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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