Rays and beams

A prism’s spectrum

Glass bends blue light more than red, because its refractive index is higher for shorter wavelengths. White light through a prism fans out into a spectrum, as Newton showed in 1666.

What you are looking at

A white lamp, drawn as many wavelength slices, passes a flint glass prism. The beams leave at slightly different angles and spread across the screen.

How the bench is set up

A collimated white lamp, an F2 flint prism, and a screen about 300 mm beyond it.

The theory

The index of glass follows the Sellmeier equation

n^2(\lambda) = 1 + \sum_i \frac{B_i\lambda^2}{\lambda^2 - C_i},

and the deviation at the angle of minimum deviation $\delta_m$ through a prism of apex $A$ satisfies

n = \frac{\sin\big((A+\delta_m)/2\big)}{\sin(A/2)}.

What to measure

Deviation against wavelength, from the glass’s Sellmeier index

Open the screen card: the colours run from red to violet.
Hover over individual beams after the prism to read their wavelengths.
Turn the prism until the deviation is smallest and use the formula to find n.

What the model leaves out

The lamp is represented by a number of wavelength samples; set more in the lamp’s settings for a smoother spectrum.

Things to try

Change the glass to BK7 and see the spectrum narrow: crown glass disperses less.
Add a second prism the other way round to recombine the colours.

References

E. Hecht (2017), Optics, 5th edition, Pearson.
I. Newton (1672), A letter containing his new theory about light and colors, Philosophical Transactions 6, 3075.

More experiments, and how the lab works, are in the laser lab’s overview.