AST 111 Lecture 10

 Just like the “hierarchy” of objects in the universe, so is there one in the structure of matter

 Matter is NOT infinitely divisible ◦ It is made up of discrete particles called atoms.  Evidence: ◦ Brownian motion ◦ Gases combine in whole-number ratios

 We can interact with individual atoms.  STM image of gold is shown

 Interest in matter surged in 1700’s ◦ Alchemists wanted to turn lead into gold  Experiments done by Benjamin Franklin and Robert Millikan showed: ◦ Matter contains two types of charge: + and – ◦ Like charges repel, opposites attract

 J. J. Thomson: ◦ Matter is like plum pudding ◦ Negative charge is embedded in positive charge

 Ernest Rutherford (1900’s): ◦ Shoot a beam of + particles at the “plum pudding atom”, should go right through since + particles all spread out

Most of the beam particles went through the target foil. A few of them bounced right back toward where they came from!

 Dense nucleus holds the + charge  Electrons exist in clouds around the dense nucleus ◦ This cloud is mostly empty space Bohr Model Quantum Mechanics

 Nucleus consists of: ◦ + charges called protons  Neutral charges called neutrons  Number affects stability of nucleus  If like charges attract, why do + charges in the nucleus stick together?

 Electrons can “jump” from one cloud to another  Different clouds can have different energies  By conservation of energy, the atom emits a “photon” (or “energy packet”)  The process can occur in reverse

 Larger: ◦ Molecules  Electron clouds of atoms combine and the atoms form a molecule ◦ Can rearrange in chemical reactions ◦ This is the domain of chemistry  Smaller: ◦ Quarks combine to make up nuclear particles ◦ Forces carried by messenger particles ◦ This is the domain of particle physics

 Plasma ◦ Free electrons move among + ions  Gas ◦ Atoms / molecules do not interact much  Liquid ◦ Atoms / molecules share weak bond, move freely  Solid ◦ Atoms / molecules held in place, often in crystal structure

 For everyday purposes: ◦ Light travels as RAYS ◦ Our eye picks up just a few of these rays

Does the tree in the previous slide create light? If not, how do we see it?

 Light is created ◦ It then interacts with matter, and it is:  Absorbed, or  Transmitted, or  Reflected

White light is a combination of all colors of visible light. If I shine white light on a shirt and see that it’s green, what does that say about the light absorbed by the shirt? What does it say about the light reflected by the shirt? How do we know if the shirt transmits light?

 Double-slit experiment ◦ So far, light sounds like a particle, but… ◦ If light were a particle, what would we expect to see if we shined light on a pair of slits?

 A wave repeats itself in time and space.  What is waving? ◦ It’s an electromagnetic disturbance  The wave moves at the speed of light: ◦ c = 3.0 x 10 8 m/s

Blue light: 400 nm, 750 trillion crests / second Red light: 700 nm, 428 trillion crests / second

 Last century, it was shown (primarily by Einstein) that light acts as either a particle or a wave, depending on the circumstances.  The experiments involved are extremely complicated.  We’ll choose whatever works for the given situation!

 The energy of a photon decreases as wavelength increases ◦ Which has more energy: a blue photon or a red photon? ◦ Which has more energy: an x-ray photon or a blue photon? ◦ Which could cause an electron to jump to a higher level: an x-ray photon or a red photon?

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 Visible light makes up only a small part of the EM spectrum! ◦ IT IS THERE! Our eyes simply don’t respond to it. ◦ Certain materials and devices are found to respond to, say, IR or UV radiation (i.e. radiation that we cannot see).  These are our “eyes” for that part of the spectrum.