What role does bulk of a prism play in dispersion?

Question

I know that a prism deviates light because the refractive index of glass is a function of the wavelength of light used. However what role does bulk of the prism play in dispersion? i.e only the glass layer prism is there with air inside. So what would happen to dispersion effects if one were to remove the bulk

My thinking:

The dispersion effects would still be there but not so prominent. I think it has something to do with optical path length

edit: I have read this answer Can a hollow prism disperse light when immersed in water? But I don't think this answer is quantitative enough

Answer 1

Let's do some ray tracing. Here's an example of rays passing through a filled prism (blue lines denote its surface; the darker rays are reflections):

And here's the case when the prism is made of two glass slabs each with parallel surfaces:

See the animation below where the drawings alternate to compare them. Notice how the initially refracted rays are identical, while the hollow prism makes extra refraction on the surface where the first glass slab ends. Now this extra-refracted ray is actually parallel to the incident ray, and continues in such a form until it crosses the surface of the second glass slab, where it refracts—only to refract back on exit.

The end result is that the only effect you get from such a hollow prism is shifting the light ray and getting a very small, barely visible, dispersion.

So the role of the bulk is, as mmesser314 said, to keep light travelling in a straight line, i.e. to avoid additional refractions inside the prism.

Answer 2

The surface of a prism bends light because of a sudden change in index of refraction. Dispersion separates colors, as you noted. The index is different for each color, so the angle of bending is different.

The bulk is uniform. Once inside, each color has no further bending. The glass is the same everywhere. So each color has its own index, but that index doesn't change as the color advances. Refraction is bending of light due to changes in index of refraction. So the role of the bulk is to keep light traveling in a straight line.

The other factor in a prism is the triangular shape. For a slab with parallel sides, all the colors are unbent by the same angle as they were bent. They all become parallel. They are combined back into white light, or perhaps a narrow band of colored light. The triangular shape changes the angle of unbending so they continue to spread out.