Quantum Mechanical Ideas

Photons and their energy When electromagnetic waves are exhibiting their “particle-like” nature, we call those little mass-less bundles of energy PHOTONS. There are photons of visible light, photons of UV, photons of microwaves, photons of IR, etc.

Sometimes the wavelengths of photons are measured in meters, sometimes in nanometers and sometimes in Angstroms, where one Angstrom = 1 x meters Also, the ENERGY of electrons is often given in “electron-Volts”, eV, instead of Joules, where one eV = 1.6 x Joules a very tiny amount of energy! New Units of measurement

All electromagnetic photons carry energy as they travel along at “the speed of light”. The energy of a photon, in eV, is given by E = hf where f is the frequency of the photon, measured in Hertz h is a constant called Plank’s constant. h = 4.14 x eV·s

The different frequency of electromagnetic waves (photons) determines if they are visible light, radio wave, microwaves, etc. higher frequency = more energy!

Which photon has more energy- an X-ray photon or a microwave photon?

The different frequencies of visible light correspond to different colors of light. Blue light has a higher frequency than yellow light. Which color of light has the highest energy?

How can you produce different colors of light?

What makes one atom different from another?atom The amazing colors produced in fireworks are a result of the different types of atoms that are used to make the fireworks.

Each atom has its own unique number of protons, neutrons, and electrons. Each electron in every element is in an “orbital” about the nucleus and has a unique energy. That unique energy determines the amazing colors seen in fireworks!

A Quantum is a discreet unit of a physical quantity. For example: our money is measured in a quantum of one cent. You can have 1 cent, 2 cents, 8 cents, etc., but you can’t have 1.24 cents or cents! You must jump from 1 to 2 to 3 to 4, etc. Electric charge, which ultimately comes from either a proton or an electron, is QUANTIZED. There is no such thing as a half of an electron or a fifth of a proton, so everything that has electrical charge must have some multiple of the charge of an electron or proton- 5 electrons, 8 protons, etc. That’s why electric charge is QUANTIZED.

The electrons, in their orbitals about the nucleus, have QUANTIZED levels of energy that are determined by which orbital they are in. The orbitals are numbered with “n” numbers, the “principle quantum number”: n = 1, n = 2, n = 3, etc. where the orbital closest to the nucleus is n = 1. The “n-number” for each atom’s electrons determine that electron’s energy. The larger the “n”, the larger the energy.

What does the energy of an electron in its orbital have to do with the colors of fireworks???

When an electron absorbs energy from an external source in any form (heat, electricity, a collision, etc.), it jumps to a higher orbital- called an “excited state”. Energy

When the electron falls back down to its original orbital, called its “rest state” or “ground state”, it must give up that extra energy. The energy is emitted in the form of a photon! Some of those emitted photons are visible light of different colors- some photons are not visible to us, like UV or IR or microwaves or X-rays energy photon

If an atom is continually absorbing energy, all kinds of transitions between higher and lower orbital levels are possible, resulting in many different types of emitted photons of many different colors.

MetalColor StrontiumRed CopperBlue BariumGreen SodiumYellow/Orange CalciumOrange GoldIron What elements are used in fireworks to produce different colors of light?

Atomic Spectra Since the electrons’ energy are unique for each element, each element produces a unique spectra of colors when supplied energy. We may see with our eyes only many overlaping colors of light. To see all the distinct colors in the atom’s spectra requires a “diffraction grating”. Spectra for Neon

Each element produces a unique spectra of colored lines when viewed through a diffraction grating Argon Helium Nitrogen Mercury

Because each element produces a unique emission spectra, scientists use “spectral analysis” to determine the composition of unknown substances. The spectra is like a fingerprint- absolutely unique for each element! Argon

Astronomers use “spectral analysis” to determine the composition of stars as well. However… the line spectra is shifted toward the red end of the spectra. This is called “red shift” and is an example of the Doppler shift due to the stars moving away from us. The “red shift” is one of the primary evidences of an expanding universe!

Using a Spectrometer to determine the identity of a elemental gas 1.The gas will not glow until it is energized. Energy can be provided in the form of heat or by applying a high voltage. 2.If you look at the glowing tube with just a diffraction grating, the emission spectrum lines of color are visible.

Using a Spectrometer to determine the identity of a elemental gas 1.If you look at the glowing tube through a “spectrometer”, which contains a diffraction grating, you can actually precisely measure wavelengths of the spectral lines. 2.Since each element emits only certain wavelengths, the gas can be identified.

Hydrogen The emission spectrum of Hydrogen is the most studied spectrum because it is also the simplest. Hydrogen has only ONE electron. But that ONE electron can be energized to many different orbitals, “excited states” and will emit photons as it returns to its “ground state”.

Light behaves like a wave AND like a particle The first clear demonstration of the particle-like behavior of light was in The Photoelectric Effect Albert Einstein won the Nobel Prize in Physics for his study of the Photoelectric Effect.

The Photoelectric Effect The ejection of electrons from certain metals when light falls upon them

Shining light on a metal can liberate electrons from its surface. The light has to have enough energy (high enough frequency) for this effect to occur. The energy of the “photoelectrons” liberated from the surface depends on the frequency (the energy) of that incident light- NOT its intensity! Increasing the intensity of the light increases the number of photoelectrons emitted, but not the energy of each photoelectron.

When will Photoelectrons be produced?Photoelectrons PHet simulation

If no electrons are ejected, you must… …increase the frequency of the light If only a few electrons are ejected and you want more, your must….. …increase the intensity of the light If you want to increase the kinetic energy of the electrons, you must… …increase the frequency of the light.

Practical applications of the photoelectric effect Garage door automatic openers Or Burglar alarm systems Auto light meters in camera flashes Solar cells to produce electricity