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Chapter 3: Radiation. Information from the Cosmos. Information from the Cosmos. Until recently, our knowledge of the universe was obtained only by studying the visible light that happened to arrive on Earth. Since the 1930’s, possible to study other types of radiation and particles ---
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Chapter 3: Radiation Information from the Cosmos
Information from the Cosmos • Until recently, our knowledge of the universe was obtained only by studying the visible light that happened to arrive on Earth. • Since the 1930’s, possible to study other types of radiation and particles --- • radio waves, X-rays, gamma rays, cosmic rays, neutrinos, and gravitational radiation. • To understand the methods used to study the cosmos, we must understand the basic nature and behavior of light.
So, what is light? The Historical View • Greeks • 5th century B.C., Socrates and Plato speculated that light was made up of streamers emitted by the eye, acting like antennae (you “see” when antennae make contact with an object). • Pythagoreas believed that light traveled from luminous objects to the eye in the form of tiny particles. • Empedocles taught that light traveled in waves. • Newton championed particle theory of light. • Huygens (Newton’s contemporary) stated that light was a wave; supported the assertion with experimental data showing that, under certain circumstances, light will spread out (diffract) like a wave. • Einstein published a theory (photoelectric effect) in 1905 that proposed light to be massless bundles of concentrated electromagnetic energy - called photons.
So, what is light?continued Is it a particle? • The particle or ray model of light is illustrated by the properties of reflection and refraction. • The wave model of light is illustrated by the properties of reflection, refraction, diffraction, interference, and polarization. Is it a wave? • But there are problems: if light is a wave, and waves need a “medium” such as air or water to carry them, then how can light travel through empty space? It is neither,but it’slike both • The solution was to decide that light was neither a wave nor a particle, but something else which sometimes behaved like them.
Waves and Information • Most of the information around us gets to us in waves. • Sound energy that travels to our ears is in one form of a wave. • Light is energy that comes to our eyes if the form of another type of wave.
What is a Wave? • Wave motion isNOTa mechanical phenomenon because a wave is not amaterial object but a form. • It cannot be assigned a mass, and the concept of acceleration cannot be applied to a wave. • The motion of a wave is vastly different from the motion of the medium in which it travels. In fact, a wave can exist without any movement of matter at all! • So, what is a wave? It is a pattern or form that moves. • It can be • a deformation of a material object (music string or waves on the surface of a body of water) • OR • it can be a pattern in a field ( light or radio waves).
Waves: Standard Dimensions In physics, waves are described by a few standard dimensions. Amplitude A= height of wave above “rest position” Wavelength = length of one cycle Velocity v= speed of wave v = f x Frequency f = how often wave crest passes,longer wavelength means lower frequency
Frequency and Period Frequency: how often a vibration occurs in some interval of time, # vibrations (or cycles) per unit time. units are Hertz (Hz) 1-Hz = 1 vibration/sec = 1 cycle/sec 103 Hz = kHz (AM radio frequencies) 106 Hz = MHz (FM radio frequencies) Period: the time to complete one vibration (or cycle), the inverse of the frequency period = 1 / frequency OR frequency = 1 / period
Questions:Frequency and Period Frequency= 1 / period period= 1 /frequency 1. What is the frequency in cycles per second of a 50-hertz wave? A 50-hertz wave vibrates 50 times per second. 2. The Sears Building in Chicago sways back and forth at a vibration frequency of about 0.1 Hz. What is its period of vibration? The period is 1/frequency = 1/(0.1 Hz) = 1/(0.1 vibrations/s) = 10 s. Thus, each cycle (or vibration) takes 10 seconds.
Wave Speed • The speed of some waves depends on the medium through which the wave travels. • Sound waves travel at speeds of 330 m/s to 350 m/s in air, and about four times as fast in water. • The speed of the wave is related to the frequency and wavelength of the wave. • Wave speed = frequency x wavelength
Wave Types • Two types of waves • transverse • longitudinal Cheerleader demo
Types of waves Transverse waves: the motion of the medium (rope) is at right angles to the direction in which the wave travels. Examples: stretched strings of musical instruments, waves on the surfaces of liquids, some of the waves produced in earthquakes. Although they require no “medium” to travel, electromagnetic waves are also transverse waves. Longitudinal waves: the particles in the medium move along the direction of the wave; travel in solids, liquids, and gases. Examples: sound waves, one type of Slinky wave shown in class, some of the waves produced in earthquakes.
Light as a Wave • Light is a type of electromagnetic radiation that travels through space as a wave. • Electromagnetic waves are fundamentally different from many other waves that travel through material media (sound or water waves). • Electromagnetic waves require NO material medium to travel from place to place. • The wave speed of all types of electromagnetic radiation in a vacuum is called the speed of light. c = 300,000 km/sec
Creating Electromagnetic Waves • All matter is made up of atoms. • Atoms are, in turn, made up of smaller particles: protons, electrons, and neutrons. • Two of the elementary particles that make up atoms possess a property described as electrical charge. • The charges on each are equal and opposite. • electron: - charge • proton: + charge
Charged Particle Interactions Any electrically charged object exerts a force on other charged objects. • Electrons • negatively charged • Protons • positively charged Like charges repel one another. Unlike charges attract.
