Light & Atoms. Electromagnetic [EM] Waves. Light and several other forms of radiation are called electromagnetic waves or electromagnetic radiation.

Transcription

1 Light & Atoms Electromagnetic [EM] Waves Light and several other forms of radiation are called electromagnetic waves or electromagnetic radiation. These have both and electric part and a magnetic part to them We call them waves from the standpoint that they create an electrical disturbance in space All EM waves can travel in space (a vacuum). Sound is not an EM wave and cannot travel in a true vacuum; requiring a medium (collection of atoms) in order to travel. Interestingly enough, real outer space is not a true vacuum and does contain atoms that allow sound to travel in space...however, it is unlikely that you'd be able to hear anything with your ears. Not enough atoms would strike our eardrums to allow the poor sensitivity of our ears to pick anything up. 1

2 Waves The wavelength is the distance between the crests (tops) of the wave The inverse of the wavelength is the frequency; as one goes up the other goes down. The amplitude describes the intensity of the wave Wavelengths are unique to every color. So as the wavelength changes so does the color 2

3 If we combine all the colors of light together we get WHITE Note the Prism used to separate the light! 3

4 Wavelengths of light are VERY small 700 x 10-9 meters ( 700 nm) for RED 400 x 10-9 meters for (400 nm) VIOLET So small in fact that 50 wavelengths of light can fit across the thickness of a sheet of plastic wrap!! The universal symbol for wavelength is: l The Electromagnetic Spectrum As mentioned there are many types of EM waves other than visible light gamma rays, X-rays, ultraviolet rays, visible light, infrared waves, radio waves (including microwaves) 4

5 The list forms what is called the Electromagnetic Spectrum Atmospheric Opacity 5

6 The Speed of Light The velocity (speed) of light in a vacuum is 300,000,000 meters/second (3 x 10 8 m/s) or 186,451 miles/sec. Its universal symbol is c It is the same speed for ALL of the EM waves! So... Light from the Travel time Light takes TIME to get from place to place! Sun to Earth 8.5 minutes Sun to Jupiter 43 minutes Sun to Pluto 5.4 hours Sun to the nearest star 4.3 years Sun to the furthest stars 14 billion years We re basically looking back in time with each observation of an astronomical object! 6

7 Distance vs. Time Since speed and time are related by distance... The speed of light can also be used to measure distance! A light year (ly) is the distance that light travels in one year = 9.5 x meters = 6 trillion miles. We cannot see the individual wavelengths of light since they are so very small. But we can detect them with instruments. What we actually detect are called Photons which are bundles of waves acting together 7

8 Photons carry energy E = hc λ What happens as the wavelength goes down? So what kind of EM wave has the smallest wavelength? What happens as the wavelength goes up? So what kind of EM wave has the largest wavelength? What does this mean for colors?? They are very small Atoms <1 x 10-9 meters Several million could fit across a period! Take note of the seemingly circular orbits of the electrons!! 8

9 The circular orbits of the electrons are not really true...they are more like 3-D electron clouds! The nucleus has a positive electrical charge and the electrons have a negative electrical charge. Electricity is the force that binds them together. Atomic Numbers (Protons) If you examine a nucleus of you find that it, generally, contains several protons; particles that carries one unit of positive electrical charge. The number of protons in a nucleus is called the atomic number and determines which kind of element (sometimes called chemical element) the atom is. 9

10 Several examples include: There are 92 naturally occurring elements, in addition to a few (~20) artificial ones that have been made in laboratories. Number of protons (Atomic number) Element 1 Hydrogen 2 Helium 6 Carbon 8 Oxygen 92 Uranium So what happens if an atom loses or gains a proton? What about the electrons? Electrically speaking, the negative electron has the same charge as the positive proton. In a neutral atom, the number of electrons and protons are the same This gives the atoms a balance 10

11 Ions So what happens if an atom loses or gains an electron? Have you changed what it is??? These are called ions Can you do the same thing by changing the number of protons? Neutrons The nucleus of most atoms also contains neutrons. There are generally the same number of these as the number of protons and the number of electrons A neutron an elementary particle with roughly the same mass as a proton, but it has no charge. So what happens if an atom loses or gains a neutron? Have you changed what it is??? Have you changed its charge (more positive or more negative)? These are called isotopes 11

12 For Example... Carbon has three isotope forms, of which carbon- 12 (6 protons + 6 neutrons) is by far the most common. # of protons # of electrons # of neutrons Carbon Carbon Carbon Only the usual kind of hydrogen does not contain any neutrons in its nucleus. Normal hydrogen consists simply of one proton and one electron. Molecules We sometimes find material in the form of a combinations of two or more atoms. We call these molecules An example of a molecule would be carbon monoxide, which is a combination of one carbon atom and one oxygen atom. The science of how elements interact with each other is called chemistry. 12

