The Nature of Light

Anatomy of a wave

What is a photon?

A particle representing a quantum of light

Carries energy proportional to the

radiation frequency

Has zero rest mass

Anatomy of a wave

Amplitude – Distance from peaks to middle of wave

Crest – Top of wave

Trough – Bottom of wave

Wavelength (m) – Distance between crests

Equal to one full cycle of the wave

Frequency (1/s or s-1) – How many wave cycles pass through a given point in a

time interval

Wave speed (how fast its going) = wavelength x frequency

V = λ x f

Speed of light (c) = 3.00 x 108 m/s C = λ x f

Depends on the medium it is traveling through

Anatomy of a wave

Rarefaction –

Stretching of the wave

Compression –

Shortening of the wave

Energy of a wave

ALL WAVES transport energy as they travel

Total Energy – ½spring constant x amplitude squared

E = ½k x A2

This means for a wave caused by simple harmonic motion, that

every particle in the wave will have the same total energy

This tells us that a waves total energy is proportional to the

amplitude squared

Therefore, if you double the waves amplitude, you get 4x the

energy

If you triple the amplitude, you get 9 times the energy

More wave properties

Intensity = power/area

Power = energy/time

Therefore, changing the amplitude of a

wave can change its energy

Intensity is proportional to A2

This can help us figure out how much

damage can be caused by an earthquake

Wave

Interference

Constructive Interference –

When two crests overlap to create on crest with

double the amplitude

Destructive Interference –

One crest and one trough with the same amplitude

overlap and cancel the wave

Wave Property Connections

Longer wavelength = Lower frequency, Lower energy

Shorter wavelength = Higher frequency, Higher energy

Electromagnetic Spectrum

Electromagnetic Spectrum

Electromagnetic Spectrum

Wavelength

Visible Spectrum

White light is refracted (bent) or passed through a prism

Separates the light by color – continuous spectrum

Different colors of light are separated by

their different wavelengths

Color Wavelength (nm)

Red 740-660

Orange 660-620

Yellow 620-580

Green 580-500

Blue 500-415

Violet 415-380

Spectrums

Two types of light spectrums

Continuous

Bright line

States there are mandatory energy levels for atoms

Different atoms have different energy levels, therefore give

off different light combinations

Fingerprint of all atoms, elements, compounds

The Hydrogen Atom

Bohr

Excited State (based off electron placement

around nucleus)

Proposed that an electron moves into an orbit or

higher energy level further from the nucleus when

an atom absorbs energy

Ground State

Electron returns here (home) after being excited

The Hydrogen Spectrum

The Hydrogen Spectrum

Lower energy levels are closer to the

nucleus

Lines converge at higher energies

Energy levels are closer together at higher

energies

Highest energy: n=∞

No longer in atom atom has been ionized

Absorption and Emission

Atoms emit photons

of certain energy and

lines of certain

frequencies

Electrons occupy

specific orbits

Hence, energy of the

atom is quantized

Or approximated

Light as a particle

Energy is released

as electrons fall into

lower energy levels

Energy is released

as ‘light’ from

electromagnetic

radiation

Energy of a photon

is proportional to

frequency

Energy of a photon

E = hf

E = energy of a photon

Units: J = Joule

h = planck’s constant

(6.63x10-34J*s)

f = frequency (1/s or s-1)

Electromagnetic Spectrum

Recap!

What is a photon?

A quantized packet of energy called light

When do atoms emit visible light?

When an electron falls from a higher energy level to

its ground state

What is the relationship between frequency,

wavelength, and energy?

Longer wavelength, lower frequency, lower energy

Shorter wavelength, higher frequency, higher energy