THE ENERGY OF LIGHT

HOW LIGHT IS ‘MADE’

• Today we are going to take a look at how light is made and how we can determine the energy of light

• In order to understand this, we must understand the atom and how it really works

THE ATOM• If this is your view of the atom, I’ve got

news for you…

• Most of what you think is wrong

• We have explained much of what the atom is like in a way to simplify it for younger students.• You are now at a level where we can

complicate a few things to help you see a more realistic view of how the atom works

• This view we will take is STILL cutting some corners and simplifying some things for you.

THE ATOM• The Bohr Model is the best model to explain

what we are looking at today• This model is not really used anymore because we

use the quantum mechanical model

• The nucleus contains the protons and neutrons and provides mass to the atom

• The electrons can be found outside the nucleus• Electrons only exist in certain ORBITALS around

the nucleus

ELECTRONS

• You were probably told that electrons ‘orbit’ around the nucleus

• This isn’t exactly true

• Electrons are both particles and waves• They are standing waves

ELECTRONS• The standing wave orbitals of electrons actually

cause destructive interference in the areas between them

• This means that electrons can ONLY exist at certain distances (orbitals) from the nucleus

• The further the orbital is from the nucleus, the higher the energy of the electron

• Electrons want to occupy the lowest energy orbitals• If lower energy orbitals are full, the electrons

will occupy the next highest energy orbital

ELECTRON ORBITALS• Electrons can be pushed into higher energy orbitals

• This can only be done by adding more energy into the atoms

• This can be done with light energy, electricity,

• Electrons can only absorb the energy if it is equal to the energy gap between two orbitals• This will move the electron out to the higher energy level

ELECTRON ORBITALS• Electrons in an EXCITED STATE (higher

energy orbitals) will immediately drop to a lower energy orbital

• When the do this, they must release energy• This is released as a photon

• The energy they release must be equal to the energy difference in the gaps of orbitals

ELECTRON ORBITALS

• These energy gaps in orbitals is different for every different element

• This means that each element releases its own unique energy photons

ENERGY OF A PHOTON

• So what does the energy of a photon tell us?

• The energy of a photon is directly proportional to the frequency of the photon

• To make this equation equal, a constant is used• Plank’s Constant = h = 6.26*10-34 J*s

ENERGY OF A PHOTON• Because a photon is a particle of light, it moves at the speed of light

• c = 3.00*108 m/s

• Think back to the velocity of light equation

• c is a constant• Therefore if the frequency changes so does the wavelength of light

• Higher Frequency Light = Higher Energy Light = Shorter Wavelength Light

• Lower Frequency Light = Lower Energy Light = Longer Wavelength Light

ENERGY OF A PHOTON• We know from before that:

Therefore:

So we can now combine our energy equation with this new equation:

WAVELENGTH OF LIGHT• The wavelength of light tells us what ‘colour’ light is

• Shorter wavelengths are closer to blue and violet

• Longer wavelengths are closer to red

ELECTROMAGNETIC SPECTRUM• Light is just another name for Electromagnetic Waves

• What we see as light is actually just a very small range of wavelengths of light• There is only a small range of energies of electromagnetic waves that our eyes can detect

• There is actually a broad range of light that is invisible to our eyes!

ELECTROMAGNETIC SPECTRUM

ENERGY OF A PHOTON

• Example #1A photon of blue light has a wavelength of 458 nm. What is the energy of the photon?

Knownh = 6.262*10-34 J*sc = 3.00*108 m/sλ = 458 nm = 4.58*10-7 m

RequiredE = ?

4.34*10-19 J

ENERGY OF A PHOTON• Example #2

A radio station sends out their signal using photons with a frequency of 96.3 MHz. What is the energy of the photons released by the radio station?

Known

h = 6.262*10-34 J*sF = 96.3 MHz = 9.63*107

Hz

RequiredE = ?

ATOMIC SPECTRA

• Each element gives off its own unique energy photons

• The energy of photons tells can tell us the ‘colour’ or type of light that we see

• So we can actually analyze the energy of light given off by an object, check to see what spectra it matches, and figure out what elements it is made of!

HOW WE CREATE LIGHT!• Almost everything we use to make light takes advantage

of the Bohr Model explanation given earlier

• We input energy (usually electricity) into a sample of an element (sometimes a compound)

• The atoms absorb energy and increase to the higher energy orbitals followed by dropping down and releasing light

• Depending on what element(s) are in the material, the colours will be different