# What role does bulk of a prism play in dispersion?

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

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.

• What is this golden ray in the pictures? Commented May 9, 2020 at 11:59
• @DDD4C4U originating at the first surface? The reflection. The main ray is white simply because it's much brighter. Commented May 9, 2020 at 12:09

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.

• Wait ... you first say that the index changes for each color, then you say that there is no change in the index. The bulk can be uniform, but the index still changes with frequency. Commented May 8, 2020 at 20:52
• @garyp - Yes, Each color has its own different index. Each color bends a different amount at the surface. Each color keeps its unvarying index while in the bulk, and therefore travels in a straight line. Commented May 8, 2020 at 21:00
• "role of bulk is to keep light travelling in a straight line" can you elaborate on this Commented May 9, 2020 at 6:08
• Ok, But I recommend that you reword the phrase "there is no change in index of refraction". As you just said in your comment, the index does change from color to color. You meant the index does not change spatially. I understand what you mean now, but the original wording leaves open an incorrect interpretation. Commented May 9, 2020 at 13:42
• @DDD4C4U - I updated the answer. But Ruslan's ray tracing is perhaps more helpful. Commented May 9, 2020 at 15:34