Photons

Particle Physics Lesson 3

Learning ObjectivesLearning Objectives

To know what a photon is.To know what a photon is.

To know how to calculate the energy To know how to calculate the energy of a photon.of a photon.

To be able to calculate how many To be able to calculate how many photons are emitted by a light photons are emitted by a light source every second.source every second.

Part of E-M Spectrum

Wavelength Range

Visible 400-700 nm

Electromagnetic Electromagnetic SpectrumSpectrum

Part of E-M Spectrum

Wavelength Range

Radio > 0.1mMicrowave 0.1m to 1mmInfrared 1 mm to 700

nmVisible 400-700 nmUltraviolet 400 nm – 1 nmX-rays < 1 nmGamma-rays < 1 nm

Electromagnetic Electromagnetic SpectrumSpectrum

Mnemonic?Mnemonic?

Rabbits Rabbits Radio Radio MateMate Microwaves Microwaves In In Infrared Infrared Very Very Visible Visible Unusual Unusual Ultraviolet Ultraviolet X-rated X-rated X-rays X-rays Gardens Gardens Gamma Rays Gamma Rays

Highest energy? Highest frequency?Highest energy? Highest frequency?

VideoVideo

In Search of Giants (11 of 15) The In Search of Giants (11 of 15) The Weird Quantum World.wmvWeird Quantum World.wmv

Wave Equation (GCSE)Wave Equation (GCSE)

Recall from GCSE:-Recall from GCSE:-

Electromagnetic radiation travels at Electromagnetic radiation travels at the speed of light which is 3.00 × the speed of light which is 3.00 × 101088 m s m s-1-1 in a vacuumin a vacuum..

(m)Wavelength Hz)Frequency( )s (m Speed Wave -1

Wave Equation (AS Wave Equation (AS Physics)Physics)

In AS we write this equation using In AS we write this equation using symbols:-symbols:-

where c = the speed of light in a where c = the speed of light in a vacuum (m svacuum (m s-1-1))

f = the frequency of e-m radiation (Hz)f = the frequency of e-m radiation (Hz) λ = the wavelength of e-m radiation (m)λ = the wavelength of e-m radiation (m)

f c

Photon EnergyPhoton Energy We can work out the energy of an We can work out the energy of an

incoming photon using this equation:-incoming photon using this equation:-

Where E = Energy of Photon in Joules (J)Where E = Energy of Photon in Joules (J) f = Frequency of the Radiation in Hertz f = Frequency of the Radiation in Hertz

(Hz)(Hz) h = Planck’s constant = 6.63 x 10h = Planck’s constant = 6.63 x 10-34-34 Js Js

Or in words:-Or in words:-Radiation ofFrequency ConstantsPlanck'EnergyPhoton

hfE

Photon EnergyPhoton Energy Recall from GCSE that f = c/Recall from GCSE that f = c/λλ so we so we

can substitute this into the photon can substitute this into the photon energy equation E=hf to get:energy equation E=hf to get:

Or in words:-Or in words:-

(m)RadiationofWavelength

)(msLight of Speed(Js)ConstantsPlanck'(J)EnergyPhoton

1

hc

E

Worked Example

Q: What is the photon energy for UV Q: What is the photon energy for UV radiation with a wavelength 400 nm?radiation with a wavelength 400 nm?

λλ = 400 nm = 400 = 400 nm = 400 ×× 10 10-9-9 m m

E = ?E = ?

h = 6.63 h = 6.63 ×× 10 10-34-34 J s J s

c = 3 c = 3 ×× 10 1088 ms ms-1-1

J19-9-

-18-34

10 97.4m 10 400

ms 10 x 3 Js 10 6.63

hcE

Light from a Laser Light from a Laser (GCSE)(GCSE)

IntenseIntense

In PhaseIn Phase

Same FrequencySame Frequency

the last one also means same wavelengththe last one also means same wavelength which means monochromatic (one which means monochromatic (one

colour)colour)

Laser BeamLaser Beam

Light from a laser is monochromatic.Light from a laser is monochromatic.

So each photon has the same freq and So each photon has the same freq and energy.energy.

If we call the no. of photons leaving the laser If we call the no. of photons leaving the laser per second, n, then:-per second, n, then:-

Because this gives you the energy emitted Because this gives you the energy emitted per second which is power.per second which is power.

nhfbeam theofpower the

Worked ExampleWorked Example

A 5 mW laser pointer emits a beam A 5 mW laser pointer emits a beam of red light of wavelength 670 nm, of red light of wavelength 670 nm, how many photons are emitted per how many photons are emitted per second?second?

P = 5 mW = 5 P = 5 mW = 5 ×× 10 10-3-3 W W λλ = 670 nm = 670 = 670 nm = 670 ×× 10 10-9-9 m m n = ?n = ?

nhc

P

1161-834-

-9-1-3

10 68.1)s m 10 3 Js 10 (6.63

m) 10 670 Js 10 (5P

shc

n