AST101H Chapter 5 Light and Matter Notes

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

October 10, 2011 Chapter 5: Light and Matter 5.1 Light in Everyday Life  Light is a form of energy  The energy that light carries is called radiative energy  We measure energy in units of joules  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 o 1 watt = 1 joule/s - Light comes in different forms called colors - The basic colors in a rainbowlike spectrum are red, orange, yellow, green, blue, and violet - We see white when the basic colors are mixed in roughly equal proportions - Light from the sun or a light bulb is called white light, because it contains all the colors of the rainbow - Black is what we perceive when there is no light and hence no color - Primary colors include red, green, and blue light; they are the colors 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 - Light can interact with matter in four basic ways: 1. Emission: a light bulb emits visible light; the energy of the light comes from electrical potential energy supplied to the light bulb 2. Absorption: i.e. when you place your hand near an incandescent light bulb, your hand absorbs some of the light, and this absorbed energy warms your hand 3. Transmission: some forms of matter, such as glass or air, transmit light, which means allowing it to pass through 4. Reflection/scattering: light can bounce off mater, leading to reflection (when the bouncing is all in the same general direction) or scattering (when the bouncing is more random) - Materials that transmit light are transparent, materials that absorb light are opaque - Materials can also affect different colors of light differently (i.e. red glass transmits red light but absorbs other colors) - All the information that light brings to Earth from the universe was encoded by one of the four basic interactions between light and matter common to our everyday experience 5.2 Properties of Light - Newton guessed that light is made up of countless tiny particles, but scientists demonstrated that light behaves like waves - A particle of matter can sit still or it can move from one place to another - A particle is a thing, while a wave is a pattern revealed by a its interaction with particles - Waves are characterized into three basic properties: 1. Wavelength: the distance from one peak to the next (or one trough to the next) 2. Frequency: the number of peaks passing by any point each second o Cycles per second are often called hertz (Hz) 3. Speed: how fast the peaks travel October 10, 2011 - The waves carry energy; therefore the speed tells us how fast the energy travels from one place to another - Wavelength x frequency = speed - The vibrations of matter allow the waves to transmit energy from one place to another - Fields associated with forces, such as electric and magnetic fields, describe the strength of the force that any particle would experience at any point in space - Light waves are vibrations of both electric and magnetic fields caused by the motions of charged particles - Light is an electromagnetic wave - The vibrations of the electric field in an electromagnetic wave will cause an electron to bob up and down - 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/s - The longer the wavelength, the lower the frequency, and vice versa - Light behaves as both a wave and a particle - Light comes individual pieces called photons, that have properties of both particles and waves - Photos of light can be counted individually and can hit a wall one at a time; each photon is characterized by a wavelength and a frequency - Each photon of light carries a specific amount of radiative energy - The shorter the wavelength of the light, the higher the energy of the photons - The spectrum of visible light that splits into the rainbow of color is only a tiny part of the complete range of light’s wavelengths - Visible light differs from other forms of light only in wavelength and frequency of photons - The complete spectrum of light is called the electromagnetic spectrum, light itself is called electromagnetic radiation - Light with wavelengths somewhat longer than red light is called infrared - Radio waves are the longest wavelength light (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 millimeters, is called microwaves - Light with wavelengths somewhat shorter than blue light is called ultraviolet - Light with even shorter wavelengths is called X rays, the shortest-wavelength light is called gamma rays THE ELECTROMAGNETIC SPECTRUM Gamma rays | X rays | Ultraviolet | Visible | Infrared | Radio Wavelength -- shorter longer -- Frequency -- higher lower --- October 10, 2011 5.3 Properties of Matter - Atoms = indivisible - All ordinary matter is 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 - Atoms are made of particles called protons, neutrons, and electrons - Nucleus contains most of the atom’s mass, because protons and neutrons are each about 2000 times as massive as an electron - The properties of an atom depend mainly on the electrical charge in its nucleus - 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 - Proton = positive, electron = negative, neutron = no charge - Oppositely charged particles attract and similarly charged particles repel - Electrons do not orbit the nucleus - Atomic number = the number of protons in a nucleus - 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 - The number of different material substances is far greater than the number of chmical elements because atoms can combine to form molecules - Substances composed of molecules with two or more different types of atoms are called compounds - Three phases of matter: solid, liquid, gas - Chemical bonds = interactions between electrons that hold the atoms in a molecule together - 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 (h
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