Fall 2014Astron 1Instructor: Babar Ali Visible spectrum of the sun.

Lecture 10 Astron 1 Instructor: Dr. Babar Ali Book: Chapter 5 Fall 2014Astron 1Dr. Ali

Today's Topics Light Fall 2014Astron 13

Light is important to astronomers “Light from the sky is a treasure that links you to the rest of the Universe” -- taken from an older textbook, All objects in the Universe emit, interact with, or affect light. Modern observational astronomy is primarily the study and analysis of light from extra-terrestrial sources. Solar System is the only place where direct material from extra-terrestrial sources is available. It is important to understand the nature of light. Fall 2014Astron 14

James Clerk Maxwell ( ) Scottish physicist. Investigated the phenomena of Electricity and Magnetism. Mathematically unified both in 4 elegant equations. Fall 2014Astron 15

When charged particles oscillate, They create electromagnetic disturbances That move through space in the form of waves. Light! Fall 2014Astron 16 James Clerk Maxwell’s Equations

Synthesized: –laws of Electricity, –laws of Magnetism –laws of the behavior of light. Fall 2014Astron 17 Magnetic cause Electric Fields Current

Light is Radiation Radiation is the transport of energy in the form of Electromagnetic waves. Light is electromagnetic radiation. Maxwell established light as a wave that requires no medium to be present for its propagation. Fall 2014Astron 18

Light behaves like a wave Maxwell’s findings were consistent with the commonly held beliefs of the time. The observed experiments and behavior indicated that light behaves as a wave: –Interference –Diffraction Fall 2014Astron 19 Image: courtesy of wiki commons

Properties of a light wave Fall 2014Astron 110 Peak = Crest Trough

Properties of a light wave Fall 2014Astron 111 Wavelength Wavelength is a distance. Units of wavelengths are units of distance.

Properties of a light wave Fall 2014Astron 112 Direction of propagation at the speed of light

Properties of a light wave Fall 2014Astron 113 Frequency = how many peaks pass by a fixed point in one second. Unit of frequency is Hertz (Hz), which is inverse seconds (1/second).

Properties of light wave Wavelength is related to frequency: The wavelength/frequency determines the color of light. Fall 2014Astron 114 X = c

Light as a particle Earth 20 th century experiments showed that light also acts like a moving particle Photoelectric effect –Photons provide discreet quanta of energy which can be absorbed by electrons. Compton scattering –Photons carry momentum = a property of particles, not waves. Fall 2014Astron 115

Light as a particle Photon A massless particle that moves like wave. Light = E&M wave stream of photons. Energy of photon: E = h Quantum physics and later refinements explain the wave-particle duality of all objects. Fall 2014Astron 116

Propagation of light Apparent Brightness of a source decreases with increasing distance in proportion to d 2 = Inverse Square Law Fall 2014Astron 117 B ~ 1/d 2 B 1AU ~ 1/(1) 2 = 1/1 B 2AU ~ 1/(2) 2 = 1/4 B 3AU ~ 1/(3) 2 = 1/9

Propagation of light Principle of least time. Light will choose a path that takes the least time to move from point A to point B in space. This is not always a straight line! Fall 2014Astron 118

E&M Spectrum Fall 2014Astron 119

E&M Spectrum  (Gamma) Rays:  < 0.01 nm (1 nm = m) –Gamma = 3 rd letter of Greek alphabet, 3 rd kind discovered from radioactive atoms –Dangerous: brakes up living tissue –(generated in interior of  ’s, death of  ‘s, merger of  ’s, galaxies X Rays: 0.01 nm < < 20 nm – penetrates soft tissue to image shadow of bones. –Stopped by large # of atoms in  ‘s atmosphere UV (Ultraviolet): 20 nm < < 400 nm –Mostly blocked by Ozone –Causes sunburn & skin cancer Fall 2014Astron 120

E&M Spectrum Visible: 400 nm <  < 700 nm (1 nm = m) –where  gives off greatest amount of radiation –Penetrates  ’s atmosphere effectively –In 1672, Newton found (w/prism) that sunlight, (white) = made up of  colors in rainbow IR (infrared=Heat): 1000 nm < < 10 6 nm –Nerve endings = sensitive to this band –Absorbed by H 2 O & CO 2 in  ’s atmosphere Radio Waves: few mm < < 100’s Km –(microwaves, radar, FM & TV, AM) –communication, ovens, airport, military, AM related to Ionosphere Fall 2014Astron 121

E&M Spectrum Fall 2014Astron 122

Review Topics Properties of light: wavelength, frequency, speed. What determines the color of light. E&M spectrum. All parts of it. How does light propagate? Fall 2014Astron 123