1. Intro to Modern Physics Aim: How did we arrive to our most recent physics discoveries?

2. Physics Phoundation Since 585 BCE scholars and scientists have been understanding the world around them through the study of physics Up until 1900 CE, scientists developed a solid foundation to explain just about everything that occurs around them – Kinematics and Newton’s Laws could explain motion – Electricity and circuitry was better understood and harnessed – Light was understood to be a wave due to refraction, defraction and dispersion

3. However! Scientists couldn’t explain why when certain EM waves interacted with certain metals, electrons were emitted from the metal (PHOTOELECTRIC EFFECT) This phenomena couldn’t be possible if light was only a wave – it must have particle properties in order for this to occur Therefore EM radiation has BOTH WAVE & PARTICLE PROPERTIES (WAVE-PARTICLE DUALITY)

4. Quantum Theory Quantum theory assumes that EM energy is emitted from and absorbed by matter in DISCRETE AMOUNTS or packets Each packet is called a QUANTUM of energy A packet of light is called a PHOTON The energy of each individual photon depends on the light’s frequency and a constant value

5. Photon Energy Units: J -OR- eV Planck’s Constant 6.63×10 -34 J∙s Photon Frequency Units: Hz Speed of Light 3.0×10 8 m/s Photon Wavelength Units: m 1eV = 1.6×10 -19 J

6. Photoelectric Effect Each metal has a different threshold that must be matched in order for an electron to be ejected Increasing the amplitude (brightness) of the light DOES NOT increase the energy of each electron but ejects MORE of them Increasing the FREQUENCY (color) of the light will give the ejected electrons MORE energy

7. Max Karl Ernst Ludwig Planck April 23, 1858 – October 4, 1947 Won Nobel Prize in Physics in 1918 for his Quantum Theory Sad Trivia: His son, Erwin Planck, was executed in 1945 by the Gestapo for his part in an assassination attempt on Adolf Hitler

8. Example #1 How much energy does a photon of green light have when its frequency is 6.0×10 14 Hz?

9. Example #2 Determine the wavelength of a photon that has 5eV of energy

10. Photon-Particle Collisions Even though photons are considered massless particles they still have ENERGY and MOMENTUM When a photon collides with an electron, the photon loses energy to the electron, causing it move At the same time, the photons frequency decreases (increase wavelength) due to the transfer of energy However, BOTH the energy and momentum is conserved between the two particles

11. Summary Which phenomena shows light has wave properties? Which phenomena shows light has particle properties? Explain the Photoelectric Effect Describe Quantum Theory What is a photon? How do you calculate the energy of a photon? What is the value of Planck’s Constant?