# PH2.3- Photons

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• Created by: Catrin
• Created on: 11-03-12 12:02

## The structure of atoms

Both protons and neutrons are known as nucleons

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## Composition f the nucleus

Number of protons= atomic number Z

Number of protons and neutrons= mass number A

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## Isotopes

Same number of protons, different number of neutrons.

E.G. 1 H 2 H 3 H

1 1 1

2H= deuterium

3H= titrium

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## The Electromagnetic spectrum

Consists of a large number of different waves, all of which travel at the same speed in a vacuum (3x108ms-)

Electromagnetic waves are an inseperable combination of varying electrical and magnetic fields which are perpendicular to eah other and to the direction of the wave, ans do are classed as transverse waves.

Typical wavelengths:

Y-rays: 10-13 +

X-rays: 10-8 - 10-13

Ultrviolet: 10-6.5 - 10-8

Visable: 7.8x10-7 - 3.9x10-7

Infrared: 10-3 - 10-6

Microwaves: 10-1 - 10-3

The larger the wavelength, the smaller the frequency: c=f x /|

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## Energy of a photon

E= H x f  OR E= h x c / /|

Where:

E= energy of 1 photon (quantum)

f= frequency (Hz) Unit s-1 (vibraions per second)

h= plans constant= 6.6x10-34Js-

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## The electron volt

The energy gained by an electron when it is accelerated through a potential difference of 1V

1ev= 1.6x10-19J

ev x 1.6x10-19 = J

J/ 1.6x10-19= eV

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## Energy levels and the origin of spectra

The higher energy levels are still there even if they're not occupied by any electrons.

Ground state= all of the electrons occupy the lowest availale energy level

Energy levels are given a number, n, called the quantum number

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An electron has a negative value for its PE whilst in orbit around an atom since work must be done to remove it (to infinity, where PE=0)

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## Excitation

Atoms aren't always in their "ground state". If an elecron in orbit is given just the right amount of energy, it may move to a higher energy level. The atom is now said to be in an excited state. This can be done by:

1.Fast electrons

2.Photons of certain frequency

(3. Heat)

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## What happens next?

The electron will stay in the higher energy level for a short time, before it returns to the ground state.

This happens either directly or via intermediate energy levels.

The electron emits the difference in energy, E2-E1, between the higher energy level, E2, and the lower energy level, E1, as a single photon

E2- E1= hf

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## Spectra

White light source= all the wavelengths (continuous spectrum), filament light bulb, spectrometer.

Absorption spectrium= continuous spectrum that contains some dark bands. Absorption of a photon excites the electron.

Fraunhafer= darker lines on the colour, produced by the suns atoms absoribing the specific wavelenghts.

Dark lines remains because atoms emit the absorbed light in random directions so the continuous spectrium doesn't absorib the colours back.

Emission spectrum= produce specific colours/wavelenghts in random directions

Used for: protein/DNA identification, flame tests.

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## Line Spectra

Consists of very few narrow lines on a dark background. Each line corresponds to a specific wavelength.

Produced by low-density gases and vapours.

The atoms in these gases and vapours are so far apart that they don't interact, and their energy levels don't "overlap".

Any excited electron can only move to lower energy levels within that 1 atom, and there is a very limited amount of different wavelengths produced.

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## Band Spectra

Several lines grouped together into bands.

The line within each band are closer together at one side, making this side sharper and brighter.

Produced by gases and vapours whose molecules contain more than 1 atom.

Since these atoms are bonded together, their energy levels "overlap".

This means that there are a greater number of transitions available to any excited electron since there are energy levels available from the other neighbouring atoms.

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## Continuous Spectra

Procuced by hot solids and liquids and high-density gases.

Atoms are so close that it interacts with many others, and any 1 atom may find many other energy levels overlapping into its own energy levels.

Any excited electrons have a limitless number of availbale energy levels that they can move down into.

Almost every wavelenght of light is produced.

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## X-Ray Spectra

Very short wavelenght electromagnetic waves (1x10-10m)

X rays are produced in an X-ray tube ("Tube" because its a glass container and we can get the air out of it.

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## How an X-ray tube works

Setting an electron up to a very high speed.

The heat of the filament knocks the electrons out, leaving positive ions, the electrons are attracted to the positive ions and so move back.

Try and attract the electrons back out with another charge- anode

the electrons start accelerating towards the Tungsten target.

X-rays can only come out of one small point due to the Lead shielding

There is a partial vacuum to prevent the electrons colliding with the air.

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## Properties of X-rays

They travel in straight lines at the speed of light (in a vacuum)

They have a wavelength of 1x10-10m

Ionising capabilities

Effect photographic film

Can penetrate through materials.

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## Calculating the kinetic energy for an electron str

Work done = Kinetic energy gained

Since W.D= e x V,

e x V = KE gain

If the energy is needed in eV, then divide the answer withe 'e'. KEgain (in eV)= V

KE= 1/2 mv2

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## X-ray spectra

A whole range of different wavelengths are produced

usual way of showing this range of x-rays is to plot a graph of intensity vs wavelength

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