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Astronomy & Astrophysics
Ian Shelton

Chapter 5: Light and Matter; Reading Messages from the Cosmos 5.1: Light in Everyday Life Energy and Power - The rate of energy flow is called power, which we measure in units called watts - a power of 1 watt means an energy flow of 1 joule per second: 1 watt = 1 joule/s Light and Power - when we pass white light through a prism, it disperses into a rainbow of colour that we call a spectrum - the basic colours in a rainbowlike spectrum are red, orange, yellow, green, blue and violet - we see white when the basic colours are mixed in roughly equal proportions - light from the sun, or a light bulb is often called white light, because it contains all the colours of the rainbow - black is what we perceive where there is no light and hence no colour - huge range of colours by combining only red, green, and blue light; these three colours are often called the primary colours of vision because they are the colours directly detected by cells in your eyes - you can produce a spectrum with either a prism or a diffraction grating, which is a piece of plastic or glass etched with many closely spaced lines How do light and matter interact? - Emission: a light bulb emits visible light; the energy of the light comes from electrical potential energy supplied to the light bulb - Absorption: when you place your hand near an incandescent light bulb, your hand absorbs some of the light and this absorbed energy warms your hand - Transmission: some forms of matter, such as glass or air, transmits, which means allowing it to pass through - Reflection/ scattering: light can bounce off matter leading to what we call reflection (when bouncing is all in the same general direction) or scattering (when the bouncing is more random) - Materials that transmit light are said to be transparent - Materials that absorb light are called opaque - Materials can also affect different colours of light differently. For example, red glass transmits red light but absorbs other colours, while a green lawn reflects (scatters) green light but absorbs all other colours 5.2 Properties of Light What is Light? Particles and Waves in Everyday Life - Marbles, balls, and individual atoms and molecules are all examples of particles - a particle of matter can sit still or it can move from one place to another - tossing a pebble into a pond generates waves. The waves carry energy outward, but matter does not travel outward. Matter such as a floating leaf or the water itself, bobs up and down and sloshes back and forth as the waves pass by - in essence, a particle is a thing, while a wave is a pattern revealed by its interaction with particles - the wavelength is the distance from one peak to the next (or one trough to the next) - their frequency is the number of peaks passing by any point each second o we then say that the waves have a frequency of three cycles per second o cycles per second often are called hertz (Hz), so we describe this frequency as 3 Hz - the speed of the waves tells us how fast their peaks travel across the pond o because the waves carry energy, the speed essentially tells us how fast the energy travels from one place to another - wavelength x frequency = speed Light as an Electromagnetic Wave - fields associated with forces, such as electric and magnetic fields, describe the strength of the force that nay particle would experience at any point in space o we can describe the forces that changes particles exert on one another in terms of electric fields and magnetic fields - lights waves are vibrations of both electric and magnetic field caused by the motions of charged particles , thus light is an electromagnetic wave - all light travels through empty space at the same speed – the speed of light (represented by the letter c) which is about 300, 000 km per second - because the speed of any wave is its wavelength times its frequency, we find a very important relationship between wavelength and frequency for light: the longer the wavelength, the lower the frequency and vice versa Photons: “Particles” of Light - light behaves as both a wave and a particle - we therefore say that light comes in individual “pieces” called photons, that have properties of both particles and waves - just as a moving baseball carries a specific amount of kinetic energy, each photon of light carries a specific amount of radiative energy - the shorter the wavelength of the light (or equivalently, the higher its frequency), the higher the energy of the photons - to sum up, light is both a particle and wave, an idea we describe by saying that light consists of individual photons characterized by wavelength, frequency, and energy. The wavelength, frequency, and energy are related in a very specific way because all photons travel through space at the same speed –the speed of light What is electromagnetic spectrum? - The spectrum of visible light that splits into the rainbow of colour is only a tiny part of the complete range of light’s wavelengths - Visible light differs from other forms of light only in the wavelength and frequency of the photons - Because we sometimes describe light as an electromagnetic wave, the complete spectrum of light is usually called the electromagnetic spectrum - light itself is often called electromagnetic radiation - visible light has wavelengths ranging from about 400 nanometers at the blue or violet end of the rainbow to about 700 nanometers at the red end (a nanometer is a billionth of a meter) - light with wavelengths somewhat longer than red light is called infrared, because it lies beyond the red end of the rainbow - radio waves are the longest wavelength light. That is, radio waves are a form of light, not a form of sound - the region near the border between infrared and radio waves, where wavelengths range from micrometers to milometers, is sometimes give the name microwaves - light with wavelengths somewhat shorter than blue light is called ultraviolet, because it lies beyond the blue (or violet) end of the rainbow - light with even shorter wavelengths is called