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Please turn off your phone

Please turn off your phone. Visible spectrum of the sun. Lecture 10 Astron 1 Instructor: Dr. Babar Ali Book: Chapter 5. Today's Topics. Light. Light is important to astronomers. “Light from the sky is a treasure that links you to the rest of the Universe”

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  1. Please turn off your phone Visible spectrum of the sun. Astron 1

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

  3. Today's Topics • Light Astron 1

  4. 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. Astron 1

  5. James Clerk Maxwell (1831-79) • Scottish physicist. • Investigated the phenomena of Electricity and Magnetism. • Mathematically unified both in 4 elegant equations. Astron 1

  6. James Clerk Maxwell’s Equations • When charged particles oscillate, • They create electromagnetic disturbances • That move through space in the form of waves. • Light! Astron 1

  7. James Clerk Maxwell’s Equations • Synthesized: • laws of Electricity, • laws of Magnetism • laws of the behavior of light. Magnetic cause Electric Fields Current Astron 1

  8. 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. Astron 1

  9. 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 Image: courtesy of wiki commons Astron 1

  10. Properties of a light wave Peak = Crest Trough Astron 1

  11. Properties of a light wave Wavelength Wavelength is a distance. Units of wavelengths are units of distance. Astron 1

  12. Properties of a light wave Direction of propagation at the speed of light Astron 1

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

  14. Properties of light wave • Wavelength is related to frequency: • The wavelength/frequency determines the color of light. lXn= c Astron 1

  15. Light as a particle • Earth 20th 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. Astron 1

  16. Light as a particle • Photon A massless particle that moves like wave. Light = E&M wave stream of photons. • Energy of photon: E = hn • Quantum physics and later refinements explain the wave-particle duality of all objects. Astron 1

  17. Propagation of light • Apparent Brightness of a source decreases with increasing distance in proportion to d2 = Inverse Square Law B~ 1/d2 B1AU~ 1/(1)2= 1/1 B2AU~ 1/(2)2= 1/4 B3AU ~ 1/(3)2= 1/9 Astron 1

  18. 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! Astron 1

  19. E&M Spectrum Astron 1

  20. E&M Spectrum • g (Gamma) Rays: l < 0.01 nm (1 nm = 10-9 m) • Gamma = 3rd letter of Greek alphabet, 3rd 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 < l < 20 nm • penetrates soft tissue to image shadow of bones. • Stopped by large # of atoms in ‘s atmosphere • UV (Ultraviolet): 20 nm < l < 400 nm • Mostly blocked by Ozone • Causes sunburn & skin cancer Astron 1

  21. E&M Spectrum • Visible: 400 nm < l < 700 nm (1 nm = 10-9 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 < l < 106 nm • Nerve endings = sensitive to this band • Absorbed by H2O & CO2 in ’s atmosphere • Radio Waves: few mm < l < 100’s Km • (microwaves, radar, FM & TV, AM) • communication, ovens, airport, military, AM related to Ionosphere Astron 1

  22. E&M Spectrum Astron 1

  23. 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? Astron 1

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