Question

(c) Tryptophan can absorb UV light around 280 nm and fluoresces around 355 nm. Explain why the fluoresced light is of a longer wavelength than the light absorbed. Include an energy level diagram to illustrate your answer. 14 marks

Answer 1

Uv light wavelength = 280 nm its energy = hc/lambda 3.626 times 10^- 34 times 3 times 10^8/280 times 10^- 9 J = 7.1 times 10^- 19 J = 4.44 ev \r\n emitted light wavelength = 355 nm Energy = hc/lambda = 6.26 times 10^- 34 times 3 times 10^8/355 times 10^- 9 J = 5.6 times 10^- 19 J = 3.5ev i.e. absorbed energy is more than emitted energy. This happens because, when an electron absorbs light, it goes to a higher energy level but in case of material, it does not immediately come back to ground state. Instead it comes down to a meta stable state before finally reaching the ground state This meta stable state is at a lower energy level as compared to the excited level. Hence, emitted light is of lower energy, or in other