1. NATS 1311 - From the Cosmos to Earth Nuclear Fission Neutron strikes nucleus - breaks it apart into two separate atoms - different elements - releases another two neutrons + energy - if enough material is present (critical mass) - run away reaction leads to explosion - atomic bomb. Controlled reaction - nuclear reactor. In reactor, fuel doesn’t produce as many neutrons - neutron absorber is also used to absorb some of the neutrons - so reaction doesn’t run away. A nuclear reactor cannot explode!

2. NATS 1311 - From the Cosmos to Earth Nuclear Fusion In nuclear fusion - two atoms collide and combine to form single atom/element - tremendous amount of energy released. Velocity or KE of atoms and/or pressure must be large enough to overcome repulsive electromagnetic force of outer cloud of electrons - such as in center of Sun. Mass of resultant atom slightly less (~ 3%) than two original atoms together - mass converted to energy. Note that the fusion reaction in a hydrogen bomb is started by an atomic bomb.

3. NATS 1311 - From the Cosmos to Earth Light

4. NATS 1311 - From the Cosmos to Earth Four Ways in Which Light can Interact with Matter 1. emission – matter releases energy as light 2. absorption – matter takes energy from light 3. transmission – matter allows light to pass through it 4. reflection – matter repels light in another direction

5. NATS 1311 - From the Cosmos to Earth Mirror reflects light at angle equal to incoming angle - Most materials reflect light randomly - scattering. Movie screen scatters light into array of beams that reaches every member of the audience

6. NATS 1311 - From the Cosmos to Earth Materials that transmit light are transparent Materials that absorb light are opaque In general - some combination of reflection, absorption, and transmission Red glass transmits red light - absorbs all other colors Green grass reflects green light - absorbs all other colors

7. NATS 1311 - From the Cosmos to Earth Light What is light? - A vibration in an electromagnetic field through which energy is transported. Light as a wave Light as a particle E = hf photon f = c The dual nature of light or wave-particle duality:

8. NATS 1311 - From the Cosmos to Earth Light as a Wave A wave is a pattern which is revealed by its interaction with particles. Waves on a Pond Animation Wave is moving up and down but not outward - carries energy but not matter.

9. NATS 1311 - From the Cosmos to Earth Anatomy of a Wave Animation

10. NATS 1311 - From the Cosmos to Earth Wavelength and Frequency Animation

11. NATS 1311 - From the Cosmos to Earth Properties of Waves Period: time to complete one cycle of vibration - from crest to crest or trough to trough Frequency (f): number of crests passing a fixed point per second Frequency= 1/period Example: Period = 1/100 = 0.01 sec. Frequency = 100 hertz (cycles/sec.) Amplitude (a): maximum displacement from equilibrium Wave length (l): distance between successive crests Speed (of a wave) (s)= wave length x frequency s= l x f

12. NATS 1311 - From the Cosmos to Earth TYPES OF WAVES Transverse: Vibration or oscillation is perpendicular to direction of propagation of wave. Examples: water wave, vibrating string, light Longitudinal: Vibration or oscillation is in the same direction as propagation of wave. Examples: sound waves, mass on a spring, loudspeaker

13. NATS 1311 - From the Cosmos to Earth ELECTROMAGNETIC WAVES (LIGHT WAVES) Velocity186,000 miles/second 300,000 kilometers/second 3 x 10 6 m/second It takes 1 1/3 second for light to travel from the earth to the moon. It takes 8 1/3 minutes for light to travel from the sun to the earth.

14. NATS 1311 - From the Cosmos to Earth Remember: Light is a vibration in an electromagnetic field through which energy is transported. So electrons can be manipulated by light. Electrons wiggle up and down as light passes by - like the leaf on the pond.

