Light is a wave. Light is also a particle! 3/23/09

Transcription

1 Wavelength (λ) is the distance between successive peaks of a wave Frequency (ν) is the number of wave peaks that pass by an observer per second λ ν = the speed of a wave ( = c, for light) Light is a wave Light is also a particle! Light particles are called photons The energy of a photon increases as the wavelength of the light decreases A 100-watt light bulb E photon = h f emits photons per second! 1

2 Which travels fastest? a) Gamma rays b) Ultraviolet c) Visible light d) Radio waves e) They all travel at the same speed Blue light is (compared to red light) a) Faster speed b) Longer wavelength c) Higher energy photons d) Only A and C e) All of the above Which of the following would be true about comparing gamma rays and radio waves? a) The radio waves would have a lower energy and would travel slower than gamma rays. b) The gamma rays would have a shorter wavelength and a lower energy than radio waves. c) The radio waves would have a longer wavelength and travel the save speed as gamma rays. d) The gamma rays would have a higher energy and would travel faster than radio waves. e) The radio waves would have a shorter wavelength and higher energy than gamma rays. 2

3 What is the structure of matter? Ancient Greek philosopher Democritus thought all material were composed of four basic elements: fire, water, earth and air Atoms indivisible not true! More than 100 chemical elements Atomic Structure Protons atomic number Neutrons electrons atomic mass number 3

4 An atom consists of a small, dense nucleus (containing protons and neutrons) surrounded by electrons - Model Proposed by Niels Bohr 1913 A nucleus is about m in size and the first electron orbits out at m from the center of the atom The size of the electron orbit is 100,000 times greater than the size of the nucleus Atoms are mostly empty space 4

5 So if a nucleus the size of an orange (10 cm) was located at the center of the football field, where would the electron be? End Zone? Grandstands? On Campus? In Bowling Green? If the electron s orbit is 100,000 times bigger than the nucleus then the electron would be 10,000 m or 6.21 miles away from the center of the Football Field! TCCW Greenwood mall is about 3.6 miles The electron should be thought of as a distribution or cloud of probability around the nucleus that on average behave like a point particle on a fixed circular path 5

6 Types of Spectra Continuous spectrum Absorption line spectrum Emission line spectrum A Spectral Curve We represent the composition of light with a spectral curve Wavelength (or color) of the light on the horizontal axis Intensity (or energy output per second) on the vertical axis Which color of light is most intense in the figure above? Continuous Spectrum The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption 6

7 Absorption Line Spectrum A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum Emission Line Spectrum A low-density cloud of gas emits light only at specific wavelengths, producing a spectrum with bright emission lines In-class Activities: Types of Spctra Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don t understand what the question is asking, ask me. 7

8 The Sun s spectrum Exam #2 on Thursday Sections Bring pencils and a calculator Where is the Moon in the orbit? Sunlight Sunlight Sunlight Third 8

9 E Sunset W W E Light Midnight Noon from the E W Sun W Sunrise E When does the Moon rise? The position of each phase tells us when its highest in the sky The moon will rise about 6 hours before its highest in the sky and set about 6 hours later Example: The waning crescent moon is high in the sky around 9:00 AM local time Eclipses Solar Eclipses Lunar Eclipses 9

10 We see apparent retrograde motion when we pass by a planet Although Mars moves steadily along its orbit, it appears (from Earth s perspective) to slow to a stop and move westward (retrograde) relative to the background stars as Earth passes it Kepler s First Law: The orbit of each planet around the Sun is an ellipse with the Sun at one focus. The heavens do not move in perfect circles! Kepler s Second Law: As a planet moves around its orbit, it sweeps out equal areas in equal times. 10

11 Kepler s Third Law The size of the orbit determines the orbital period more distant planets orbit the Sun at slower average speeds, obeying the relationship p 2 = a 3 p = orbital period in years a = avg. distance from Sun in AU From Kepler s third law, an asteroid with an orbital period of 8 years lies at an average distance from the Sun equal to (p 2 =a 3 )? a) 2 AU. b) 4 AU. c) 8 AU. d) 16 AU. e) It depends on the asteroid s mass. 11

12 Newton s First Law A body remains at rest or moves in a straight line at a constant speed unless acted upon by an outside (net) force. A rockets will coast in space along a straight line at constant speed. A hokey puck glides across the ice at constant speed until it hits something What are Newton s three laws of motion? Newton s 2nd law of motion: The acceleration of an object is proportional to the force acting on it and inversely proportional to its mass. F = ma A baseball accelerates as the pitcher applies force by moving his arm What are Newton s three laws of motion? Newton s 3rd law of motion: For every force, there is always an equal and opposite reaction force A rocket is propelled upward by a force equal and opposite to the force with which gas is expelled out the back 12

13 Conservation of Momentum Newton s first law The total momentum of interacting objects cannot change unless an external force is acting on them Interacting objects exchange momentum through equal and opposite forces Conservation of Angular Momentum Angular momentum = m v r Orbital Kepler s 2 nd law Conservation of Energy Energy can be neither created nor destroyed. It can change form or be exchanged between objects. The total energy content of the Universe was determined in the Big Bang and remains the same today. 13

14 The Universal Law of Gravitation Note that position B is at the midpoint between the Moon and Earth. At which location would the net force be least? Moon Earth A B C 14

15 Note that position B is at the midpoint between the Moon and Earth. If a spaceship is traveling from Earth to the Moon, At which position would the acceleration of the ship be greatest? At which position would the acceleration of the ship be least? Earth A B C Moon How is mass different from weight? Mass the amount of matter in an object Weight the force that acts upon an object You are weightless in free-fall! Newton s Law and Gravitation Projectile motion Velocity Acceleration Force 15

16 Center of Mass Objects orbiting each other actually revolve around their mutual center of mass Objects of equal mass orbit a point halfway between them Objects of unequal mass orbit a point closer to larger object Escape Velocity If you want to leave Earth never to return, you must give your spaceship a high enough velocity so it will follow an open orbit This velocity is called the escape velocity At Earth s surface, the escape velocity is 11 km/s (40,000 km/hr) Tides and Phases Size of tides depends on phase of Moon 16