Prism and light dispersion lab

What the prism shows

Refraction

Light changes direction when it crosses into a material where it travels at a different speed. The refractive index n is how many times slower light travels in the material than in a vacuum. At each face, Snell’s law links the angles measured from the face’s normal (the line at right angles to the surface):

n₁ sin θ₁ = n₂ sin θ₂

The lab bends every ray twice, once where it enters and once where it leaves, using the actual face it meets and that face’s normal. When light inside the glass meets a face at more than the critical angle (sin θ_c = n₂/n₁), none of it escapes: it reflects back inside. This is total internal reflection, and the lab follows such rays for up to four reflections.

Dispersion

In glass the refractive index depends on wavelength: it is higher for violet than for red, so violet bends more. That is dispersion, and it is why a prism spreads white light into a spectrum. The real glasses use the Sellmeier formula with SCHOTT’s published coefficients (wavelength in micrometres):

n²(λ) − 1 = Σ B_i λ² / (λ² − C_i)

The teaching glass uses Cauchy’s simpler approximation n = A + B/λ², with values chosen to make the spread easy to see. It is not a real named glass.

“Crown” and “flint” are optical-glass terms for glasses with low and high dispersion. They have nothing to do with any paint brand.

Two controls

Dispersion off gives the glass one refractive index for every wavelength. The beam still bends, but all colours leave in the same direction, so no spectrum appears. Equal index gives the prism the same index as the air around it, so the beam passes straight through without bending at all.

Not the same as diffraction or interference

A prism separates colours because the refractive index changes with wavelength. Diffraction gratings separate them by interference between many narrow openings, and the colours of oil films and soap bubbles come from interference between light reflected from the top and bottom of a thin film. Those are different effects and are not modelled here.

What the model leaves out

This is educational geometric optics in two dimensions. Each ray is a line with no width. Partial reflection at each face (Fresnel losses), absorption in the glass and polarisation are not modelled, and the brightness and glow of the drawn rays are not measured intensities. The screen positions are in the bench’s own units, not millimetres. When the colours land close together, the magnified view enlarges the screen, never the angles.

White light can be modelled two ways: equal energy, where every sampled wavelength has the same power, or the CIE D65 standard daylight model. Neither is a measurement of the light where you are.