Photons. Particle Physics Lesson 3. 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. Electromagnetic Spectrum. Electromagnetic Spectrum. Mnemonic?.
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Particle Physics Lesson 3
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.
Gardens Gamma Rays
Highest energy? Highest frequency?
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Recall from GCSE:-
Electromagnetic radiation travels at the speed of light which is 3.00 × 108 m s-1in a vacuum.
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)
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:-
Recall from GCSE that f = c/λ so we can substitute this into the photon energy equation E=hf to get:
Or in words:-
λ = 400 nm = 400 × 10-9 m
E = ?
h = 6.63 × 10-34 J s
c = 3 × 108 ms-1
the last one also means same wavelength
which means monochromatic (one colour)
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.
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 = ?