xrays and the shortest-wavelength light is called gamma rays - in general, certain types of matter tend to interact more strongly with certain types of light, so each type of light carries different information about distant objects in the universe. That is why astronomers seek to observe light of all wavelengths, from radio waves to gamma rays 5.3 Properties of Matter What is the structure of matter? - All ordinary matter is indeed composed of atoms and the properties of ordinary matter depend on the physical characteristics of their atoms - Atoms come in different types and each type corresponds to a different chemical element Atomic Structure - Each chemical element consists of a different type of atom, and atoms are in turn made of particles that we all protons, neutrons, and electrons - Protons and neutrons are found in tiny nucleus at the center of the atom - The rest of the atom’s volume contains electrons, which surround the nucleus - Although the nucleus is very small compared to the atom as a whole, it contains most of the atom’s mass because proton’s and neutrons are each about 2000 times as massive as an electrons - The properties of an atom depend mainly on the electrical charge in its nucleus o Electrical charge is a fundamental physical property that describes how strongly an object will interact with electromagnetic fields total electrical charge is always conserved, just as energy is always conserved Atomic Terminlogy - Each different chemical element contains a different number of protons in its nucleus. This number is its atomic number - The combined number of protons and neutrons in an atom is called its atomic mass number - Versions of an element with different numbers of neutrons are called isotopes of that element Molecules - The number of different material substances is far greater than the number of chemical elements because atoms can combine to form molecules - Substances composed of molecules with two or more different types of atoms are called compounds. Thus water is a compound What are the phases of matter? - Chemical bond, the name we give to the interactions between electrons that hold atoms in a molecule together Phase Changes in Water - At low temperatures, water molecules have a relatively low average kinetic energy. Each molecule is therefore bound tightly to its neighbors, making the solid structure of ice. As long as the temperature remains below freezing, the water molecules within this rigid structure area always vibrating and higher temperature means greater vibrations - The melting point is the temperature at which the molecules finally have enough energy to break the solid bonds of ice o The molecules can then move much more freely among one another, allowing the water to flow as a liquid - If we continue to heat the water, the increasing kinetic energy of the molecules will ultimately break the bonds between neighboring molecules altogether. The molecules will then be able to move oreely and freely moving particles constitute what we call a gas. Above the boiling point (100 C), all the bonds between adjacent molecules are broken so that the water can exist - How is that our atmosphere can contain water in the gas phase? In other words, some gas (water vapor) is always present along with solid ice or liquid water. The process by which molecules escape from a solid is called sublimation, and the process by which molecules escape from a liquid is called evaporation. Higher temperatures lead to higher rates of sublimation or evaporation Molecular Dissociation and Ionization - The molecules in a gas move freely, but they often collide with one another. As the temperature rises, the molecules move faster and the collisions become more violent. At high enough temperatures, the collisions will be so violet that they can break the chemical bonds holding individual water molecules together. The molecules then split into pieces, a process we call molecular dissociation - At still higher temperatures, collisions can break the bonds holding electrons around the nuclei of individual atoms, allowing the electrons to go free. The loss of one or more negatively charged electrons leaves the remaining atom with a net positive charge. Such charged atoms are called ions. The process of stripping electrons from atoms is called ionization. - At temperatures of several thousand degrees, what once was water becomes a hot gas consisting of freely moving electrons and positively charged ions of hydrogen and oxygen. This type of hot gas, in which atoms have become ionized, is called a plasma - Solid phase –atoms or molecules are held tightly in place - Liquid Phase –atoms or molecules remain together but move relatively freely - Gas Phase –atoms or molecules move essentially unconstrained - Molecular Dissociation –molecules break apart into component atoms - Plasma Phase –free electrons move among positively charged ions - Fully Ionized Plasma –atoms in plasma become increasingly ionized Phases and Pressure - Temperature is the primary factor determining the phase of a substance and the ways in which light interacts with it, but pressure also plays a role - Pressure is the force per unit area pushing on an object’s surface - on earth, enough liquid water has evaporated from the oceans to make water vapour an important ingredient of our atmosphere. Some of these atmospheric water vapour molecules collide with the ocean surface, where they can “stick” and rejoin the ocean –essentially the opposite of evaporation - the greater the pressure created by water vapour molecules in our atmosphere, the higher the rate at which water molecules return to the ocean. This direct return of water vapour molecules from the atmosphere helps keep the total amount of water in Earth’s ocean’s fairly stable How is energy stored in atoms? - To produce light, these objects must somehow transform energy contained in matter into the vibrations of electric and magnetic fields that we call light - Atoms actually contain energy in three different ways: o 1 : by virtue of their mass, they
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