Chapter 2: Light and Matter Electromagnetic Radiation (How we get most of our information about the cosmos) Examples of electromagnetic radiation: Light Infrared Ultraviolet Microwaves AM radio FM radio TV signals Cell phone signals X-rays

Question 2 a) wavelength b) frequency c) period d) amplitude e) energy The distance between successive wave crests defines the ________ of a wave. Answer: a

Question 2 a) wavelength b) frequency c) period d) amplitude e) energy The distance between successive wave crests defines the ________ of a wave. Light can range from short-wavelength gamma rays to long-wavelength radio waves.

Properties of a wave Context: Example from Sound waves wavelength (l) crest amplitude (A) velocity (v) trough Context: Example from Sound waves Amplitude => Volume Frequency (n) => Pitch, determines which note you hear Shape of wave => what makes a piano sound different from a trumpet

Waves bend when they pass through material of different densities. Things that waves do 1. Refraction Waves bend when they pass through material of different densities. air water swimming pool prism glass air air

2. Diffraction Waves bend when they go through a narrow gap or around a corner.

3. Interference Waves can interfere with each other

4. Wave Speed depends on medium – mechanical waves must have a medium to travel in Example: Speed of sound In air 340 m/s or 760 MPH at 20 deg C (varies with temperature) In Water 1497 m/s at Sound speed depends on elasticity (stiffness) of the material => Bell Jar Demonstration

Applies to all kinds of waves, not just radiation. The Doppler Effect Applies to all kinds of waves, not just radiation. at rest velocity v1 velocity v1 velocity v2 you encounter more wavecrests per second => higher frequency! velocity v1 velocity v3 fewer wavecrests per second => lower frequency!

Demo: buzzer on a moving arm Demo: The Doppler Ball Doppler Effect Demo: buzzer on a moving arm Demo: The Doppler Ball

The frequency or wavelength of a wave depends on the relative motion of the source and the observer.

} Radiation travels as Electromagnetic waves. That is, waves of electric and magnetic fields traveling together. Examples of objects with magnetic fields: a magnet the Earth Clusters of galaxies Examples of objects with electric fields: Power lines, electric motors, … Protons (+) Electrons (-) } "charged" particles that make up atoms.

Scottish physicist James Clerk Maxwell showed in 1865 that waves of electric and magnetic fields travel together => traveling “electromagnetic” waves.

The speed of all electromagnetic waves is the speed of light. c = 3 x 10 8 m / s or c = 3 x 10 10 cm / s or c = 3 x 10 5 km / s light takes 8 minutes Earth Sun c = l n or, bigger l means smaller n

A Spectrum Demo: White light and a prism Demo: Spectrum of the Sun

Refraction of light All waves bend when they pass through materials of different densities. When you bend light, bending angle depends on wavelength, or color.

The Electromagnetic Spectrum 1 nm = 10 -9 m , 1 Angstrom = 10 -10 m c = l n

Clicker Question: Compared to ultraviolet radiation, infrared radiation has greater: A: energy B: amplitude C: frequency D: wavelength

Clicker Question: The energy of a photon is proportional to its: A: period B: amplitude C: frequency D: wavelength

Many objects (e.g. stars) have roughly a "Black-body" spectrum: We form a "spectrum" by spreading out radiation according to its wavelength (e.g. using a prism for light). What does the spectrum of an astronomical object's radiation look like? Many objects (e.g. stars) have roughly a "Black-body" spectrum: Asymmetric shape Broad range of wavelengths or frequencies Has a peak Brightness Frequency also known as the Planck spectrum or Planck curve.

