Light Light, or visible light, is electromagnetic radiation of a wavelength that is visible to the human eye (about 400–700 nm). Light travels in straight lines called rays. Light is also a particle Photons: “pieces” of light, each with precise wavelength, frequency, and energy. Light Facts: Speed of light- 3x108m/s, 300,000,000 m/s or 186,000mps Traveling the speed of light -1.3 sec to the moon -8 minutes to the sun -4 hours to Pluto - 2 months to edge of solar system -4.2 years to closest star -100,000 years across our galaxy -2.5 million years to Andromeda Light Year A light-year or light year (symbol: ly) is a unit of measurement of length, the distance that light travels in a vacuum in one year. •1 lightyear = 9.4605284 × 1015 meters •6 trillion miles= 1.2 x 1012 Closest stars Distance (LY) Star 0.00 Sun (Sol) 4.24 Proxima Centauri (Alpha Centauri C) 4.36 Alpha Centauri (A&B) 5.96 Barnard's Star (Proxima Ophiuchi) 7.78 Proxima Leonis (Wolf 359, CN Leonis) 8.29 Proxima Ursae Majoris (Lalande 21185) 8.58 Sirius (A&B, Alpha Canis Majoris) 8.73 Proxima Ceti (A&B, Luyten 726-8, UV & BL Ceti) 9.68 Proxima Sagittarii (Ross 154, V1216 Sagittarii) 10.32 Proxima Andromedae (Ross 248 , HH Andromedae) 10.52 Epsilon Eridani (Proxima Eridani) 10.74 Proxima Piscis Austrini (Lacaille 9352) 10.92 Proxima Virginis (Ross 128, FI Virginis) 11.27 Proxima Aquarii (A,B&C, EZ Aquarii) 11.37 Proxima Cygni (A&B,61 Cygni) 11.40 Procyon (A&B, Alpha Canis Minoris) 11.52 Proxima Draconis (A&B, Struve 2398) 11.62 GX/GQ Andromedae (Groombridge 34 A&B) 11.82 Epsilon Indi (Proxima Indi) 11.83 Proxima Cancri (DX Cancri ) 11.89 Tau Ceti 11.99 Proxima Horologii (GJ 1061) 12.10 YZ Ceti (LHS 138) Reflection Reflection is the change in direction of a wave front at an interface between two different media so that the wave front returns into the medium from which it originated.. The angle coming in (incidence) must equal the angle out (reflection). Why is the sky blue? It is because blue light from the sun strikes the air molecules and scatters and our eyes perceive it as blue. Refraction Refraction is the change in direction of a wave due to a change in its speed. This is most commonly seen when a wave passes from one medium to another. Why we see the light! A Bohr model of the hydrogen atom, showing an electron jumping between fixed orbits and emitting a photon of energy with a specific frequency Why do we see? Electromagnetic (EM) radiation, also called light even though it is not always visible, is a self-propagating wave in space with electric and magnetic components. Electromagnetic radiation is classified into types according to the frequency of the wave: these types include, in order of increasing frequency; radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays Wavelength wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek letter lambda (λ). Frequency Frequency is a measure of the number of occurrences of a repeating event per unit time. The period is the duration of one cycle in a repeating event. Amplitude The amplitude is a measure of a wave's magnitude of oscillation, that is, the distance between the trough and the crest during one wave cycle. Spectra The word ' spectrum' (the plural of which is 'spectra') comes from a Latin word, spectare, which means 'to make a display out of something'. Thus, emission spectra are produced by thin gases in which the atoms do not experience many collisions (because of the low density). The emission lines correspond to photons of discrete energies that are emitted when excited atomic states in the gas make transitions back to lower-lying levels. A continuum spectrum results when the gas pressures are higher. Generally, solids, liquids, or dense gases emit light at all wavelengths when heated. An absorption spectrum occurs when light passes through a cold, dilute gas and atoms in the gas absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in the same direction as the absorbed photon, this gives rise to dark lines (absence of light) in the spectrum. The spectrograph, fitted to the base of the telescope (seen at the top), breaks light into its component colors and records the spectrum. Light from a star goes straight to the collimator at the bottom of the instrument and is reflected back up to a diffraction grating in the middle. It is then reflected down and to the left to a digital detector for storage and display on a video terminal. University of Arizona Steward Observatory spectrograph photo by J. B. Kaler Fe Text H .