# Calculating particle properties of a wave

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1 Calculating particle properties of a wave A light wave consists of particles (photons): The energy E of the particle is calculated from the frequency f of the wave via Planck: E = h f (1) A particle can act like a wave: The momentum p of the particle is calculated from the wavelength via de Broglie: p = h/ (2) Frequency f and wavelength can be converted into each other by the wave equation: f = v (3) Ch. 12

2 Energy of a photon Consider light with a wavelength = 500 nm Use (1),(3), v=c E hf hc J s m (1) (3) m/s J = 2.5 ev A practical energy unit in quantum physics is an ev, the energy of an electron accelerated by 1 Volt: ev = electron-volt = ( electron charge e ) (1 Volt) = ( Coulomb) (1 Volt) = Joule Conversion of Joule to ev : J = ev

3 Conversion between wavelength and energy Wavelength and energy are inversely proportional with the proportionality constant hc: E = hc/ = hc/e hc = 1240 ev nm Green light for example: E = 1240 ev nm / 550 nm = 2.25 ev

4 How many photons can you see? In a test of eye sensitivity, experimenters used 1 millisecond flashes of green light. The lowest light power that could be seen was Watt. How many green photons (550 nm, 2.25 ev) is this? A. 10 photons B. 100 photons C. 1,000 photons D. 10,000 photons 10-3 s W = J = ev = 250 ev; 250eV/2.25eV = 111 photons

5 Detecting single photons A photon can be converted into a pulse of electrons: For ultraviolet light, X-rays, Gamma rays: In a Geiger counter a photon knocks electrons from gas molecules, creating a miniature spark. For visible light: In a photomultiplier a photon kicks an electron out of a solid (the photocathode). The electron is multiplied into a million electrons by multiple bounces.

6 Kamiokande particle detector lined with photomultipliers Wed. Feb 27, 2006 Phy107 Lecture 16 6

7 The wave character of electrons h p h mv e v Js s kg velocity The wavelength is inversely proportional to the velocity v of the electron and to its mass m e. An electron with low velocity (low kinetic energy) has a long wavelength, which is easier to detect. Ch. 12.4

8 Wavelength of an electron from its kinetic energy 1) Velocity v from the kinetic energy E kin : E kin = ½ m e v 2 v = 2 E kin /m e 2) Wavelength via de Broglie and p=m e v: = h/m e v For a kinetic energy E kin = 1eV one obtains: 1) v = J / kg (ev = J ) = m/s (m e = kg) 2) = Js / kg m/s (h = Js) = m = 1 nm

9 Typical wavelength of an electron The wavelength of an electron is very short and hard to measure with macroscopic tools. The spacing of nickel atoms in a crystal was needed to demonstrate electron diffraction. A typical electron energy in a solid with 1eV energy has a wavelength of only 1 nanometer. Nanotechnology has the capability to shape electron waves and thereby influence the behavior of electrons in a solid (Lect. 24a).

10 Wavelength of a football NFL guidelines: Weight 14 to 15 ounces = 0.4 kg Brett Favre can throw the ball 70 miles/hour = 30 m/s mvv 0.4 kg 30 m /s 12 kg m /s h p J s 12 kg m /s m nm That is similar to the Planck length where space-time falls apart, times smaller than the smallest measurable length (Lect. 3). Conclusion: Macroscopic objects don t show measurable quantum effects.

11 When are quantum phenomena important? (optional) 1) Quantum physics is important at small distances d, smaller than the de Broglie wavelength =h/p : d < Example: Nanotechnology for shaping electron waves. 2) Quantum physics is important for large energy quanta E = hf: Example: Planck s radiation law cuts the spectrum off when the energy to create a photon exceeds the available thermal energy ( E therm 0.1 ev at T=300K ) : E > E therm Example: The photoelectric effect occurs only when the photon energy exceeds the minimum energy to knock out an electron ( = work function 4eV ) : E >

12 Criteria 1) and 2) are connected by the wave equation: f = v ( v = velocity of the wave ) If the wavelength is small the frequency f is large and the energy quantum E=hf is large. Example: Quantum computing requires that quantum effects are dominant. Three approaches to quantum computing are pursued in the Physics Department (silicon quantum dots, superconducting junctions, and cold atoms in traps). All of them use very low temperatures to keep the quantum effects undisturbed by thermal noise (Criterion 2).

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