ASTRONOMY 202 Spring 2007: Solar System Exploration

Instructor: Dr. David Alexander Web-site: www.ruf.rice.edu/~dalex/ASTR202_S07

Class 18: Properties of Light [3/2/07]

Announcements

Light and Matter

• Thermal Radiation

The Doppler Effect

Guess the celebrity Chapters 5 & 6

Now Playing: Longview – Green Day Thermal Radiation Thermal Radiation: Rules

Rule 1: Stefan-Boltzmann Law Emitted power per square metre is proportional to the 4 th power of the temperature. Hotter objects emit more total radiation per unit surface area . P = σσσT4

σ = Stefan-Boltzmann constant = 5.7 x 10 -8 Watt/m 2/K 4

Rule 2: Wien’s Law 6 λ ≈ 9.2 x10 max nm Hotter objects emit photons T with higher average energy. λ max = wavelength of maximum intensity Thermal Radiation

Stefan-Boltzmann Law defines intensity P = σσσT4 9.2 x10 6 Wien’s Law defines wavelength of peak of spectrum λ ≈ nm max T Emission and Absorption A complete spectrum

The spectrum of the planet Mars Doppler effect

The Doppler effect only occurs when the motion is along the direction of travel of the light. Doppler effect

An object moving away from us has the waves ‘stretched out’ to a longer wavelength and is said to be redshifted .

An object moving towards us has the waves ‘bunched up’ to a smaller wavelength and is said to be blueshifted .

v λ − λ velocity along line of sight = wavelength shift = 0 speed of light rest wavelength λ c 0 e.g. a star moving away from us at 230 km/s has its Hydrogen-alpha line (656.3 nm) shifted by ~0.5 nm to 656.8 nm. Key Features of Telescopes

Light collecting area Size of telescope defined by its diameter (e.g. 16”, 2m, etc) and the collecting area scales with the square of the diameter. Collecting Area = πd2 Largest optical telescope in the world is 10.4m (Gran Telescopio Canarias )

Angular Resolution Angular resolution is the key to distinguishing features on a distant object or one star from another in a close binary. Larger telescopes tend to have better angular resolution.

Because light is a wave and can interfere there Diff. limit = 2.5x10 5 x ( λ/D) arc seconds is a physical limit to the angular resolution of any telescope (the diffraction limit ). λ – wavelength of light D – diameter of telescope atmosphere Ground-based vs Space-based

Not all wavelengths can penetrate the Earth’s atmosphere.

Different wavelengths penetrate to different depths requiring telescopes in space, particularly to observe the higher energy wavelengths. Angular Resolution

Angular separation of two points is related to their actual separation and r their distance from the observer. r s – separation 360 o dr -- distance distance α = s × 2πr

Angular Resolution of human eye is about one arc minute (60 arc seconds) Round 1 1 2 3

4 5 Round 2 1 2 3

4 5 Round 3 1 2 3

4 5 Round 4 1 2 3

4 5 Answers 1 2 3

Carson Daly Beyonce Ashton Kutcher

4 Renee Zellweger Jon Stewart 5 Telescopes across the spectrum Uranus

Crab nebula Galaxy

Gamma Rays X-ray Europa Sun Infrared

Visible

Extreme Ultraviolet Radio

Radio Galaxy: Fornax A