# Does the order in which light comes into a rectangular prism affect its dispersion?

I read about how when light comes out of a rectangular prism, there are violet and red fringes on either side of the light spectrum. The website didn't give the specifics in this subject, so I am not sure about how it exactly works.

My "order" I mean if violet or red is on top of the light spectrum coming in.

I pictured both situation and found out no matter which order, light always comes out the same.

situation1 If purple/violet is on top:

I am assuming purple will bend more than red, so the order reverses.

situation2 if red is on top:

Purple naturally bends more, but this time, the order isn't reversed.

On each ray, the red and the violet are strictly on top of each other (together, of course, with all the other colours of the rainbow, to make white light). At the prism, they get displaced by different amounts so each red ray is displaced from the violet ray it sat on top of before the prism.

Image source

In the middle of the beam, this doesn't matter, because there are other rays (originally right next to the initial ray) which combine with each component to again make white light.

At the edges of the beam, however, this is no longer the case, so you get a slight red edge and a slight violet edge.

• What if red is to the left and violet to the right? – most venerable sir Aug 6 '15 at 15:35
• How do you mean? What, exactly, changes in the physical situation? The input (angle of incidence, type of glass slab), or the output (colours of the outgoing beam)? As you might guess, the output cannot change if the input does not change either. – Emilio Pisanty Aug 6 '15 at 15:48
• I might have a conceptual misunderstanding. I imagine light as a flat plane made of difference color. So say red is on the very left(one extreme), violet is on the very right(the other extreme, since all color lines are placed side by side). When I mean the order is changed, I mean that the plane is reversed. – most venerable sir Aug 6 '15 at 15:52
• Yes, that is a misconception. The beam of white light is made up of a lot of parallel rays, and each ray contains all the colours. – Emilio Pisanty Aug 6 '15 at 16:42
• Well if that's the case, why when I shoot a beam of light into a prism, there are only one violet and one red fringes? If you say there are many rays, wouldn't there be fringes not only on the edge but in the beam itself? – most venerable sir Aug 6 '15 at 18:21