Matter, Light, and their Interactions

Matter, Light, and their Interactions A Planetary Model of the Atom Almost all astronomical information is obtained through the light we receive from cosmic objects

Announcements n Grades for EXAM 1 are now posted on the OWL Gradebook; for clarifications, please see me. n Homework # 3 starts today; it is due on Thursday October 20 th n The schedule for the Final exam is posted on SPIRE (also announced on the course website) n Quiz # 3 will take place on Thursday, October 20 th

Assigned Reading n Complete Unit 20 n Units 4, 21, and 22

Angular Momentum and Its Conservation

Angular Momentum n It is the `quantity of motion of a spinning (rotating) object. L = m. v. r = P. r (units: kg m 2 s -1 ) n Depends on the geometry, the mass, and the rotational velocity of an object. n Angular momentum is conserved. A spinning wheel wants to keep spinning. A stationary wheel wants to keep still. Conservation is the tendency of a spinning object to keep spinning with the rotation axis in a constant direction n Angular momentum is also a vector quantity this means that the direction of the axis of rotation is resistant to change.

Everyday Examples of the Conservation of Angular Momentum n Riding a bike n Spinning a basketball on your finger n Steering a satellite n A spinning ice skater

Figuring out orbital velocities with angular momentum n The angular momentum of an object (like a planet) moving in a circle (like an orbit!) is: L = m v r = constant v m = mass of planet v = velocity of planet r = orbital radius of planet m r

Kepler s Second Law of Orbits 2. As a planet moves around it s orbit, the closer the planet to the Sun, the higher its speed. 1 month

How to think about the conservation of angular momentum. The angular momentum before is equal to the angular momentum afterwards. For a planet or satellite: Angular Momentum when close Angular Momentum when distant L 1 = m 1 v 1 r 1 L 2 = m 2 v 2 r 2 but L 1 = L 2 Since m 1 =m 2, v 1 r 1 = v 2 r 2 This is Kepler s Second Law

Survey Question L = m v r If Earth orbited the Sun at a distance of ½ AU but with the same angular momentum that it now has, how much faster/slower would its orbital velocity be? 1) ¼ its current value 2) ½ its current value 3) the same as its current value 4) 2 times its current value 5) 4 times its current value

Tools of Astronomy - Part 2 q So far we have investigated the tools that enable us to explain the motions of planets (and stars, galaxies, etc.) q Now we need to investigate the tools that enable us to understand how: q stars shine q we measure the size of the Universe q we understand the life cycles of stars q etc., etc., etc.

Tools of Astronomy: In other words we need to investigate: 1. Matter 2. Light 3. And how they interact with each other

Let s start with Matter n Matter comes in different forms (phases): Solid Liquid Gas Plasma (gas of elementary particles) The particular form we see depends on the temperature and the kind of matter we re looking at. But all matter is made of very simple particles, called atoms.

Atoms A Planetary Model of the Atom Atoms are made of: q electrons (swarming around like planets around the sun), q protons, and neutrons (together they form the nucleus, at the center of the atom like the Sun). The bounding force: the attractive Coulomb (electrical) force between the positively charged nucleus and the negatively charged electrons.

Atomic View of Matter Electrons have negative charge; protons have positive charge; neutrons have no charge; electrons and protons attract each other

Hydrogen Atom Magnified by 10 12 (nucleus)

Energy Levels Electrons can be in different orbits of certain energies, called energy levels. Different atoms have different energy levels, that are called quantized. Quantum means discrete!

Excitation of Atoms v Atoms can do what planets cannot do: change energy levels v Electrons can actually absorb energy and move to a more energetic (further away) state v Similar to a satellite changing orbit. v If the energy can remove the electron from the atom, the atom is called ionized. Now you have an ion (net positive charge) v The least energetic state is called ground state. This is a form of light-matter interaction

Elements and Isotopes Changing the number of protons in the nucleus changes the element (H, He, Li, etc.) Changing the number of neutrons in the nucleus changes the isotope

Survey Question If you could add a proton to an atom to create a new stable, isolated atom, you would have created: 1) an ion 2) an isotope of the original atom 3) a fission reaction 4) a different element

Survey Question If you could add a proton to an atom to create a new stable, isolated atom, you would have created: 1) an ion 2) an isotope of the original atom 3) a fission reaction 4) a different element

States of Matter Solid molecules are pretty much fixed together Liquid molecules move around a bit more Gas molecules move about freely

Atomic View of States of Matter Higher speeds mean higher temperatures

Optical Light

Light is: n Energy Sometimes called radiant energy Think solar power, photosynthesis, the fireplace n It can travel through empty space! n Information the signal received by your car radio the signals received by telescopes staring at stars the signals received by your eyes right now! n It travel at an incredible speed: c=300,000 km/s!

What else is light? n Light is a wave, specifically an electromagnetic wave (remember the `electromagnetic force?); hence: Light= electromagnetic radiation n Light is also a particle, more properly a `bundle of energy called a quantum (a photon) Photons are massless, but they carry energy and react to gravitational fields

Light as a wave n Waves you can see: e.g., ocean waves n Waves you cannot see: sound wave electromagnetic waves Light is an electromagnetic wave

Properties of Waves n Wavelength the distance between crests (or troughs) of a wave. n Frequency the number of crests (or troughs) that pass by each second. n Speed the rate at which a crest (or trough) moves. For light in general: λ ν = c wavelength speed of light = 3x10 5 km/s in vacuum frequency

Visible Light Shorter Wavelength Longer Wavelength

n White light is made up of nearly equal amounts of all frequencies in the visible portion of the spectrum.

But visible light isn t the whole story. It s just a small part of the entire electromagnetic spectrum Short Wavelength (high frequency) (high energy) Long Wavelength (low frequency) (low energy)

Electromagnetic Radiation Short wavelength Long wavelength

Infrared versus visible light Infrared Visible

Light as particles Light comes in quanta of energy called photons little packets of energy. Photons are massless, but they have momentum and they react to a gravitational field.

Intensity More intense = more photons of same frequency More energetic = higher frequency A single photon's energy depends on the wavelength (or frequency) only, not the intensity. But the energy you experience depends also on the intensity (total number of photons). A lot of infrared radiation will make your skin feel warmer than just a little infrared radiation

Wave-particle duality All types of electromagnetic radiation act as both waves and particles. "The two views are connected by the relation E=hν = hc / λ h is the Planck's constant = 6.626 10-34 m 2 kg / s More intense = more photons of same frequency More energetic = higher frequency

Properties of Light n All light travels through (vacuum) space with a velocity = 3x10 5 km/s n The frequency (or wavelength) of photon determines how much energy the photon has (E=hν). n The number of photons (how many) determines the intensity n You cannot have fractions of photons (i.e., you cannot divide the energy at will) n Light can be described in terms of either energy, frequency, or wavelength.

Survey Question: Compared to visible light, radio waves have: n higher energy and longer wavelength n higher energy and shorter wavelength n lower energy and longer wavelength n lower energy and shorter wavelength n all light has the same energy

Sun seen in optical and Ultraviolet Optical X-ray

Survey Question: When exposed to the Sun for too long, you get sunburned because: The infrared radiation from the Sun is too intense The infrared radiation from the Sun is too energetic The visible radiation from the Sun is too energetic The UV radiation from the Sun is too energetic The UV radiation from the Sun is too intense

Survey Question: When exposed to the Sun for too long, you get sunburned because: The infrared radiation from the Sun is too intense The infrared radiation from the Sun is too energetic The visible radiation from the Sun is too energetic The UV radiation from the Sun is too energetic The UV radiation from the Sun is too intense