Charged Particles and Electric Fields An electric field extends outward in all directions from any charged particle. Electric field strength proportional to 1/r2 . If a charged particle moves, its electric field changes. The resulting disturbance travels through space as a wave.
Electromagnetism • A changing electric field produces a magnetic field. • The vibrating electric and magnetic fields are always oriented perpendicular to one another and move together through space. • These fields do not exist as independent entities; rather, they are different aspects of a single phenomenon: electromagnetism(EMR). Together, they constitute an electromagnetic wave that carries energy and information from one part of the universe to another.
Electromagnetic radiation • Light is just one type out of many types of “electromagnetic radiation” (EMR). Electrons accelerate and decelerate Electrons drop to lower energy levels releasing energy in the form of EMR • EMR is produced when electrons decelerate and lose energy (e.g. in a radio transmitter) releasing energy in the form of EMR • or drop from a high energy level in an atom to a lower one and lose energy.
Observing EMR • When EMR is absorbed or detected (e.g. by a leaf, an eye, a telescope or photographic film) the reverse happens. EMR is absorbed by electrons EMR is absorbed by electrons allowing a reaction to take place and is turned into an electrical signal • The energy of the EMR is absorbed by electrons and converted to electrical energy • or it causes an electron to jump to a higher energy level, allowing a chemical reaction to take place
Properties of Light:Reflection and Refraction • An isolated light beam travels in a straight line. • Light can change directions under certain conditions: • Reflection from a surface, • mirrors, objects • Refraction (or bending of a ray of light) as the ray travels from one transparent medium to another. • pencil in a clear glass of water • light through a piece of glass
Properties of Light: Dispersion • Electromagnetic waves interact with the charged particles in matter and travel more slowly in transparent media than in a vacuum. • The change in speed of the light wave causes the wave to refract. • Since the speed of an EM wave in a medium changes with wavelength, the amount of refraction depends on the wavelength. • This effect is called dispersion.
Color of electromagnetic radiation • The human eye interprets difference in frequency as “colour”, and calls the range of frequencies that we can see “visible light”. • 700 nm • 450 nm • wavelength • ultraviolet • infra-red • frequency • 6 x 1014 Hz • There are of course an infinite number of possible frequencies (and wavelengths) for light, but humans can see only a very small “band” of them between the ultraviolet and the infra-red.
Red Orange Yellow Green Blue Violet Visible Spectrum
Properties of Light: Interferenceand Superposition • What happens if two waves run into each other? • Waves can interact and combine with each other, resulting in a composite form. • Interference is the interaction of the two waves. • reinforcing interaction = constructive interference • canceling interaction = destructive interference • Superposition is the method used to model the composite form of the resulting wave.
Interference of Waves Interference: ability of two or more waves to reinforce or cancel each other. Constructive interference occurs when two wave motions reinforce each other, resulting in a wave of greater amplitude. Destructive interference occurs when two waves exactly cancel, so that no net motion remains.
Radiation and Temperature • What determines the type of electromagnetic radiation emitted by the Sun, stars, and other astronomical objects? Temperature • Electromagnetic radiation is emitted when electric charges accelerate, changing either the speed or the direction of their motion. • The hotter the object, the faster the atoms move in the object, jostling one another, colliding with more electrons, changing their motions with each collision. • Each collision results in the emission of electromagnetic radiation- radio, infrared, visible, ultraviolet, x-rays. How much of each depends on the temperature of the object producing the radiation.
Measuring Temperature • Atoms and molecules that make up matter are in constant random motion. • Temperature is a direct measure of this internal motion. • The higher the temperature, the faster (on average) the random motion of particles in matter. • Temperature of an object represents average thermal energy of particles that make up that object.
Opacity of the Atmosphere Half-Absorption Altitude (km) Wavelength (angstroms) • Only a small fraction of the radiation produced by astronomical objects actually reaches our eyes because atoms and molecules in the Earth's atmosphere absorb certain wavelengths and transmit others. • Opacity is proportional to the amount of radiation that is absorbed by the atmosphere.
Doppler Effect • Observers in a fast-moving spacecraft will see the stars ahead of them seem bluer than normal, while those behind are reddened. • The stars have not changed their properties— the color changes are the result of the motion of the spacecraft relative to the stars.
Red-shift and Blue-shift • Wave motion from a source toward an observer at rest with respect to the source. • Waves from a moving source "pile up" in the direction of motion and be "stretched out" on the other side. • An observer situated in front of the source measures a shorter-than-normal wavelength— a blueshift— while an observer behind the source sees a redshift. Apparent wavelength= true frequency = 1 +recession velocity true wavelength apparent frequency wave speed
Review - Chapter 3 • Define the following wave properties: period, wavelength, amplitude, and frequency. • State relationship between wavelength, frequency and wave speed. • Define/describe the following terms as they relate to waves: diffraction, superposition, interference, and dispersion. • What is “c”and why is it special? • What colors combine to make white light? • What do radio waves, infrared radiation, visible light, ultraviolet radiation, X rays, and gamma rays have in common? How are they different? • What is opacity? How does it affect observations on Earth? • What is the Doppler effect? How does it alter observations? • What is temperature?
