Lecture6

Lecture6 - What is light? Light: The Cosmic Messenger Basic...

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1 Basic Properties of Light and Matter Our goals for learning: What is light? What is matter? How do light and matter interact? Almost everything we know about the universe beyond the solar system comes from the study of “light” and its interaction with matter. Light: The Cosmic Messenger What is light? Newton showed that white light is composed of all the colors of the rainbow. But just as there are sounds we cannot hear, there are colors we cannot see. Visible light is only a tiny part of a vast electromagnetic spectrum of radiant energy. Light is an electromagnetic wave If you could line up electrons they would wiggle up and down as a light wave passed by The Scottish mathematical physicist James Clerk Maxwell predicted the existence of electromagnetic waves that would travel at the same speed as light, but could have different wavelengths . Radio waves and X-rays are just like light. Both are electromagnetic waves. Wavelength x Frequency = c c = 300,000km/s The Electromagnetic Spectrum • Gamma rays – shortest wavelength; highest energy • X-rays – wavelengths about size of atoms • Ultra Violet (UV) • Visible light – the wavelengths our eyes can “detect” • Infra Red (IR) • Microwaves – wavelengths of a few millimeters • Radio waves – longest wavelength; lowest energy All travel at same speed in outer space Only visible light, radio and some infrared waves pass through Earth’s atmosphere
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2 Light is also a particle! From measurements of the photoelectric effect in which light seems to knock electrons out of atoms, Einstein came up with the idea of photons. Photons : “pieces” of light (radiant energy), each with precise wavelength, frequency, and energy . The more compact the photon (smaller wavelength) the higher its energy : in fact, E = h x frequency or E = hc/wavelength Where h is a constant called Planck’s constant. Photons come from electrically charged particles (usually electrons in atoms). What is matter? Atomic structure: atoms have a nucleus and orbiting electrons PROTONS NEUTRONS ELECTRONS Atomic Terminology • Atomic Number = # of protons in nucleus • Atomic Mass Number = # of protons + neutrons Isotopes Isotope: same # of protons but different # of neutrons. ( 4 He, 3 He) Molecules : consist of two or more atoms (H 2 O, CO 2 )
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3 How do light and matter interact? Emission photons are produced Absorption photons are consumed Transmission photons pass through freely Reflection or Scattering photons are re- directed, in the same (reflection) or random (scattering) directions Terminology: • Transparent: transmits light • Opaque: blocks (absorbs) light Interactions of light and matter Thought Question Why is the rose red? a) The rose absorbs red light.
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This note was uploaded on 02/20/2012 for the course ASTRO 3 taught by Professor Mclean during the Spring '11 term at UCLA.

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Lecture6 - What is light? Light: The Cosmic Messenger Basic...

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