Interaction Of Light With Matter-Classical Physics-Handouts, Lecture notes of Classical Physics

Download Interaction Of Light With Matter-Classical Physics-Handouts and more Lecture notes Classical Physics in PDF only on Docsity! PHYSICS –PHY101 VU © Copyright Virtual University of Pakistan 98 Summary of Lecture 32 – INTERACTION OF LIGHT WITH MATTER 1. In this lecture I shall deal with the 4 basic ways in which light interacts with matter: a) Emission - matter releases energy as light. b) Absorption - matter takes energy from light. c) Transmission - matter allows light to pass through it. d) Reflection - matter repels light in another direction. 2. When an object (for example, an iron rod or the filament of a tungsten bulb) is heate o o o d, it emits light. When the temperature is around 800 C, it is red hot. Around 2500 C it is yellowish-white. At temperatures lower than 800 C, infrared (IR) light is emitted but our eyes cannot see this. This kind of emission is called blackbody radiation. Blackbody radiation is continuous - all wavelengths are emitted. However most of the energy is radiated close to the peak. o As you can see in the graph, the position of the peak goes to smaller wavelengths (or higher frequencies) as the object becomes hotter. The scale of temperature is shown in degrees Kelvin ( )K o o. To convert from C to , simply add 273. We shall have more to say about the Kelvin scale later. K 3 max Where exactly does the peak occur? Wien's Law states that 2.90 10 m K. We can derive this in an advanced physics course, but for now you must take this as given. Tλ −= × docsity.com PHYSICS –PHY101 VU © Copyright Virtual University of Pakistan 99 3. In the lecture on electromagnetic waves you had learnt that these waves are emitted when charges accelerate. Blackbody radiation occurs for exactly this reason as well. When a body is heated up, the electrons, atoms, and molecules which it contains undergo violent random motion. Light may emitted by electrons in one atom and absorbed in another. Even an empty box will be filled with blackbody radiation because the sides of the box are made up of material that has charged constituents that radiate energy when they undergo acceleration during their random motion. 4. When can you use Wien's Law? More generally, when can you expect a body to emit blackbody radiation? Answer: only for objects that emit light, not for those that merely reflect light (e.g. flowers). The Sun and other stars obey Wien's Law since the gases they are composed of emit radiation that is in equilibrium with the other materials. Wien's law allows astronomers to determine the temperature of a star because the wavelength at which a star is brightest is related to its temperature. 5. All heated matter radiates energy, and hotter objects radiate more energy. The famous Stefa 4 n-Boltzman Law, which we unfortunately cannot derive in this introductory course, states that the power radiated per unit area of a hot body is , where the Stefan- Boltzman constant is = P Tσ σ = -8 -2 -4 4 sun 5.67 10 W m K . 6. Let us apply for finding the temperature of a planet that is at distance from the sun. The sun has temperature T and radius . In equilibrium, the energy resun P T R R σ × = 4 2 0 2 ceived from the sun is exactly equal to the energy radiated by the planet. Now, the total energy radiated 1 by the sun is 4 . But on a unit area of the planet, only of this is re 4sun T R R σ π π × 4 2 0 2 4 0 ceived. 1 So the energy received per unit area on the planet is 4 . This must be 4 equal to . 7. The above was for blackbody radiation where the emitted light has a sun sun T R R RT T T R σ π π σ × × ⇒ = continuous spectrum. But if a gas of identical atoms is excited by some mechanism, then only a few discrete wavelengths are emitted. Each chemical element produces a very distinct pattern of colors called an emission spectrum. So, for example, laboratory hydrogen gas lamps emit 3 lines in the visible region, as you can see below. Whenever we see 3 lines spaced apart in this way, we immediately know that hydrogen gas is present. It is as good as the thumbprint of a man! docsity.com