1. Photons Particle Physics Lesson 3

2. Learning Objectives To know what a photon is. To know how to calculate the energy of a photon. To be able to calculate how many photons are emitted by a light source every second.

3. Electromagnetic Spectrum

4. Electromagnetic Spectrum

5. Mnemonic? Rabbits  Radio Mate  Microwaves In  Infrared Very  Visible Unusual  Ultraviolet X-rated  X-rays Gardens  Gamma Rays Highest energy? Highest frequency?

6. Video In Search of Giants (11 of 15) The Weird Quantum World.wmv

7. Wave Equation (GCSE) Recall from GCSE:- Electromagnetic radiation travels at the speed of light which is 3.00 × 108 m s-1in a vacuum.

8. Wave Equation (AS Physics) In AS we write this equation using symbols:- where c = the speed of light in a vacuum (m s-1) f = the frequency of e-m radiation (Hz) λ = the wavelength of e-m radiation (m)

9. Photon Energy We can work out the energy of an incoming photon using this equation:- Where E = Energy of Photon in Joules (J) f = Frequency of the Radiation in Hertz (Hz) h = Planck’s constant = 6.63 x 10-34 Js Or in words:-

10. Photon Energy Recall from GCSE that f = c/λ so we can substitute this into the photon energy equation E=hf to get: Or in words:-

11. Worked Example • Q: What is the photon energy for UV radiation with a wavelength 400 nm? λ = 400 nm = 400 × 10-9 m E = ? h = 6.63 × 10-34 J s c = 3 × 108 ms-1

12. Light from a Laser (GCSE) Intense In Phase Same Frequency  the last one also means same wavelength  which means monochromatic (one colour)

13. Laser Beam Light from a laser is monochromatic. So each photon has the same freq and energy. If we call the no. of photons leaving the laser per second, n, then:- Because this gives you the energy emitted per second which is power.

14. Worked Example A 5 mW laser pointer emits a beam of red light of wavelength 670 nm, how many photons are emitted per second? P = 5 mW = 5 × 10-3 W λ = 670 nm = 670 × 10-9 m n = ?