Reversibility of Light Diffraction

A glass prism is taken in vacuum (refractive index = 1 exactly). A parallel beam of polychromatic light is incident on it perpendicularly (that is angle of incidence = 0). Inside the prism there will be no refraction (since angle of incidence = 0). While making exit from the prism, there will be refraction following Snell's Law, since then the light is incident obliquely. Will there be dispersion, forming spectrum? Possibilities: 1. There will be dispersion with formation of spectrum, since different colors of light have different refractive indices and will be deviated by different angles. But it is also known that all colors of light travel with equal speed in the emergent medium, vacuum. Then? 2. There will be no dispersion and no spectrum will form, since all colors of light travel with equal speed in vacuum. But, does this not violate the principle of reversibility of light, because if we send the emergent polychromatic beam back into the prism using a plane mirror placed by perpendicular incidence then there will be dispersion and spectrum inside the prism ? Then? Which one is correct?
- Ujan Chakraborty (age 15)
Kolkata,West Bengal, India

You're comment about the reversibility of the light propagation is exactly right. Since the speeds of the different colors change by different amounts between the glass and the vacuum, you get exactly the opposite effect of what you get when light travels from the vacuum to the glass. There will be  a dispersion of the spectrum.

Mike W.

(published on 10/29/2012)