Photons Edexcel International A Level Physics revision
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In plain words
Light is a wave, but it delivers its energy in separate packets, like rain arriving as drops rather than as a steady stream. Each packet is called a photon. The energy of one photon depends only on the light's frequency: every blue photon carries more than every red one.
4 things to know
- Electromagnetic radiation behaves as a wave when it travels (it diffracts and interferes) and as particles when it is emitted or absorbed. A photon is one quantum, or packet, of electromagnetic energy.
- Photon energy E = hf = hc ÷ λ, where h is the Planck constant, 6.63 × 10⁻³⁴ J s.
- The electronvolt is a small unit of energy: the energy gained by an electron moved through a p.d. of 1 V. 1 eV = 1.60 × 10⁻¹⁹ J.
- A photon has momentum even though it has no mass: p = E ÷ c, which is the same as h ÷ λ.
Worked example
Find the energy, in joules and in electronvolts, of a photon of light of wavelength 500 nm.
- E = hc ÷ λ = 6.63 × 10⁻³⁴ × 3.00 × 10⁸ ÷ 5.00 × 10⁻⁷.
- = 3.98 × 10⁻¹⁹ J.
- In electronvolts: 3.98 × 10⁻¹⁹ ÷ 1.60 × 10⁻¹⁹ = 2.5 eV.
Tips and tricks
- Joules to electronvolts: divide by 1.60 × 10⁻¹⁹. Electronvolts to joules: multiply by it.
- Brighter light means more photons each second, not more energy in each photon.
It lands in your notebook with its questions as flashcards.
Photons: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the energy of a photon proportional to?
E = hf.
What is 1 eV in joules?
The energy gained by an electron moved through 1 V.
Which photon has the most energy?
It has the highest frequency.
A light is made brighter without changing its colour. What changes?
The same colour means the same energy per photon.
A photon has an energy of 4.8 × 10⁻¹⁹ J. What is that in electronvolts?
4.8 × 10⁻¹⁹ ÷ 1.60 × 10⁻¹⁹.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 6 marks. Write your answers on paper, then check them.
Photons
Edexcel International A Level Physics WPH · 6 marks · papermunch.org
Name ______________________________ Date ______________
Find the energy of a photon of frequency 6.0 × 10¹⁴ Hz.[2]
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4.0 × 10⁻¹⁹ J. 6.63 × 10⁻³⁴ × 6.0 × 10¹⁴.
A laser gives out 2.0 mW of light as photons of energy 3.0 × 10⁻¹⁹ J. Find the number of photons it emits each second.[2]
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6.7 × 10¹⁵. 2.0 × 10⁻³ ÷ 3.0 × 10⁻¹⁹.
Find the momentum of a photon of energy 3.0 × 10⁻¹⁹ J.[2]
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1.0 × 10⁻²⁷ kg m/s. 3.0 × 10⁻¹⁹ ÷ 3.00 × 10⁸.
Answers: Photons
- 1. 4.0 × 10⁻¹⁹ J. 6.63 × 10⁻³⁴ × 6.0 × 10¹⁴.
- 2. 6.7 × 10¹⁵. 2.0 × 10⁻³ ÷ 3.0 × 10⁻¹⁹.
- 3. 1.0 × 10⁻²⁷ kg m/s. 3.0 × 10⁻¹⁹ ÷ 3.00 × 10⁸.