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Lecture 5

Astronomy Week 5 Readings: 5.1-5.4

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

Astronomy Week 4 Readings 10/8/2012 8:35:00 PM 5. Light and Matter 5.1 Light in Everyday Life How do we experience light?  Warmth of sunlight tells us that light is a form of energy  Energy that light carries is called Radiative energy  Energy and Power o Rate of energy flow is called power, which we measure in units called watts  1 watt = 1 joule/s  Light and Color o Spectrum = prism split light into the rainbow of light  See basic colors, ROYGBV  While light is made up of basic colors mixed  Black is perceived when there’s no light, dark o Primary colors of vision are red, green and blue  Directly detected by eyes o Can also produce spectrum with a diffraction grating = piece of plastic or glass etched with many closely spaced lines  CD or DVD How do light and matter interact?  Light interacts with matter in 4 diff. ways: o Emission = process by which matter emits energy in the form of light  Ex. light build emits visible energy o Absorption = process by which matter absorbs Radiative energy  Ex. hands get warm when put near a light bulb o Transmission = process by which matter allows light to pass through  Ex. transparent objects like glass or air transmit light  Opaque objects block (absorb) light o Reflection/scattering = Light can bounce off matter leading to reflection (when the bouncing is all in the same direction) or scattering (when the bouncing is random)  Mirror reflects light on single path  Angle it strikes is the same angle its reflected  Movie screen scatters light  Each person sees light from all parts of the screen  Interactions b/w light and matter determine the appearance of everything around us 5.2 Properties of Light What is Light?  Particles and Waves in Everyday life o Light can act either like a wave or a particle  Particle of matter can sit still or move around from one place to another  Wave is a pattern of motion that can carry energy w/o carrying matter along with it  Waves consist of peaks and troughs  Ex. Leaf in pond rises and falls with tossing a pebble  Waves consist of 3 basic properties:  wave length (distance from one peak or trough to next)  frequency (number of peaks passing by any point each second)  Unit used: Hertz (Hz) = cycles per second  speed (how fast energy travels from one place to other)  wavelength x frequency x speed  Particle = thing, wave = pattern revealed by its interaction with particles  Light as an Electromagnetic Wave o Light wave is a vibration of electric and magnetic fields, in which light interacts with charged participles thru the fields  Concept of field = abstract concept describing how a particle would interact with a force  Ex. idea of gravitational field describes how a particle would react to the local strength of gravity and the idea of electromagnetic field describes how a charge particle would respond to forces from another charge particles o Light is an electromagnetic wave = light that consists of electric and magnetic fields  Ex. electrons move when light passes by, showing that light carries vibrating electric field o All light travels through empty space at speed of light (c) o Longer the wavelength, the lower the frequency & vice versa  wavelength x frequency = speed of light = constant Photons: “Particles” of Light  Light behaves as both a particle and wave o Therefore, light comes in photons = ind. “pieces” that have properties of both particles and waves  Each photo has wavelength and frequency o Energy of photon depends on its frequency Special Topic: What do Polarized Sunglasses Have to Do with Astronomy?  Polarization describes the direction in which a light wave is vibrating  Reflection can change the polarization of light  Polarized sunglasses block light that reflects off of horizontal surfaces What is the electromagnetic spectrum?  Human eyes cannot see most forms of light o Spectrum of visible light that splits into a rainbow of colors is only tiny part of complete range of lights wavelengths  Complete spectrum of entire range of wavelengths of light = electromagnetic spectrum o Light itself is often called electromagnetic radiation  I
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