Laboratory work: finding the wavelength of light with a diffraction grating
The aim is to measure the wavelength of light with a diffraction grating. Equipment: a grating (with known period d, e.g. 10 μm or 20 μm), an incandescent lamp, a black screen with a narrow slit in the middle and a long rail with a millimetre scale. Looking through the grating at the slit one sees spectra of the 1st, 2nd, … orders on both sides of the slit. We measure the distance y from the slit to the chosen colour's line of order n and the distance x from the grating to the slit screen. For small angles (y ≪ x) sinφ ≈ tanφ = y/x, so from d·sinφ = nλ we get λ = d·y/(n·x). Repeating the measurement for each colour in several orders and on the right and left sides, we find λ_mean, the mean absolute error Δλ = |λ_mean – λ| and the relative error ε = Δλ_mean/λ_mean. We record results in a table. Safety: only a school lamp; do not look at the Sun or at lasers; the teacher connects the lamp to the supply.
Draw the table template: colour, x (mm), y (mm), n, λ (nm), λ_mean, Δλ, ε. Fill it with the numbers of the example. Do the real work in the school laboratory; at home you may observe the spectrum on a CD with a lamp, but never look straight at the Sun.