light through 2 prisms.

When white light passes through a glass slab, Newton's assumption is that a slab is made of 2 glass prisms placed in opposite direction, such that dispersion of both is cancelled out and a white light is emitted back. I understood this concept well, but I am not able to understand this concept using other principles.

Paradox part of the slab.

When a light ray strikes the face 1 of the glass slab, light cannot differentiate a prism and a glass slab. The surface and the medium only matters. So violet light has to bend more than red light, whether in slab or prism. In a slab there is no air in between . So the medium is same, and using principle of rectilinear propagation of light, we can say as there is no change in medium, light must move in the same path. (In the two prisms case there was a medium change, so the argument that red and violet coincide to form a white ray seems feasable). At the face 2, due to Snelll's law, both the rays are emmitted parallel to the incident ray, so both are parallel, thus never coincident. Then how do these rays join, and we get white light in the end?


1 Answer 1


What you've missed is that the path each color traces inside the rectangular slab differs just as when white light enters a prism. When the various colors exit the slab, they all propogate in the same direction, so your eye sees them as coming from the same source location, and thus sees the "white" light again. I believe that if you start with a very narrow "pencil beam" of white light, you could block off part of the exiting light to remove desired colors

  • $\begingroup$ if the emmited rays are parallel, how do they appear to come from the same direction? $\endgroup$ Commented May 26, 2022 at 16:01
  • $\begingroup$ @VenkatSarma I can only suggest you read any introductory article on how a focussing lens works. $\endgroup$ Commented May 26, 2022 at 16:06
  • $\begingroup$ A simple spherical lens produces a separate image for each color in the incident beam (a distortion known as aberration). $\endgroup$
    – R.W. Bird
    Commented May 26, 2022 at 18:23
  • $\begingroup$ @R.W.Bird No,and no. a spherical lens made of material that has chromatic dispersion will produce a chromatic aberration. That is not a distortion. Distortion has a specific meaning in image-quality languuage. A spherical lens of zero chromatic dispersion will have no chromatic aberrations. $\endgroup$ Commented May 27, 2022 at 12:03

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