Energy Levels & Photons

Atomic & Nuclear Lesson 2

Homework

• Revise for the skills test.

• After the test, complete worksheet & read about emission spectra and absorption spectra in the textbook.

Learning Objectives

• To outline evidence for the existence of atomic energy levels.

• To know what a photon is.

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

Part of E-M Spectrum

Wavelength Range

Visible 400-700 nm

Electromagnetic Spectrum

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 Spectrum

Mnemonic?

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

• Highest energy? Highest frequency?

Looking at Emission Spectra

• Look at the spectra of a white light and a set of standard discharge lamps: sodium, neon, hydrogen and helium.

• Make notes of your observations. What are the differences between the spectra?

(Diagram: resourcefulphysics.org)

Emission spectra

• An energy input raises the electrons to higher energy levels. This energy input can be by either electrical, heat, radiation or particle collision.

• When the electrons fall back to a lower level there is an energy output. This occurs by the emission of a quantum of radiation.

• When ever possible, electrons occupy the lowest energy level called the ground state.

Emission Spectra

Each element has its own specific set of lines.

How do we get emission spectra?

Bohr proposed that electrons moving between energy levels caused the line spectra.

The energy levels and spectra series

Photons

Light is not a continuous wave but is emitted as “packets”.

These “packets” of energy are called “photons”.

The different colours of light correspond to different photon energies only certain energies are allowed.

Absorption spectra• When light of all frequencies is passed

through a gas then the gas absorbs light of the same frequency as it would emit.

• The light is radiated in all directions causing a reduction of intensity in the direction of the observer (dark lines).

• And so is seen when emitted energy is absorbed by a medium the Sun

Wave Equation (GCSE)

• Recall from GCSE:-

• Electromagnetic radiation travels at the speed of light which is 3.00 × 108 m s-1 in a vacuum.

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

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)

f c

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 JsOr in words:-

Radiation ofFrequency ConstantsPlanck'EnergyPhoton

hfE

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:-

(m)RadiationofWavelength

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

1

hc

E

Worked Example

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

λ = 400 nm = 400 × 10-9 mE = ?h = 6.63 × 10-34 J sc = 3 × 108 ms-1

J19-9-

-18-34

10 97.4m 10 400

ms 10 x 3 Js 10 6.63

hcE