Question

If you were shown three different interference patterns (double slit, single slit, and diffraction grating), how could you tell the difference between them?

Newton thought that light wasn't a wave because he couldn't see diffraction or interference. Imagine you are transported back in time to convince him otherwise. Write a 1-2 paragraph explanation for Newton explaining why he does not see these features despite the wave nature of light.

Answer 1

differences are that grating pattern has more intense bright bands, and the bright bands are narrower and more sharply peaked. While the double slit have bright bands with less intensity and wider peaks at the bright bands. This is because for diffraction grating at angles between maxima, the waves from each slit differ in phase, and interfere to produce relatively wide dark areas on the screen; the overall result is a pattern of extremely narrow maxima.

The light intensities of the double-slit bands did not stay constant, instead one saw as the wave diffracted further away from the center, the light intensities decreased, this is because in real life the width of the slits causes each slit to act like a single slit, and the non uniform single-slit pattern combines with the ideal double-slit pattern, therefore dimming occurs as it moves away from the center. The double-slit intensities will stay constant if the width of the slits is very small.

In the single slit this creates a single bright band since the magnitude of the wave decreases as it moves farther from the slit.However, in a double slit experiment, light passes through both slits and the resulting waves interfere with one another causing light and dark bands to appear.

single slit diffraction also works to demonstrate that light is a wave, it is just a little trickier to analyze mathematically because it requires doing an integral, while double slit is trivial, so Young focuses on double slit. Newton had a weird pulsating-particle model for light which could reproduce interference fringes, by assuming that light pulsates regularly between different states, and only some of the states could get through a material. Newton wanted this to be true because he believed matter was particulate, and that the particles interact through the third law, explaining the conservation laws. He couldn't bear the idea that this scheme only worked for matter, and not light, so he tried to shoehorn light into a particle model.

It was Thomas Young who showed the interference pattern produced by light passing through two pinholes that was the almost the end of the particle theory.

Later, the measurement of the speed of light in water which showed it to be lower than the speed in air was the end for Newtons particle theory. Newton’s explanation of refraction involved forces of attraction on the light corpuscles causing them to accelerate towards a material, speed up and the path bend.

Newton was well aware that he had no proof that light actually travelled faster in water than in air.