The amount of light emitted by an object in a unit of time is known as

luminosity, in 26 watts (or 3.846 × 10 33 ergs per second). Luminosity is an absolute measure of radiant power; that is, its value is independent of an observer’s distance from an object. Astronomers usually refer to the luminosity of an object in terms of solar luminosities, with one solar luminosity being equal to the luminosity of the Sun. The most luminous 17 solar luminosities. The dim

The sun is an ordinary star, one of about 100 billion in our galaxy, the Milky Way. The sun has extremely important influences on our planet: It drives weather, ocean currents, seasons, and climate, and makes plant life possible through photosynthesis. Without the sun’s heat and light, life on Earth would not exist. About 4.5 billion years ago, the sun began to take shape from a molecular cloud that was mainly composed of hydrogen and helium. A nearby supernova emitted a shockwave, which came in contact with the molecular cloud and energized it. The molecular cloud began to compress, and some regions of gas collapsed under their own gravitational pull. As one of these regions collapsed, it also began to rotate and heat up from increasing pressure. Much of the hydrogen and helium remained in the center of this hot, rotating mass. Eventually, the gases heated up enough to begin nuclear fusion, and became the sun in our solar system. Other parts of the molecular cloud cooled into a disc around the brand-new sun and became planets, asteroids, comets, and other bodies in our solar system. The sun is about 150 million kilometers (93 million miles) from Earth. This distance, called an astronomical unit (AU), is a standard measure of distance for astronomers and astrophysicists. An AU can be measured at light speed, or the time it takes for a photon of light to travel from the sun to Earth. It takes light about eight minutes and 19 seconds to reach Earth from the sun. The radius o...

This new guide will show you everything you need to know about measurement of light. It’s important to understand the different terms used to characterize light. From the measurement of light in the electromagnetic spectrum, to understanding perceived brightness to the human eye, light intensity and the tools used to measure light, this guide covers it all. Want to learn more about light measurement? Get the free PDF Get the PDF version to save to your desktop and read it when it’s convenient for you. Units of Light(Common Light Measurement Terms) The lighting industry uses several different units to measure light, depending on what information is needed. Below are a few of the most common units and terms: Flux (Luminous Flux) – Originating from the Latin word ‘Fluxus,’ meaning flow, flux is the amount of energy a light emits per second, measured in lumens (lm). When it comes to lighting, you need to consider watts (W) (energy used) versus lumens (lm) (brightness). Or electricity consumption versus light output. Lumens are weighted for human perception where as watts are not. Lumen (lm) – The SI unit of luminous flux, this is a unit of light flow. Watt (W) – The unit of measuring electrical power, this is a radiometric measurement. Intensity of Light – the Quantity of visible light that is emitted in unit time per unit solid angle Candela (cd) – The SI base unit of luminous intensity. It is a unit of luminous intensity of a light source in a definitive direction. 1 lumen =...

1 Science and the Universe: A Brief Tour • Introduction • 1.1 The Nature of Astronomy • 1.2 The Nature of Science • 1.3 The Laws of Nature • 1.4 Numbers in Astronomy • 1.5 Consequences of Light Travel Time • 1.6 A Tour of the Universe • 1.7 The Universe on the Large Scale • 1.8 The Universe of the Very Small • 1.9 A Conclusion and a Beginning • For Further Exploration • 3 Orbits and Gravity • Thinking Ahead • 3.1 The Laws of Planetary Motion • 3.2 Newton’s Great Synthesis • 3.3 Newton’s Universal Law of Gravitation • 3.4 Orbits in the Solar System • 3.5 Motions of Satellites and Spacecraft • 3.6 Gravity with More Than Two Bodies • Key Terms • Summary • For Further Exploration • Collaborative Group Activities • 4 Earth, Moon, and Sky • Thinking Ahead • 4.1 Earth and Sky • 4.2 The Seasons • 4.3 Keeping Time • 4.4 The Calendar • 4.5 Phases and Motions of the Moon • 4.6 Ocean Tides and the Moon • 4.7 Eclipses of the Sun and Moon • Key Terms • Summary • For Further Exploration • Collaborative Group Activities • 5 Radiation and Spectra • Thinking Ahead • 5.1 The Behavior of Light • 5.2 The Electromagnetic Spectrum • 5.3 Spectroscopy in Astronomy • 5.4 The Structure of the Atom • 5.5 Formation of Spectral Lines • 5.6 The Doppler Effect • Key Terms • Summary • For Further Exploration • Collaborative Group Activities • 6 Astronomical Instruments • Thinking Ahead • 6.1 Telescopes • 6.2 Telescopes Today • 6.3 Visible-Light Detectors and Instruments • 6.4 Radio Telescopes • 6.5 Observ...

The energy of the incident photon must be equal to the sum of the metal's work function and the photoelectron kinetic energy: E photon = KE electron + Φ \text+\Phi E photon ​ = KE electron ​ + Φ start text, E, end text, start subscript, start text, p, h, o, t, o, n, end text, end subscript, equals, start text, K, E, end text, start subscript, start text, e, l, e, c, t, r, o, n, end text, end subscript, plus, \Phi When light shines on a metal, electrons can be ejected from the surface of the metal in a phenomenon known as the photoelectric effect. This process is also often referred to as photoemission, and the electrons that are ejected from the metal are called photoelectrons. In terms of their behavior and their properties, photoelectrons are no different from other electrons. The prefix, photo-, simply tells us that the electrons have been ejected from a metal surface by incident light. To explain the photoelectric effect, 19th-century physicists theorized that the oscillating electric field of the incoming light wave was heating the electrons and causing them to vibrate, eventually freeing them from the metal surface. This hypothesis was based on the assumption that light traveled purely as a wave through space. (See If a single large wave were to shake the dock, we would expect the energy from the big wave would send the beach balls flying off the dock with much more kinetic energy compared to a single, small wave. This is also what physicists believed would happen if...