Light and MatterAST 111 Lecture 10

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

Matter

Matter is NOT infinitely divisible

◦ It is made up of discrete particles called atoms.

Evidence:

◦ Brownian motion

◦ Gases combine in whole-number ratios

Structure of Matter

We can interact with individual atoms.

STM image of gold is shown

Atoms

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

Structure of Matter

J. J. Thomson:

◦ Matter is like plum pudding

◦ Negative charge is embedded in positive charge

Structure of Matter

Ernest Rutherford (1900’s):

◦ Shoot a beam of + particles at the “plum pudding atom”, should go right through since + particles all spread out

Structure of Matter

Structure of Matter

Most of the beam particles went through thetarget foil.

A few of them bounced right backtoward where they came from!

Dense nucleus holds the + charge

Electrons exist in clouds around the dense nucleus

◦ This cloud is mostly empty space

Structure of Matter 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?

The Atomic Nucleus

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

Quantum View of the Atom

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

Structure of Matter

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

Phases of Matter

For everyday purposes:

◦ Light travels as RAYS

◦ Our eye picks up just a few of these rays

Light

A Question

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

Light is created

◦ It then interacts with matter, and it is:

Absorbed, or

Transmitted, or

Reflected

Light

More Questions

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?

Light as a Wave

Light as a Wave

Light as a Wave

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 108 m/s

Wave Properties

The Electromagnetic Spectrum

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!

Light as a Particle

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?

Energy of Light

Wave – Particle Duality

=???

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.

The Spectrum of EM Radiation