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PHY121 Ch 5 Notes - Astronomy Chapter 5 Light and...

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Astronomy Chapter 5: Light and Telescopes Light is both wave and particle Light waves are characterized by a wavelength “λ” and a frequency “v” Frequency is the number of waves that pass a stationary point in 1 second Wavelengths are measured in units of nanometers (1 nm = 10 - 9 m ) or angstrom (1 Å = 10 - 10 = . ) m 0 1 nm Light is the visible form of electromagnetic radiation, which carries energy through space as electric and magnetic waves. Use “light” to refer to electromagnetic radiation that we can see, but visible light is only a small part of a range Range includes visible light, x-rays, and radio waves travels at the speed of light “c”, which is actually the speed of electromagnetic radiation radiation refers to anything that radiates from a source; light (like all electromagnetic radiation) spreads outward from a source a particle of electromagnetic radiation is called a photon You can think of a particle of light, a photon , as a bundle of waves that acts sometimes as a particle and sometimes as a wave. Shorter wavelength photons carry more energy, and longer wavelength photons carry less (Planck’s constant “h” = 6.6262 x 10 - 34 * ) j s The energy of a photon does not depend on the intensity of the light!!! A spectrum is an array of electromagnetic radiation displayed in order of wavelength The electromagnetic spectrum includes gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, and radio waves. Colors of visible spectrum differ in wavelength – red having the longest wavelength and violet the shortest Average λ of visible light = 0.0005 mm. Because so tiny, scientists use the nanometer and angstrom. Visible light wavelengths are between 4000 Å and 7000 Å (400 – 700 nm) Infrared radiation , wavelengths range from 700nm to 1mm; eyes not sensitive to this radiation, but you can feel the heat (i.e. lamp gives off infrared radiation) Radio and infrared radiation have longer wavelengths and carry less energy X-ray, gamma, and ultraviolet radiation(waves shorter than violet) have shorter wavelengths and more energy Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object Note: recall formula for energy of a photon; extremely short wavelength photons (X-rays, gamma rays) have high energies and can be dangerous
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Earth’s atmosphere is transparent in only two atmospheric windows – visible light and radio. Astronomical telescopes use a primary (or objective) lens or mirror to gather light and focus it onto a small image, which can be magnified by an eyepiece . Primary lens bends (refracts) the light as it passes through the glass and brings it to focus on a small inverted image Primary mirror is a concave piece of glass with a reflective surface; forms a small inverted image by reflecting the light Focal length is the distance from the lens or mirror to the image of a distant light source [star]
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