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Light, Spectra, and Matter. Energy Transfer:. There are 3 ways to transfer energy from one place to another. Convection: Direct movement of material of one temperature to another location. Conduction: Energy transport by direct contact.
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Energy Transfer: There are 3 ways to transfer energy from one place to another. Convection: Direct movement of material of one temperature to another location. Conduction: Energy transport by direct contact. Radiation: Energy transported by electromagnetic radiation. Which is most efficient? Which is fastest? While energy inside the Sun is transferred in each of these ways, the primary method the Sun uses to get energy into space is light. Light itself is a bundle of energy that follows very specific rules. We call a single one of these ‘bundles’ a photon.
Photons: A photon is really an oscillating combination of electric and magnetic fields. All photons move through space with the same velocity, 3x105 km s-1. Nothing can move faster than light!
Photons: Since a photon is a carrier of energy and it’s velocity is fixed, then how is the amount of energy in a photon manifested? Photons are characterized by the spacing of wave peaks (wavelength l) and the number of oscillations per second (frequency n) The amount of energy is directly proportional to frequency and inversely to wavelength. or E n and E l-1 So how is the energy detectable?
Newton and Color: Isaac Newton is known for many things, but one obscure discovery he made was the spectral nature of sunlight. Newton used a prism to break sunlight into the colors of a rainbow and a second prism to put the colors back together again. Sunlight, it turned out, was not one color, but many… Color is the way that light`s energy can be detected. (Blue is more energetic than Red)
Herschel Thinks Outside the Box: In 1800 William Herschel made a discovery when he tried to determine the temperature of light. He noticed that a thermometer recorded energy from the Sun`s spectrum even when placed beyond the red end of the visible rainbow. He called this emission Calorific Rays and it was the first discovery that light had colors invisible to the human eye. These rays are known today as Infrared light.
The Electromagnetic Spectrum: There are many wavelengths of light that we can’t see. Combined they make up a continuum of photon energies called the electromagnetic spectrum. We use many kinds of ‘invisible’ light in our daily lives. Going back to Newton’s Prism experiment…what ‘colors’ of light does the Sun emit?
The Solar Spectrum: When we look at the Spectrum of the Sun, we see a distinct distribution of colors. Other stars have similar patterns as do most hot objects. The main difference is where the peak color is. Gustav Kirchoff (1862) called this kind of emitter a ‘Blackbody’.
Characteristics of Blackbodies: 1) They produce most of their light in a `color` that is directly proportional to their Temperature. This relationship between T and is called Wien`s Law…. Where max is given in centimeters (10-2 m). Blackbodies: Definition: A Blackbody is an object that radiates energy into space in a manner that is characteristic only of the temperature of the radiator. The Sun and all the planets release energy in a manner very close to that of a Blackbody. Red blue
Blackbodies: Definition: A Blackbody is an object that absorbs all energy incident on it and re-radiates it back into space in a manner that is characteristic only of the temperature of the radiator. The Sun and all the planets release energy in a manner very close to that of a Blackbody. Characteristics of Blackbodies: 2) The total amount of energy (E) produced by a blackbody increases with Temperature to the fourth power (T4). This relationship between T and E is called the Stephan-Boltzmann Law….
UV Visible Infrared From the peak color of a star`s spectrum we can determine its temperature. What`s wrong with this plot? Spica should be brighter than the other stars! What does this tell us about the stars we see at night?
Planets as Blackbodies: The planets are all Blackbodies too. They each have a temperature that`s determined by the energy they receive or emit. Since the Sun is providing all the energy, the Earth is acting as a `photon processor`. What does photon processing imply??? Sunlight with one spectrum enters the system, but photons with a different spectrum are re-emitted? This processing has a profound effect on our environment. This is because the Earth has air, and air is made of matter.
Fraunhofer’s Surprise: Fourteen years after Herschel, Johan von Fraunhofer made an even more interesting discovery. Using a precision dispersing prism, he discovered that the `solar blackbody` was cut by thousands of dark bands.
Fraunhofer’s Surprise: Fraunhofer tried to test whether this effect was real. 1) He tested with different optics. 2) He tested by looking at different objects (moon and planets). The result was always the same…. He had no explanation.
Iron Bunsen`s Solution: Robert Bunsen turned pyromania into one of the great discoveries of modern physics. Bunsen set fire to things in order to figure out what they were made of. He didn`t get far until his friend Gustav Kirchhoff suggested using a prism to break the light apart. They quickly discovered that burning substances produced light in narrow bands with a unique pattern.
Blueprint to Composition: Bunsen and Kirchoff`s trick was the key to finding out the composition of anything from the light it produced. Many of the lines they found had the same wavelength as those of Fraunhofer`s dark bands. They were seeing the composition of the Sun!
Kinds of Spectra: Bunsen found that he could identify the signature of different elements in the Fraunhofer spectrum of the Sun. Why were Bunsen`s spectra composed of bright lines while Fraunhofer`s were dark bands? Bunsen`s fires were stimulating emissions from thermal energy in the hot gas. So what are Fraunhofer`s bands? Absorption in (and re-emission from) a cooler gas!
Kinds of Spectra: Imagine that there is a gas between you and a hot object (the Sun!). The interaction of light with this gas is called `Radiative Transfer`. Over here we see the hot object spectrum with the transitions associated with the gas removed. This is an absorption spectrum. This is what Fraunhofer saw. Down here we see the re-emitted light as the atoms relax to their lowest energy state. This is an emission spectrum. Bunsen saw this.
Atoms and Light: Why do elements have the `discrete` interactions that Bunsen saw? Why do different elements (and molecules) have different interactions? This has to do with the nature of atoms and how they are put together.
