Electromagnetic waves penetrate metals

Big Bang 7, textbook

238tj9 20 RG 7.1 G 7.1 Competence Area Electromagnetic Waves Some Light Phenomena 29 M AXWELLS Discoveries about the origin and propagation of EM waves made optics a branch of electromagnetism. For us humans, light is special because we can see it. From the point of view of the universe, however, it is only a small part of the electromagnetic spectrum (see Fig. 28.3, p. 15). The entire EM spectrum is often referred to as light, but in this chapter only the visible part is meant by light. This has a wavelength of around 400 to 750nm. As an example, we look at some phenomena in which the electromagnetic wave character of light is in the foreground, and in some places we will generalize these findings for other EM waves as well. 29.1 Metals look shiny Reflection This is about why some materials reflect electromagnetic waves so well and some don't. The fact that metals in particular reflect light so well is due to their freely moving electrons. The actual circumstances are complicated, but the essentials are easy to explain. When light hits a metal surface, the changing electric field causes the electrons on the surface to vibrate (Fig. 29.2). When the light wave hits, the surface emits another light wave, similar to an antenna (see Fig. 28.19, p. 19). That is then the reflected wave that you see. Metals reflect light almost completely. That gives them their shine. Inside, all effects almost cancel each other out. However, light can penetrate metals by a few millionths of a meter. There is evidence of this: What is the Huygens Principle? What does the law of reflection mean? Construct the reflection of a plane wave using the Huygens principle. L For mirrors, metal or metal foils are always used (Fig. 29.1). Why do these reflect so well? Why is there glass in front of the metal layer in a normal mirror, but behind it in a precision mirror? What optical effect occurs with very thin metal layers, such as gold leaf? F1 W1 F2 S1 Fig. 29.1: The large mirror of the Hubble telescope Fig. 29.2: The incoming light wave (only the electric field is shown) excites the electrons to oscillate, which creates a new light wave (dashed line). Inside, the effects cancel each other out (almost) - the wave can penetrate a little. Very thin metal layers are transparent, such as gold leaf (thickness around 10–6 m). Everyday mirrors consist of a not-so-thin metal layer that is applied to the back of a glass plate for protection (F2). With precision mirrors (Fig. 29.1) the refractive effect of the glass would interfere and therefore the metal layer lies in front of the glass. Why is glass actually transparent? Because as an insulator it has no freely moving charges and also hardly absorbs light. This reflection mechanism also applies to other EM waves. Metal scaffolding can interfere with radio or mobile phone reception through reflection. And radar waves are reflected very well on metal, which one tries to avoid with stealth bombers (see Fig. 30.10, p. 29). Summary Metals or substances with freely moving charges reflect EM waves very well because these charges are excited to oscillate and thus generate new EM waves of the same frequency. 29.2 Lifeguards on the beach Refraction and total reflection The refraction of light plays a role in everyday life with all optical devices and of course with your eyes, which are also optical devices. But why is light actually broken? ? Question box Z For testing purposes only - property of the publisher öbv

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