Quiz 1• Each quiz sheet has a different 5-digit

symmetric number which must be filled in (as shown on the transparency, but NOT the same one!!!!!)

• Please hand in both the exam and the answer sheets with your name on both

• Question/answer sheets will be handed back on Wednesday after class

• Please remain seated until we begin collecting (20-25 minutes after start)

• Class after quiz

Why is the sky blue ?

The atmosphere scatters the blue light more than red light

Light and Matter• Light is electromagnetic energy, due to

interaction of electrical charges• Matter is made of atoms – equal number

of positive and negative particles• An atom is the smallest particle of an

element; natural element H to U• Atom Nucleus (protons + neutrons),

with ‘orbiting’ electrons• No. of protons in nucleus = Atomic

Number• Science of light Spectroscopy

Radiation and Spectroscopy

• Light is electromagnetic energy

• Propagates as both particles and waves

• Photons – particles of light

• Wavelength = Velocity / Frequency

Light is electromagnetic wave;Does not require a medium to propagate,

unlike water or sound

Wavelength is the distance between successive crests or troughs

Wavelength ()

Speed (c)

Frequency (f) (# waves/second)

Speed = wavelength x frequency c = fFrequency ‘f’ is the number of waves passing a point per second

WAVES: Frequency, Wavelength, Speed

Units of wavelength and frequency

• Frequency is the number of cycles per second• Since speed of light is constant, higher the

frequency the shorter the wavelength and vice-versa

• Wavelengths are measured in Angstroms: 1A = 1/100,000,000 cm = 1/10 nanometer (nm)

• The higher the frequency the more energetic the wave

• Wavelength (or frequency) defines radiation or color

Spectrum

Prism

WhiteLight

Prisms disperse light into its component colors: Red-Violet

Visible Light

• Forms a narrow band within the electromagnetic spectrum ranging from gamma rays to radio waves

• Human eye is most sensitive to which color?

• Yellow. Why?

Light: Electromagnetic SpectrumFrom Gamma Rays to Radio Waves

Gamma rays are the most energetic (highest frequency, shortest wavelength),Radio waves are the least energetic.

Gamma

X-Ray

UV

Visible

DecreasingWavelength ORIncreasingFrequency

Visible light spectrum: Each color is defined by its wavelength, frequency or energy

Red - Blue 7000 - 4000 Angstroms ( 1 nm = 10 A, 1 A = 10-8 cm)Blue light is more energetic than red light

Light also behaves like ‘particles’ called photons Photon energy, frequency, wavelength: E = h f = hc/ Planck’s Law (‘h’ is a number known as Planck’s constant)

Matter and Particles of Light: Quantum Theory

• Light (energy) and matter in motion behave both as waves and particles

• Wave-Particle Duality - Quantum Theory • Particles of light are called photons: E = hf = hc/• Photons of a specific wavelength may be absorbed

or emitted by atoms in matter• Matter is made of different natural elements: lightest

Hydrogen (1 proton), heaviest Uranium (92 protons)• Smallest particle of an element is atom, made up of a

nucleus (protons and neutrons), and orbiting electrons• Electrons and protons attract as opposite electrical

charges, NOT gravitationally like planets and Sun

The Hydrogen Atom

Electron orbitsDiscrete energies

Absorption of light (energy) photon by H-atom

Emission of light photon by H-atomphoton energy color

Series of spectral lines of Hydrogen

Wavelengths of series of lines from Hydrogen

SPECTRAL SIGNATURE OF ELEMENTS

Continuous, Absorption, and Emission Spectra

Brightness and Temperature

• Brightness is related to the total energy emitted, or the luminosity of an object

• The energy emitted is related to the temperature of the object

• B = T4 is a constant)

Stefan-Boltzmann Law

Color Indicates Temperature and Energy of the Source

Objects generally emit radiation at all wavelengths, but mostly at one peak Wavelength depending on their temperature (e.g. blue – hot, red – cool)

Surface T (Sun) = 5600 K “ (Mercury) = 800 K

Blackbody: Perfectabsorber and emitterOf radiation at a givenTemperature T

TEMPERATURE SCALES

Room Temp = 300 K = 27 C = 81 F

Astronomers usually use the Kelvin Scale

K = C + 273 C = (F - 32) x 5/9 ~ (F - 30) / 2 F = (C x 9/5) + 32 ~ C x 2 + 30

The Doppler Effect

• Why does the “pitch” of a police siren differ when, say, a police car is approaching you, or when you are running away from the police (not recommended) ?

• The frequency (the number of sound waves per second) is higher when approaching, and smaller when receding from the source

Doppler Effect in Sound

High Pitch(short waves)

Low Pitch(long waves)

d=1

d=2

d=3

B=1

B=1/9

B=1/4

Brightness decreases inversely as the square of the distance

The Doppler Effect

Velocity c = frequency (f) x wavelength (

Doppler Shift of Wavelengths

• What about the wavelength?• What about light?• Shorter wavelength Blue-shift, • Longer wavelength Red-shift• We can determine the velocity of

astronomical objects, moving away or towards the Earth, by measuring the wavelength of light from the object

• Observed red-shift of galaxies all over the sky shows that galaxies are moving away from one another the Universe is expanding (Hubble’s Law)

Hubble Diagram: Distribution of Galaxies

Hubble’s Law: v = HodVelocity increases with distance

Expanding Universe

• Hubble’s law Universe is expanding

• Universe had a beginning !

• How long ago?

• Age of the universe: 1/Ho (units of time)

• Big Bang !!

• How does one determine distances?

Redshift

Cosmic Microwave Background (CMB): Universal and Uniform Radiation

CMB Properties• The entire universe is filled with extremely

uniform radiation

• CMB radiation corresponds to a fixed temperature of 2.73 K (-270.3 oC or -428.9 oF)

• Blackbody: uniform temperature oven

• CMB radiation is also isotropic same in all directions

• But with extremely slight variations immediately following the Big Bang due to matter

Distribution of Matter in Galaxy

• Stars rotate about the center of galaxy

• Velocity determined by gravity: mass Mc and distance Rc from the center

• KE = PE

• ½ mstar v2 = G Mc mstar / Rc

• Velocity v should decrease with radius Rc

• Surprise !

Evidence of Dark Matter: Rotation Curves of Galaxies

Dark Matter Halo

• Rotation curves are flat out to distances beyond observable galaxies

• Ergo: Galaxies have “dark matter” haloes

• What is dark matter?