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Photons

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Photons

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.

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In Infrared

Very Visible

Unusual Ultraviolet

X-rated X-rays

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:-

- 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

Intense

In Phase

Same Frequency

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 = ?