Electrons and Light. Light’s relationship to matter Atoms can absorb energy, but they must eventually release it When atoms emit energy, it is released.

Electrons and Light

Light’s relationship to matter• Atoms can absorb energy, but they

must eventually release it• When atoms emit energy, it is

released in the form of light = emission spectrum

• Atoms don’t absorb or emit all colors, only very specific wavelengths; the spectrum of wavelengths can be used to identify the element

Line Spectra= specific wavelengths are emitted; characteristic of atoms

• Light has particle-like behavior• Photons (light) come from electrons

falling from high electron orbits to low orbits.

• Light must be a wave, because…–Light is refracted in

lenses. –Light can be

diffracted.

The Wave Description of Light

Light is a Particle• Light can travel through the

vacuum of space, but waves can’t travel in a vacuum. So light must be a particle!

• Light is Both– light is both a wave and a

particle. Packets of light we call photons.

• Visible light is a kind of electromagnetic radiation. • Electromagnetic radiation is a form of energy that exhibits wavelike

behavior as it travels through space. • Examples of EMR include X rays, ultraviolet light, infrared light,

microwaves, and radio waves. • All forms of EMR move at a constant speed.

– 3.0 x 108 m/s = speed of light (c)

1. Which color of light has a longer wavelength, green or yellow? ______________

2. Which color of light has a higher frequency? Violet or Red? ______________

3. Which color of light has the highest energy? Violet or Red?

4. What is the wavelength of red light? ______________5. What is the wavelength of violet light? _____________

Characteristics of a Wave• Significance of wave motion is its repetitive nature: waves

have measurable properties of wavelength and frequency.• Wavelength is the distance between corresponding points

on adjacent waves.• Frequency is the number of waves that pass a given point

in a specific time (usually one second).

Which wave has a higher frequency? ________

A

B

Frequency and wavelength are mathematically related

V = f x λV = velocityF = frequency λ = wavelength EX 1: A wave has a frequency of 30 Hz and a

wavelength of 5 m, what is its velocity? EX 2: A wave has a frequency of 10 Hz and a

wavelength of .01 m, what is its velocity?

• Scientists in the early 20th century, Max Planck and Albert Einstein showed that electromagnetic radiation was composed of particles we call photons (photons are particles of light energy).

• Each wavelength of light has photons that have a different amount of energy.

• the longer the wavelength, the lower the energy of the photons.

Dual Nature of Light• From Einstein's famous equation E = mc2

E = mc2

m = E c2

E= energy m= mass c2 = speed of light

• Energy has an apparent mass.• The apparent mass of a photon depends on its

wavelength.

Einstein (1905)• Explained the photoelectric effect by proposing

that EMR is absorbed by matter only in whole numbers of photons.

• Concluded that light has properties of both waves and particles “wave-particle duality”

• A photon carries a quantum of energy. • The energy of a photon is proportional to its

frequency.E = hE: energy (J, joules)h: Planck’s constant (6.6262 10-34 J·s): frequency (Hz)

Practice

EX 1: Find the energy of a red photon with a frequency of 4.57 1014 Hz. GIVEN:E = ? = 4.57 1014 Hz h = 6.6262 10-34 J·s Solve: EX 2: Find the energy of a photon with a frequency of 3.55 1014 Hz. GIVEN:E = ? = 4.57 1014 Hz h = 6.6262 10-34 J·s