This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: 1 Mon. Nov 1 Phy107 Lecture 23 From Last Time… • Light shows both particle and wavelike properties • Interference is an example of wavelike props. • Photons are particles of light in the particle-like description. • Experiments that demonstrated this are – Photoelectric effect – Blackbody radiation Mon. Nov 1 Phy107 Lecture 23 Hour exam 2 results • Average = 68% • Median = 70% • Your grade is posted at [email protected] • uwmad.courses.wisconsin.edu • Use your netid and password, then go to Physics 107 course page. Mon. Nov 1 Phy107 Lecture 23 Blackbody radiation • Blackbody radiation • Object emits light with a blackbody spectrum. • Peak position (color) changes with temperature White hot Red hot Mon. Nov 1 Phy107 Lecture 23 Photoelectric effect: PARTICLES • Einstein says that light is made up of photons, individual ‘particles’, each with energy hf . • One photon collides with one electron- knocks it out of metal. • If photon doesn’t have enough energy, cannot knock electron out. • Intensity ( = # photons / sec) doesn’t change this. Minimum frequency (maximum wavelength) required to eject electron Mon. Nov 1 Phy107 Lecture 23 …explained by quantization of light • Light comprised of discrete particles (photons) • Each photon has energy hf ( h = Planck’s constant) Mon. Nov 1 Phy107 Lecture 23 Intensity of light • Photon of frequency f has energy hf • Red light made of only red photons • The intensity of the beam can be increased by increasing the photon flux. Interaction with matter • Photons are absorbed one at a time • Interaction depends on individual photon frequency / energy. 2 Mon. Nov 1 Phy107 Lecture 23 Photons and Electromagnetic Waves • Light has a dual nature. It exhibits both wave and particle characteristics – Applies to all electromagnetic radiation • The photoelectric effect gives evidence for the particle nature of light – Light can behave as if it were composed of particles • Interference and diffraction evidence the wave nature of light Mon. Nov 1 Phy107 Lecture 23 Matter waves • If light waves have particle-like properties, matter should have wave properties. • de Broglie postulated that the wavelength of matter is related to momentum as • This is called the de Broglie wavelength. λ = h p Mon. Nov 1 Phy107 Lecture 23 Why h / p ? Works for photons • Wave interpretation of light: – wavelength = (Speed of Light) / Frequency – λ = c / f • Particle interpretation of light (photons): – Energy = (Planck’s constant) x Frequency – E = hf, so...
View Full Document