Approximate black-body spectra of astronomical objects demonstrate Wien's Law and Stefan's Law cold dust hotter star (Sun) “cool" star very hot stars frequency increases, wavelength decreases

Laws Associated with the Black-body Spectrum Wien's Law: 1 T lmax energy α (wavelength at which most energy is radiated is longer for cooler objects) Stefan's Law: Energy radiated per cm2 of area on surface every second α T 4 (T = temperature at surface) 1 cm2

Betelgeuse Betelgeuse Temp = 3600 K, 2.8 AU radius, 60000 L_sun, 15 solar masses Rigel Temp = 11,000 K, 17 solar masses, 66000 L_sun Rigel

Betelgeuse Betelgeuse Temp = 3600 K, 2.8 AU radius, 60000 L_sun, 15 solar masses Rigel Temp = 11,000 K, 17 solar masses, 66000 L_sun

The total energy radiated from entire surface every second is called the luminosity. Thus Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2) For a sphere, area of surface is 4pR2, where R is the sphere's radius.

Clicker Question: A star much colder than the sun would appear: A: red B: yellow C: blue D: smaller E: larger

Types of Spectra 1. "Continuous" spectrum - radiation over a broad range of wavelengths (light: bright at every color). 2. "Emission line" spectrum - bright at specific wavelengths only. 3. Continuous spectrum with "absorption lines": bright over a broad range of wavelengths with a few dark lines.

Kirchhoff's Laws 1. A hot, opaque solid, liquid or dense gas produces a continuous spectrum. 2. A transparent hot gas produces an emission line spectrum. 3. A transparent, cool gas absorbs wavelengths from a continuous spectrum, producing an absorption line spectrum.

Sodium emission and absorption spectra The pattern of emission (or absorption) lines is a fingerprint of the element in the gas (such as hydrogen, neon, etc.) For a given element, emission and absorption lines occur at the same wavlengths. Sodium emission and absorption spectra

Demo - Spectra Demo - Spectrum of the sun

Spectrum of Helium (He) Gas Discovered in 1868 by Pierre Jannsen during a solar eclipse Subsequently seen and named by Norman Lockyer

Example: spectra - comet Hyakutake

The Particle Nature of Light On microscopic scales (scale of atoms), light travels as individual packets of energy, called photons. c photon energy is proportional toradiation frequency: E  (or E   ) example: ultraviolet photons are more harmful than visible photons. 1 

The Nature of Atoms The Bohr model of the Hydrogen atom: electron _ _ + + proton "ground state" an "excited state" Ground state is the lowest energy state. Atom must gain energy to move to an excited state. It must absorb a photon or collide with another atom.

But, only certain energies (or orbits) are allowed: _ _ _ a few energy levels of H atom + The atom can only absorb photons with exactly the right energy to boost the electron to one of its higher levels. (photon energy α frequency)

When an atom absorbs a photon, it moves to a higher energy state briefly When it jumps back to lower energy state, it emits a photon - in a random direction

Other elements Helium Carbon neutron proton Atoms have equal positive and negative charge. Each element has its own allowed energy levels and thus its own spectrum.

So why absorption lines? . . . . . cloud of gas . . . . . . The green photons (say) get absorbed by the atoms. They are emitted again in random directions. Photons of other wavelengths go through. Get dark absorption line at green part of spectrum.

Why emission lines? hot cloud of gas . . . . . . - Collisions excite atoms: an electron moves into a higher energy level - Then electron drops back to lower level - Photons at specific frequencies emitted.

Ionization Two atoms colliding can also lead to ionization. Hydrogen _ + Energetic UV Photon _ Helium + + Energetic UV Photon _ Atom "Ion" Two atoms colliding can also lead to ionization.

Clicker Question: Astronomers analyze spectra from astrophysical objects to learn about: A: Composition (what they are made of) B: Temperature C: line-of-sight velocity D: Gas pressures E: All of the above

Clicker Question: Ionized Helium consists of two neutrons and: A: two protons in the nucleus and 1 orbiting electron B: two protons in the nucleus and 2 orbiting electrons C: one proton in the nucleus and 1 orbiting electron D: one proton in the nucleus and 2 orbiting electrons E: two protons in the nucleus and 3 orbiting electrons