Aim: How do we calculate the energy of a photon?

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Presentation transcript:

Aim: How do we calculate the energy of a photon? Do Now: How does light behave like a wave? How about a particle? (WRITE DOWN AN ANSWER)

Describe what the photon looked like Describe what the photon looked like. Describe some properties of the photon?

What do we call a particle of light? A PHOTON!! A photon is a small ‘packet’ of light Any single photon has a fixed, discrete energy. The intensity of visible light can be increased or decreased by changing the number of photons present. Although a photon is a massless particle of light, it carries energy and momentum. The same rules hold true for all electromagnetic waves outside the visible range like UV light. Sugar packets described as discrete

Quantum Theory How can we tell the intensity or strength of a wave? Normally, just Amplitude Does this apply to a photon? Intensity of a photon depends more on frequency! What phenomena shows this? Photoelectric Effect! Photons incident on a Metal surface eject electrons

Work Function The minimum energy (usually measured in electronvolts) needed to remove an electron from a solid to a point immediately outside the solid surface. What process does the Work Function exhibit?

Work Function

Sample Question The work function for a photoemmisive surface is 5eV. The maximum kinetic energy of ejected photoelectrons is 10 eV. What is the energy of the photons that struck the surface? Threshold frequency??? Frequency associated with work function of a surface.

Energy of a Photon Discrete energy is like money or sugar packets, you can only have integer multiples of a minimum amount. For money, what is this minimum amount? A photon can only carry integer numbers of a minimum energy This minimum energy is denoted by Planck’s constant h= 6.63x10-34Js Planck’s constant is modern physics’ version of the penny The energy of a photon is determined by its frequency (and wavelength) Another Example: Sugar Packets vs. box of sugar Can I have half a sugar packet? NO! The frequency tells us just how much of that energy is going to be used.

Photon Units 1eV=1.6x10-19J E is in Joules f is in hertz h = 6.63 x 10-34 Js c = 3.00 x 108 m/s  is in meters A joule is a large unit of energy. When you are talking about small electron, we use an electron volt instead. 1eV=1.6x10-19J

Envelope Activity Answer the questions regarding electronvolts. Remember: 1eV=1.6x10-19J

Ex1. A photon has 3.5eV of energy. How many Joules of energy is that? 5.6 x 10-19 J Ex2. A photon has 4.8x10-19J of energy, how many electron volts is that? 3eV

Photon Problem The energy of a photon is 1.89 electronvolts (eV). the energy of the photon in Joules (b) the frequency of the photon (c) the color of light associated with the photon.

Exit Card Use your knowledge of friction to answer the questions on your exit card. Be sure to hand in your exit card when it is complete.

A photon has a wavelength of 575nm What is the frequency of that photon? What is the color of the photon? What is the energy of that photon in Joules? What is the energy of the photon in eV?