Unit 4

Atomic Physics and Spectra

The Electromagnetic Spectrum

Waves

• type of disturbance that can propagate or travel.

wavelle lnlgth ()

crest

amplitude (A)

velocity (v)trough

Wavelength () is a distance, so its units are m, cm, or mm, etc.

Period (T): time between crest (or trough) passages

Frequency (n): rate of passage of crests (or troughs

c= n

(units: Hertz or cycles/sec)

Equilibrium position

• Shorter wavelength (l) = higher frequency (n) and therefore higher energy

Waves bend when they pass through material of different densities.

swimming pool

air

water

prismairair

glass

Refraction

All radiation (including visible light) travels as Electromagnetic waves.

Examples of objects with magnetic fields:Magnetthe Earththe Sun

Examples of objects with electric fields:Electrical appliancesLightningProtons, electrons

Electromagnetic Radiation

How we get information about the cosmos

e.g. Visible Light

nc =

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

The Electromagnetic Spectrum

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

Refraction of light

(Prism demo)

Types of Spectra

1. "Continuous" spectrum luminous solid, liquid, or dense gas, emits

light of all wavelengths, produces a continuous spectrum

2. "Emission" spectrum 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 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.

Sodium emission and absorption spectra

The Particle Nature of Light

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

photon energy is proportional to frequency:

example: ultraviolet photons are more dangerous than visible photons.

The Nature of Atoms

The Bohr model of the Hydrogen atom:

_

+

proton

electron

"ground state"

_

+

"excited state"

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

Other elements

Helium Carbon

neutron proton

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

Atmospheric blocking

Radio Waves

• Emitted by TV, radio, stars, galaxies

• Radio wave image of CO gases in our Milky Way galaxy

Microwaves

• Penetrate clouds, fog communication, radar

Infrared• Thermal (heat)

IR image of Milky Way

Visible

• Detection with eyes

Ultraviolet

• Higher energy than visible light

UV image of the Sun

X ray radiation

X ray image of the Sun

Gamma-rays

• Shortest wavelength, therefore highest E• Produced by supernovae, pulsars, black holes, …• Gamma ray bursts (source ???)

• http://science.hq.nasa.gov/kids/imagers/ems/gamma.html

• https://www.e-education.psu.edu/astro801/content/l3_p4.html