QW *Use the light kits at your tables to perform and answer the following: Shine red, blue, and green lights at glow in the dark material 1. Which one.

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Presentation transcript:

QW *Use the light kits at your tables to perform and answer the following: Shine red, blue, and green lights at glow in the dark material 1. Which one makes the glow in the dark work the best? 2. Which is the worst? 3. Which is in the middle?

The blue light has the most energy The red light has the least energy WHY?... That’s what we are learning about today!

Photoelectric Effect

Wave-Particle duality To explain some aspects of light behavior, such as interference and diffraction, you treat it as a wave To explain other aspects you treat light as being made up of particles. Light exhibits wave-particle duality, because it exhibits properties of both waves and particles.

Photoelectric Effect Einstein won the Nobel Prize for Physics not for his work on relativity, but for explaining the photoelectric effect.

How Photo Electric Effect works If you shine light of high enough energy on to a metal, electrons will be emitted from the metal. This can create a current!

The Mystery Scientists were trying to answer the question: Why does low-frequency bright light not cause electrons to be emitted but higher-frequency dim light does?

Planck found out that: Energy comes in packets He gave the name quantum to the smallest quantity of energy that can be emitted (or absorbed) in the form of electromagnetic waves

Einstein took it to the next level He used Planck’s ideas to explain the photoelectric effect Before Einstein's explanation, the photoelectric effect was a real mystery. Einstein said that the reason photoelectric effect works is because of Photons Photons have no mass, but they have momentum and they have an energy given by…

E = hf h is planck’s constant F is frequency E is energy

Photo Electric Effect Knowing that light is made up of photons, it's easy to explain now. It's not the total amount of energy (i.e., the intensity) that's important, but the energy per photon. The higher the frequency of the photon the more energy it has.

When light falls on the surface of some metals, electrons are emitted from the surface. This is called the photoelectric effect Electrons jumping levels is the source of most light.

Threshold Frequency Threshold -a level, rate, or amount at which something comes into effect. ex: "everyone has a different pain threshold" Light below a certain threshold frequency, no matter how intense, will not cause any electrons to be emitted. Light above the threshold frequency, even if it's not very intense, will always cause electrons to be emitted.

Why it works Electrons can gain energy by interacting with photons. It takes a certain energy to eject an electron from a metal surface. If the photon has enough energy the energy can be transferred to the electron and the electron will have enough energy to escape from the metal.

Different metals This energy the electrons need depends on the metal. If the metal holds on to it’s electrons really tight it will require more energy. (higher frequency)