Basic quantum physics
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Basic Quantum Physics. photons and their energies. “ Light ” previously known as all forms of electromagnetic radiation has a dual nature : Wave-like (which we studied previously) and

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Basic Quantum Physics

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Basic Quantum Physics

photons and their energies

  • “Light” previously known as all forms of electromagnetic radiation has a dual nature:

  • Wave-like (which we studied previously) and

  • Particle-like in which it carries energy in tiny mass-less packets called photons which move at the speed of light.

Units of Measurement

Energy of electrons is often given in “electron-Volts”, eV, instead of Joules, where

one eV = the work energy required to move

1 electron through 1 Volt of potential difference

1 eV = Work = qDV

1 eV = 1.6 x 10-19 J

a very tiny amount of energy!

Wavelengths of photons can be measured in:

  • meters, m,

  • nanometers, nm,

  • One nanometer = 1 x 10-9 m or

  • Angstroms, Å

  • one Angstrom = 1 x 10-10 m

Energy of a photon is directly related to frequency

Photons carry energy as they travel along at “the speed of light.”

The energy of a photon, in eV, is given by

E = hf

f is the frequency of the photon, measured in Hertz

h is a constant called Plank’s constant.

h = 4.14 x 10-15eV·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?

 Energy increases with frequency

The different frequencies (energy!) 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?

Different colors of light

  • What causes different colors of light?

The variety of colors in firework displays are from the presence of different chemical elements! What makes one element different from another?

Energy changes within atoms

  • Electronswithin an atom have unique energy levels for each element, resulting in the variety of colors.

What elements are used in fireworks to produce different colors of light?

What’s a quantum?

  • A quantum, a discreet unit of a physical quantity must occur in whole number multiples

  • Examples: pennies, charges (protons, electrons), students

Depending on which orbital they occupy, electrons have QUANTIZED levels of energy .

The orbitals have principal quantum numbers, “n” beginning with the n=1 closest to the nucleus called the “ground state”

The next energy levels going outward are n = 2, n = 3, etc.

The “n-number” for each atom’s electrons have observed values for that electron’s energy.

The larger the “n”, the larger the energy.

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”.



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

The number of energy levels an electron can jump to depends on the amount of energy an atom is absorbing , resulting in many different types of emitted photons of many different colors.

The energy of the emitted photons is the energy difference between the orbitals

Ephoton = Eoriginal – Efinal

The frequency of the emitted photon is found using

E = hf

f is the frequency of the photon, measured in Hertz

h is a constant called Plank’s constant.

h = 4.14 x 10-15eV·s

Atomic Spectra

  • Since the electrons’ energy are unique for each element, each element produces a unique spectra of colors when supplied energy.

  • Our eyes see the blend of all the frequencies present. It appears monochromatic

  • A “diffraction grating” allows the light to be broken out into all the frequencies (colors) present.

Spectra for Neon

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!


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 identify a gas

  • Using a “spectrometer” or “spectroscope” to view a glowing tube of elemental gas allows precisemeasurement of wavelengths of the spectral lines with a scale inside the spectrometer.

  • The gas can be identified by matching a reference (known) spectra to the spectra viewed in the spectrometer .


The photo electric effect

The ejection of electrons from certain matter when short wavelength light falls upon them

Nobel Prize Winner, Albert Einstein

Building on Planck’s description of discreet quanta of light, Einstein explained mathematically the absorption of photons in the photoelectric effect and won the Nobel Prize in Physics in 1921.

  • 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.

Applications of the photoelectric effect

Solar cells transform sunlight to electricity

Garage door sensors use an infrared beam to keep pets safe from crushing

Applications of the photo electric effect

Night vision devices use light intensifiers

Charged couple devices act like the “eye” on a digital cameras

PHet simulation

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