Source on the bench
Hydrogen Mercury Neon Argon Nitrogen Filament
Diffraction grating
300 l/mm
600 l/mm
1200 l/mm
Diffraction order
m = 1
m = 2
Telescope · angle read on the goniometer
ϑ
0.00°
−1°
−0.05°
+0.05°
+1°
Centre the nearest line Measure the line on the crosshair
Automatic scan Clear data
ϑ
λ (nm)
ν (1014 Hz)
E (eV)
line
What you are doing. Light from the source goes through a slit and two converging lenses that make it a parallel beam, then hits a diffraction grating. The grating sends each wavelength in a different direction: λ/d = sin ϑ , i.e. ν = c / (d sin ϑ) . The telescope turns on a graduated circle: when a line sits on the crosshair, the angle read on the goniometer is the measurement. From ϑ you get λ, then ν, then the photon energy E = hν, i.e. the atom's energy jump. With hydrogen the four visible lines (Balmer series) give the Rydberg constant and hence the whole ladder of levels. Then try the incandescent filament: no lines, just a continuum — quantisation is a property of atoms, not of light itself.