Astronomy Picture of the Day

Possible First Pic of Extrasolar Planet

http://news.yahoo.com/s/space/20080915/sc_space/possiblefirstphotoofplanetaroundsunlikestar

Question

The color of visible light is determined by its ____.

A)brightness B) amplitude C)speed D)wavelength

Question

If the wavelength of light increases, while the speed remains constant, the frequency must ____.

A)increase also B) decrease C)remain unchanged

Question

If the wavelength of light increases, while the speed remains constant, the frequency must ____.

A)increase also B) decrease C)remain unchanged

c = f

Question

The bending of light that occurs when moving between media of different densities is called ___.

A)reflection B) refraction C)diffraction D)distortion

Question

The bending of light that occurs when moving between media of different densities is called ___.

A)reflection B) refraction C)diffraction D)distortion

“The change of direction of light as it passes from one medium to another is associated with a change in velocity and wavelength. The energy of the light is unchanged as it passes from one media to another. When visible light in air enters a medium such as glass, the velocity of light decreases to 75% of its velocity in air and in other materials the decrease can be even more substantial.”

fc =

1 nm = 10 -9 m , 1 Angstrom = 10 -10 m

The Electromagnetic Spectrum

Spectroscopy and Atoms

How do you make a spectrum?

When you bend light, bending angle depends on wavelength, or color.

Refraction of light

(Prism demo)

What does the spectrum of an astronomical object's radiation look like?

Many objects (e.g. stars) have roughly a "Black-body" spectrum:

Brightness

Frequency

also known as the Planck spectrum or Planck curve.

Questions

How is temperature related to the amount of energy radiated?

How is temperature related to the color of the object?

(Blackbody Demo)

"cold" dust

"hot" stars

"cool" star

Sun

frequency increases, wavelength decreases

The wavelength of peak emission tells us the temperature of the object!

Types of Spectra

1. "Continuous" spectrum- a luminous solid or liquid, or a sufficiently dense

gas, emits light of all wavelengths and produces

a continuous spectrum

2. "Emission" spectrum- a low-density, hot gas emits light whose

spectrum consists of a series of bright emission lines that are characteristic of the composition of the gas.

1. "Absorption” Spectrum- a cool, thin gas absorbs certain wavelengths

from a continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum.

Pattern of lines is a fingerprint of the element

For a given element, emission and absorption lines occur at the same wavelengths.

Helium discovered in Sun’s spectrum before being found on Earth!

Sodium emission and absorption spectra

The Particle Nature of Light

Light interacts with matter as individual packets of energy, called photons.

cphoton energy is proportional to frequency:

E f (or E 1

example: ultraviolet photons are more harmful than visible photons.

The Nature of Atoms

The Bohr model of the Hydrogen atom:

_

+

proton

electron

"ground state"

_

+

an "excited state"

Text Website Video Clips

When an atom absorbs a photon, it moves to a higher energy state briefly

When it jumps back to lower energy state, it emits photon(s) in a random direction, conserving the total energy of the system!

(video clips)

Other elements

Helium Carbon

neutron proton

Each element has its own allowed energy levels yielding a unique spectral fingerprint.

Questions?

How Many Types of Spectra?

What Causes spectra?

Text Website Tutorial

Spectrum of the Sun

Absorption spectrum

What causes emission/absorption of light at specific wavelengths?