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Understanding Photons: Energy Calculations and Emission Rates

In this lesson, we'll explore the properties of photons, including what they are and how to calculate their energy. You'll learn to determine the number of photons emitted by a light source each second. We'll also review the electromagnetic spectrum and its mnemonic for easy recall. Engage with examples, such as calculating the energy of ultraviolet radiation and the number of photons emitted by a laser pointer. By the end of this lesson, you'll have a solid grasp of photon behavior and the underlying physics concepts.

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Understanding Photons: Energy Calculations and Emission Rates

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  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?

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  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 = ?

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