QuestionA laser pointer emits green light of wavelength 500 nm. Its output power is 1.0 mW.(a) Calculate the frequency of the light. (1)
(b) Calculate the energy of one photon, in joules and in electronvolts. (3)
(c) Calculate the number of photons emitted by the laser each second. (2)Answer(a) f = c / λ = 3.00 × 108 ÷ 500 × 10-9 = 6.00 × 1014 Hz (1)(b) E = hf = 6.63 × 10-34 × 6.00 × 1014 = 3.98 × 10-19 J (2)In electronvolts: 3.98 × 10-19 ÷ 1.60 × 10-19 = 2.49 eV (1)(c) Power is the energy emitted per second, so:number of photons per second = power ÷ energy of one photon
= 1.0 × 10-3 ÷ 3.98 × 10-19 = 2.5 × 1015 s-1
(2)Examiner tips- Convert nm to m first: 1 nm = 10-9 m. Forgetting this gives a frequency 109 times too large.
- Convert mW to W: 1.0 mW = 1.0 × 10-3 W.
- To convert joules to electronvolts, divide by 1.60 × 10-19; to convert electronvolts to joules, multiply.
- You could combine (a) and (b) using E = hc / λ.
- A huge number of photons per second is expected. This is why light from a source usually seems continuous.
(b) Calculate the energy of one photon, in joules and in electronvolts. (3)
(c) Calculate the number of photons emitted by the laser each second. (2)Answer(a) f = c / λ = 3.00 × 108 ÷ 500 × 10-9 = 6.00 × 1014 Hz (1)(b) E = hf = 6.63 × 10-34 × 6.00 × 1014 = 3.98 × 10-19 J (2)In electronvolts: 3.98 × 10-19 ÷ 1.60 × 10-19 = 2.49 eV (1)(c) Power is the energy emitted per second, so:
(2)Examiner tips- Convert nm to m first: 1 nm = 10-9 m. Forgetting this gives a frequency 109 times too large.
- Convert mW to W: 1.0 mW = 1.0 × 10-3 W.
- To convert joules to electronvolts, divide by 1.60 × 10-19; to convert electronvolts to joules, multiply.
- You could combine (a) and (b) using E = hc / λ.
- A huge number of photons per second is expected. This is why light from a source usually seems continuous.