nats 1311 - from the cosmos to earth nuclear fission neutron strikes nucleus - breaks it apart into...

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!


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.


Light


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


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


Materials that transmit light are transparent

Materials that absorb light are opaque

In general - some combination of reflection, absorption, and transmissionRed glass transmits red light - absorbs all other colorsGreen grass reflects green light - absorbs all other colors

NATS 1311 - From the Cosmos to EarthLight

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:


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.


Anatomy of a Wave Animation


Wavelength and Frequency Animation

NATS 1311 - From the Cosmos to EarthProperties 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 pointper second

Frequency= 1/period

Example:Period = 1/100 = 0.01 sec. Frequency = 100 hertz (cycles/sec.)

Amplitude (a): maximum displacement fromequilibrium

Wave length (l): distance between successive crests

Speed (of a wave) (s)= wave length x frequencys= l x f


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


ELECTROMAGNETIC WAVES (LIGHT WAVES)

Velocity 186,000 miles/second

300,000 kilometers/second

3 x 106 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.


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.


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


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)


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

whereh = Planck’s constantf = frequency of a light wave - number of crests passing a fixed point in 1 secondc = velocity of lightl = wavelength of a light wave -

distance between successive crests


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.


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:1 Composition2 Temperature3 Velocity

We can separate light into its different wavelengths (spectrum).


Electron Energy LevelsElectrons 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.


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.


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


De-excitation and Emission Animation


Excitation and Absorption Animation

NATS 1311 - From the Cosmos to EarthInteraction 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

nats 1311 - from the cosmos to earth nuclear fission neutron strikes nucleus - breaks it apart into...

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