L = 5 W
• Tested whether it was defects in the prism by putting 2 prisms next to each other (second one "reverses" refraction of first)
• Observed that 1st prism turned a circular hole into an oblong shape; 2nd prism turned oblong shape into orbicircular with same regularity as if there were no prisms
• Deduced there were no imperfections in the prism glass/surface causing the oblong shape

Quantitative approach:

• Began to measure various aspects of configuration
• Distance from hole to prism, utmost length, breadth, diameter of hole, angle of rays, vertical angle of prism, angle of refraction on both sides
• Quantified difference between L and W (L = 5 W)
• Computed refractive power of glass from Law of Sines
• For 2 rays coming from opposite sides of sun's disc, determined angle they would form, verified measurements with calculation
• Still no explanation for L = 5 W
• Playing with prism angle, rotated it on its axis (obliqueness to rays of sun)
• Observed no effect
• The difference of incidence angles of rays coming from opposite sides of sun's disc, both show that W checks out - but L = 5 W does not
• Hypothesis: rays, after passing through prism, move in curved trajectories
• Analogy: tennis ball, when struck and rotating, has curvature induced by spin, higher drag on one side of the ball causing arc, if light similarly had drag it would also curve
• Observed no curvature of light, just that difference between L and the diameter of the hole was proportional to distance between them (projected shape gets larger as the hole gets further away)

Experimentum Crucis:

• 2 boards, 2 prisms:
• Window slit --> prism --> board with hole --> board with hole --> prism
• Rotating prism 1 about its axis and casting different parts of the projection onto the first board, those rays then passed through the second board and through the second prism and were reflected on the wall behind.
• Experiments focused on where on the wall the rays passing through prism 1 and prism 2 were projected
• By examining the variation of where on the opposite wall the rays of light were reflected, observed that light from the end of hte imate toward which the refraction of P1 was made, suffered considerably greater refraction passing through P2 than light tending to the other end.
• Thus: cause of length of image was that light consists of different rays, differently refrangible
• Different rays travel at different speed
• When light hits the prism, those rays of light traveling at different speeds are refracted, slowed down, bent, and sorted in order according to how much they are slowed down.
• The result is that light coming out of one end is predominantly rays of light that pass through at a slower speed (red), and light at the other end is predominantly rays of light passing through at a higher speed (violet).

Aside on telescopes:

• After this experiment, Newton realized the perfection of telescopes was limited, not by glass manufacture, but by fact that light consists of heterogeneous, differently refrangible rays
• If a glass was designed to collect any particular kind of ray at one point, all the other kinds of rays would suffer a different diffraction
• Newton then took reflections into consideration: regular, so that angle of reflection equals angle of incidence
• Light consists of heterogeneous rays, each of which is refracted differently from others
• Here, Newton lays down the doctrine and theory he deduced, then outlines future experiments.

Deductions: 1. Changes in degree of refrangability lead to changes in color 2. A change in color is equivalent to the degree of refrangibility 3. A ray's species of color and degree of refrangibility is not mutable

• Apparent transmutations of color can be achieved by mixing different light of different colors. If by refraction or other technique rays in a mixture are separated, different colors will emerge, not from mutation but as a result of returning to original form

5. Two kinds of colors: original/simple colors, and compound colors 6. Basic and compound colors can be mixed. If they are neighbor colors they form the color lying between them. If they are non-neighbor colors they do not. 7. Whiteness: random mix of all light rays with all colors jumbled together; color comes from one ray with one refraction angle dominating 8. White is usual color of light 9. Working of prism becomes clear: each ray, with different reference speed, passes into a prism, ordered according to the relative difference in refrangibility (highest to lowest, red to violet) 10. Workings of rainbows become clear: droplets refract light differently from air, creating lots of tiny rainbows. The preponderance of light rays on one end have the highest refrangibility (red), while the preponderance of light rays on the other end have the lowest refrangibility (violet). 11. Light infused in sparkling, multicolored material can also be explained - from different positions, different parts of the material will diffract and transmit different types of light; the preponderance of rays leads to rainbows, but randomness of surface breaks that up 12. Also explains Boyle experiment: wedge of red liquid, transparent; wedge of blue liquid, transparent; put them together, opaque: if one wedge transmits only light waves with red characteristic, and one wedge transmits only light waves with blue characteristic, no rays can make it through. 13. Last example: the colors of natural bodies depend on how they are qualified to reflect light of different colors in different amounts.

Finally, Newton concludes that since colors are qualities of light, it implies light has substance.

And that this [the nature of the rays the body reflects] is the entire and adequate cause of their colors is manifest because they have no power to change or alter the colors of any sort of rays in particular incident upon it.

(At this point, Newton concludes the discussion, as he has covered the realm of fact and wishes to stay clear of the realm of conjecture.)

Further experiments:

• Discussion will lead to many experiments
• Example: passing light through a prism, then through a glass lens, reverses the rays' course, and re-focuses them at a point
• Putting a piece of paper at different points near the re-focal point shows how the rays of light re-combine
• Observation: if colors are intercepted at the lens, whiteness changes into a mixture of all the other colors (also, be careful colors don't fall outside of the lens, as those will change the whiteness of final re-focused rays)