Calculation the ionisation energy of rubidium per atom, if light of wavelength 5.84 × 10^-8 m produces electrons with a speed of 2.450 × 10^6 ms^1 .

E = energy in the light rays = h ν = h c /λ = 6.67 * 10×⁻³⁴ * 3 * 10⁸ / (5.84*10⁻⁸) J = 3.42 * 10⁻18 JKinetic energy of electrons = 1/2 m u^2 = 1/2 * 9.1 * 10⁻³¹ * (2.45 * 10⁶)² J = 2.73 * 10⁻¹⁸ J The ionization energy = (3.42 - 2.73) * 10⁻¹⁸ J = 6.9 * 10⁻¹⁹ J

They can measure  the amount of energy gained or lost