{"id":28,"date":"2018-01-14T20:17:21","date_gmt":"2018-01-14T20:17:21","guid":{"rendered":"http:\/\/134.169.6.169\/milq\/?page_id=28"},"modified":"2026-04-10T08:40:41","modified_gmt":"2026-04-10T06:40:41","slug":"1-photonen","status":"publish","type":"page","link":"https:\/\/www.milq.info\/en\/mehr\/1-photonen\/","title":{"rendered":"Lesson 1: Photons"},"content":{"rendered":"
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1.1 Photoelectric effect<\/a>\u00a0–\u00a01.2 Explanation<\/a>\u00a0–\u00a01.3 Experimental verification<\/a>\u00a0–\u00a01.4 Momentum<\/a>\u00a0–\u00a01.5 Progress check<\/a>\u00a0–\u00a01.6 Summary<\/a><\/p>\n

The aim of the milq course is to make you familiar with the content of the Internet portal for quantum physics. You will certainly find new ideas here, but also information you recognize from your lessons.
\nThe course starts with the photoelectric effect. If you are very familiar with it, read the Summary (Section 1.6) of the first lesson on this website just to be sure, and then proceed to \u201cLesson 2: Preparation”.
\nIf you want to work through \u201cLesson 1: Photons” in more detail,
\nyou can download\u00a0Chapter 1 of the teaching materials as a pdf file<\/a>.<\/p>\n

1.1. The photoelectric effect: Light releases electrons from metal surfaces<\/h3>\n

Experiment 1.1 (Hallwachs experiment):<\/strong><\/p>\n

\"photoeffekt1\"
\nHallwachs experiment; photoelectric effect<\/p>\n

The light-induced emission of electrons from a metal surface is called the external\u00a0photoelectric\u00a0effect<\/strong>. One example for this is the\u00a0Hallwachs experiment<\/strong>\u00a0(see diagram). A charged zinc plate is discharged by light knocking electrons out of the plate.
\nAttempts to explain the effect using the wave theory of light run into several difficulties. The experiment is therefore deemed to be evidence for the photon hypothesis.<\/p>\n

What is the problem with using the wave theory of light to explain the photoelectric effect?<\/strong><\/a><\/div>\n
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1.2 Using photons to explain the photoelectric effect<\/h3>\n

According to the concept of the photon, light does not flow away from a light source as electromagnetic energy with a spatially continuous distribution but rather as a multitude of energy portions<\/strong>. In this sense we can call it a \u201cstream of particles\u201d.
\nThe energy equation for the photoelectric effect is:<\/p>\n\n\n\n\n
\"h \cdot f\"<\/strong><\/em><\/td>\n\"=\"\u00a0<\/strong><\/em><\/td>\n\"E_{kin}\"<\/strong><\/em><\/td>\n\"+\"\u00a0<\/strong><\/em><\/td>\n\"W_A\"<\/strong><\/em><\/td>\n\"+\"<\/strong><\/em><\/td>\n\"W_S\"<\/strong><\/em><\/td>\n<\/tr>\n
Photon energy<\/td>\n=<\/td>\nkinetic energy
\nof the electron<\/td>\n		+<\/td>\nwork function<\/td>\n+<\/td>\nenergy released
\nby collisions<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The maximum energy of the electrons released is therefore: \"E_{kin, max} = h \cdot f - W_A\"(Einstein\u2019s equation).<\/p>\n

In the photoelectric effect, photons impinge on a metal surface and are absorbed.
\nThe energy \"h \cdot f\" of a photon is transferred to an electron; the electron can leave the metal with the energy it has gained.<\/strong><\/p>\n

Here are two\u00a0worksheets<\/a> for you to download.<\/p>\n

1.3 Experimental verification of the energy equation and determination of Planck’s constant<\/h3>\n

The energy equation can be checked experimentally with the retarding potential method. To carry out the experiment below with the aid of the interactive on-screen experiment (IBE), download the “IBE zum Photoeffekt-deutsch.exe<\/a>“.<\/p>\n

Experiment 1.3::<\/strong><\/h4>\n

\"\"\"\"<\/p>\n

Planck\u2019s constant \"h\" can be determined from the gradient of the straight line through the measured points (precise measurements yield: \"h = 6.6262 \cdot 10^{-34} \text{Js}\").<\/p>\n

With different materials, the straight lines obtained have the same gradient but different intercepts on the axes. This comes about because they have different, material-dependent work functions.<\/p>\n

Determination of the work function for a metal surface<\/a><\/p>\n

Ways of determining\u00a0h experimentallyh<\/em><\/a><\/p>\n

1.4 Photon momentum<\/h3>\n

The successful interpretation of the photoelectric effect suggests a particle theory for light. This is also supported by the experimental finding that photons have a\u00a0momentum<\/strong>,\u00a0which they can transfer to other particles (e. g. to an electron in the Compton effect).<\/p>\n

A photon of frequency f has <\/strong><\/p>\n

an energy \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0\u00a0\"E = h \cdot f\" <\/strong><\/p>\n

and a momentum \u00a0\u00a0\u00a0\"p = h \cdot f / c\" .<\/strong><\/p>\n

Light exhibits\u00a0particle\u00a0behavior<\/strong>\u00a0under certain circumstances (e. g. in the photoelectric effect), under other circumstances wave\u00a0behavior<\/strong> e.\u00a0g. in interference phenomena).<\/p>\n

\"\"
\nLight – particle or wave?<\/p>\n

Quantum mechanics (or quantum electrodynamics in the case of photons) naturally provides a\u00a0mathematical model<\/strong>\u00a0with which all previous experiments on quantum objects could be described correctly. A mathematical model does not facilitate a graphic understanding, however.<\/p>\n

In the following chapters we will show that the so-called \u201cwave-particle duality\u201d loses much of its mysterious character with Born\u2019s probabilistic interpretation, however.<\/p>\n

1.5 Progress check<\/h3>\n

The following points were important in this chapter:<\/p>\n