13 In an atom an electron can only be in certain "orbits" which correspond to certain values of energy that the electron has Because of the importance of the energies of different orbits, we refer to these electron locations as "Energy Levels" Energy Levels The lowest energy state that an electron can have within an atom is called its ground state and when one of its electrons is in an energy level higher than the ground state we say that the electron is in an excited state. 13

14 Moving Up Energy Levels In order to move an electron from a small orbit, near the nucleus, to a larger orbit, farther away from the nucleus, requires you to give the atom energy. Moving Down Energy Levels If an electron jumps from a large orbit to a smaller one the atom gives out energy. The energy comes out in the form of a photon of electromagnetic radiation. Only certain wavelengths of light can be produced as a result of electrons changing from one orbit to another. 14

15 In Summary... Light and Atoms Since each different chemical element has an entirely different set of energy levels, each element has its own set of wavelengths that it can absorb and emit. Therefore, we can identify a gas by the wavelengths it gives out. 15

16 We can separate the photons into their different colors or wavelengths by using certain devices (prisms or diffraction gratings). Seeing the colors Devices (like these) capable of separating photons into their different colors or wavelengths are referred to as spectroscopes. Spectroscopy Hydrogen Helium We call these Emission Line Spectra...we ll revisit this shortly! 16

17 Temperature Temperature is a measure of the random motions of atoms. As the temperature of something rises the atoms move faster. This motion is why there are different states of matter as well Solid, liquid, gas and plasma Which contribute to the density of the object Solids Particles in a solid are tightly packed (high density), usually in a regular pattern and vibrate (very slow movement) but generally do not move from place to place. 17

18 Particles in a liquid are close together (medium density) with no regular arrangement and vibrate (a bit faster than solids), move about, and slide past each other. Liquids Particles in a gas are well separated (low density) with no regular arrangement and move freely at high speeds. Gases 18

19 The Plasma As the temperature of a solid rises the vibrating of the atoms becomes more violent. Eventually, the vibrations shake the atoms apart from being tightly packed and become first a liquid and then a gas. At the highest temperatures the collisions are so violent that electrons are knocked off of the atoms and the gas becomes ionized. Ionized gas is sometimes called plasma. Temperature Scales In the United States we use the Fahrenheit temperature scale The rest of world uses the Celsius or centigrade temperature scale In astronomy we use the Kelvin, or absolute, temperature scale In the Kelvin scale, the random motions (vibrations) of an atom are proportional to its temperature (like it should be)! Temperature Fahrenheit Celsius Kelvin Symbol F C K Boiling point of water Average human body temperature Average room temperature 68 to to to 298 Melting point of ice Absolute Zero

20 Emission Line Spectrum As we ve seen, gases (low-density substances) radiate emission lines when they are HOT. This is called an emission line spectrum. Astronomical Example Nebula 20

21 Very dense materials (solids, liquids and dense gasses) are too compact to give off emission line spectra when they are hot. These substance s densities cause them to give off a continuous spectrum (a complete rainbow). Continuous Spectrum As the density of a material increases the energy levels of the atoms become changed. Causing the wavelengths of the emission lines to also change. Instead of sharp emission lines we get a broader (fatter) line. As the density increases even more these fat lines blend together to give a continuous spectrum. Why? 21

22 Absorption Spectrum If you have a cool gas in front of a hot continuous source of radiation you get an absorption line spectrum. This is exactly opposite of the corresponding emission spectrum Absorption line spectra are very important in astronomy; these are what the spectra of stars look like. Stars have a cooler gaseous atmosphere in front of a hotter core, which defines how an absorption spectrum is produced. In addition, the atmospheres of planets that receive reflected light can be studied with this technique as well. 22

23 Our Sun Spectra Summary 23

24 An emitting object gives off a range of wavelengths. One of which is more intense than the others Note that as frequency increases the wavelength gets smaller Plank s Law So what does this mean? Wien s Law 24

25 Wien s Law Animation So...what about our Sun? What temperature would correspond to a peak wavelength in the yellow part of the visible spectrum? 25

26 Yep...about 6000 K or about o F Stephan s Law Power = s * R 2 * T 4 Large objects give off more power than small objects Hot objects give off more power than cold objects 26

27 Doppler Effect Motion-induced change in the observed wavelength (or frequency) of a wave (sound or electromagnetic wave) For Example... 27

28 In terms of color... The away motion (yielding longer than normal wavelengths) is called a red-shift The toward motion (yielding shorter than normal wavelengths) is called a blue-shift In terms of sound... A moving race car A car horn at 30 mph 28

29 End Topic Light & Atoms 29