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 10 8 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 λ V = velocity F = 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 20 th 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 = mc 2 E = mc 2 m = E c 2 E= energym= mass c 2 = 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 = h E: energy (J, joules) h: Planck’s constant (  J·s) : frequency (Hz)

Practice EX 1: Find the energy of a red photon with a frequency of 4.57  Hz. GIVEN: E = ? = 4.57  Hz h =  J·s Solve: EX 2: Find the energy of a photon with a frequency of 3.55  Hz. GIVEN: E = ? = 4.57  Hz h =  J·s