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# Many Paths

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### Many Paths

1. 1. How can light be represented by phasors? Quantum Behaviour
2. 2. <ul><li>Write down something that shows light behaving as: </li></ul><ul><ul><li>A wave </li></ul></ul><ul><ul><li>A particle (quanta) </li></ul></ul>
3. 3. But how does energy of a photon differ with frequency?
4. 4. Photons and Energy <ul><li>A photon carries light energy </li></ul><ul><li>A red photon has less energy than a violet photon </li></ul>Red Violet You could think of them like this – in terms of energy Energy = Planck’s Constant x frequency E = hf h = 6.6 x 10 -34 Js
5. 5. Measuring Energy – Joules and eV We measure energy in Joules However, when this is very small, Joules are not the most useful unit to use. We use another unit – the electron Volt – eV Energy in eV = Energy in Joules / Charge on electron (e) eV = Joules/Coulombs Note ‘e’ = 1.6 x 10 -19 C
6. 6. Measuring Energy – Joules and eV e.g. what is the energy in eV of 1 x 10 – 19 J? Energy in eV = Energy in Joules / Charge on electron (e) eV = 1 x 10 -19 / 1.6 x 10 -19 = 0.63 eV This is much more manageable…!
7. 7. Measuring Energy in LEDs
8. 8. Results e = 1.6 x 10 -19 C is Voltage needed to just light LED Striking Voltage c = 300 000 000 Frequency = c /  Notes                                                             Energy (eV) (e x Voltage) Striking Voltage (V) LED Frequency (Hz) LED Wavelength LED Colour
9. 9. Plot a graph of energy against frequency Frequency (Hz) Energy (eV) Calculate the gradient – what should it equal??? Hint – think E/f = ?