15. NATS 1311 - From the Cosmos to Earth Light as a Wave For a wave, its speed: s = l x f But the speed of light is a constant, c. For light: l x f = c The higher f is, the smaller l is, and vice versa. Our eyes recognize f (or l) as color! Visible Light Waves Animation

16. NATS 1311 - From the Cosmos to Earth Visible light ranges through 7 major colors from long wavelengths (low frequency - red) to short wavelengths (high frequency - violet) - Red, orange, yellow, green, blue, indigo, violet (Roy G Biv)

17. NATS 1311 - From the Cosmos to Earth Light as a Particle (Photon) Light propagates as quanta of energy called photons Photons move with speed of light have no mass are electrically neutral Energy of a photon or electromagnetic wave: E = hf = h c/ l where h = Planck’s constant f = frequency of a light wave - number of crests passing a fixed point in 1 second c = velocity of light l = wavelength of a light wave - distance between successive crests

18. NATS 1311 - From the Cosmos to Earth The Electromagnetic Spectrum Most wavelengths of light can not be seen by the human eye. The visible part of the electromagnetic spectrum lies between ultraviolet and infrared light (between about 400 and 700 nm). The higher the frequency (shorter the wavelength), the higher the photon energy. Radio waves are at the long wavelength end of the spectrum and gamma rays are at the short wavelength end of the spectrum.

19. NATS 1311 - From the Cosmos to Earth Light as Information Bearer Spectrum of a distant object - a spectrum is the amount of energy or intensity at different wavelengths. By studying the spectrum of an object, we can learn its: 1Composition 2Temperature 3Velocity We can separate light into its different wavelengths (spectrum).

20. NATS 1311 - From the Cosmos to Earth Electron Energy Levels Electrons can not have just any energy while orbiting the nucleus. Only certain energy values are allowed. Electrons may only gain or lose certain specific amounts of energy. Each element (atom and ion) has its own distinctive set or pattern of energy levels - holds the key to studying of distant objects in the universe. This diagram depicts the energy levels of Hydrogen. 1 eV (electron volt) = 1.6 X 10 -19 J Electron jumps to higher energy levels can only occur with addition of the particular amounts of energy representing differences between possible energy levels. Energy levels are quantized - study of electron energy levels called quantum mechanics. Atom gains this energy either from KE of another atom colliding with it or from absorption of energy carried by light - falls to lower energy level by emitting light or transfer of energy by collision.

21. NATS 1311 - From the Cosmos to Earth Absorption and Emission. When electrons jump from a low energy shell to a high energy shell, they absorb energy. When electrons jump from a high energy shell to a low energy shell, they emit energy. This energy is either absorbed or emitted at very specific wavelengths, which are different for each atom. When the electron is in a high energy shell, the atom is in an excited state. When the electron is in the lowest energy shell, the atom is in the ground state.

22. NATS 1311 - From the Cosmos to Earth The Hydrogen Atom. The hydrogen atom is the simplest of atoms. Its nucleus contains only one proton which is orbited by only one electron. In going from one allowed orbit to another, the electron absorbs or emits light (photons) at very specific wavelengths. Note - wavelength is often written as and the unit used is an angstrom (A) = 10 -8 m

23. NATS 1311 - From the Cosmos to Earth De-excitation and Emission Animation

24. NATS 1311 - From the Cosmos to Earth Excitation and Absorption Animation

25. NATS 1311 - From the Cosmos to Earth Interaction of Light with Matter So each electron is only allowed to have certain energies in an atom. Electrons can absorb light and gain energy or emit light when they lose energy. It is easiest to think of light as a photon when discussing its interaction with matter. Only photons whose energies (colors) match the “jump” in electron energy levels can be emitted or absorbed. Hydrogen So visible emission spectrum is created when a gas is heated and collisions in gas continually bump electrons to higher energy levels - emit photons of specific wavelength as they fall back to lower levels. Absorption spectrum is produced when white light is passed through cloud of cool gas. Photons of specific wavelengths absorbed as electrons jump to higher energy levels. Emission Spectrum Absorption Spectrum