Chapter 5 Light: The Cosmic Messenger

What is light? Newton showed that white light is composed of all the colors of the rainbow.

Our first key idea is that visible light is only a small part of the complete spectrum of light. You may wish to spend some time explaining the various things shown in this figure… You may also want to repeat this slide at various points to summarize other ideas.

Light is an electromagnetic wave. Use this slide to define wavelength, frequency, speed of light.

You can also go over wavelength, frequency, and speed with this tool from the Light and Spectroscopy tutorial. Anatomy of a Wave

Wavelength and Frequency wavelength  frequency = speed of light = constant

The Electromagnetic Spectrum Use this tool from the Light and Spectroscopy tutorial to go over the inverse relationship between wavelength and frequency. Electromagnetic Spectrum

Particles of Light Particles of light are called photons. Each photon has a wavelength and a frequency. The energy of a photon depends on its frequency.

Wavelength, Frequency, and Energy l  f = c l = wavelength, f = frequency c = 3.00  108 m/s = speed of light E = h  f = photon energy h = 6.626  10−34 joule  s = photon energy

Thought Question The higher the photon energy, the longer its wavelength. the shorter its wavelength. Energy is independent of wavelength.

Thought Question The higher the photon energy, the longer its wavelength. the shorter its wavelength. Energy is independent of wavelength.

What is matter? Use this figure to define the nucleus; protons, neutrons, electrons; scale of atom and “electron cloud.”

Atomic Terminology Atomic Number = # of protons in nucleus Atomic Mass Number = # of protons + neutrons

Atomic Terminology Isotope: same # of protons but different # of neutrons (4He, 3He) Molecules: consist of two or more atoms (H2O, CO2)

How do light and matter interact? Emission Absorption Transmission: Transparent objects transmit light. Opaque objects block (absorb) light. Reflection or scattering

Reflection and Scattering Mirror reflects light in a particular direction. Movie screen scatters light in all directions.

Interactions of Light with Matter Interactions between light and matter determine the appearance of everything around us.

Thought Question Why is a rose red? The rose absorbs red light. The rose transmits red light. The rose emits red light. The rose reflects red light.

Thought Question Why is a rose red? The rose absorbs red light. The rose transmits red light. The rose emits red light. The rose reflects red light.

What are the three basic types of spectra? Continuous Spectrum Emission Line Spectrum Absorption Line Spectrum Spectra of astrophysical objects are usually combinations of these three basic types.

Introduction to Spectroscopy This tool from the Light and Spectroscopy tutorial. Introduction to Spectroscopy

Three Types of Spectra Illustrating Kirchhof's Laws

Continuous Spectrum The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption.

Emission Line Spectrum A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines.

Absorption Line Spectrum A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.

How does light tell us what things are made of? Spectrum of the Sun

Chemical Fingerprints Each type of atom has a unique set of energy levels. Each transition corresponds to a unique photon energy, frequency, and wavelength. Energy levels of hydrogen

Chemical Fingerprints Downward transitions produce a unique pattern of emission lines.

Production of Emission Lines This tool from the Light and Spectroscopy tutorial. Production of Emission Lines

Chemical Fingerprints Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths.

Production of Absorption Lines This tool from the Light and Spectroscopy tutorial. Production of Absorption Lines

Production of Emission Lines This tool from the Light and Spectroscopy tutorial. Production of Emission Lines

Chemical Fingerprints Each type of atom has a unique spectral fingerprint.

Composition of a Mystery Gas This tool from the Light and Spectroscopy tutorial. Use it to show unique chemical fingerprints. Composition of a Mystery Gas

Chemical Fingerprints Observing the fingerprints in a spectrum tells us which kinds of atoms are present.

Example: Solar Spectrum You can use this slide as an example of real astronomical spectra made from a combination of the idealized types. Here we have the continuous (thermal) spectrum from the solar interior; dark absorption lines where the cooler solar atmosphere (photosphere) absorbs specific wavelengths of light.

Thought Question Which letter(s) labels absorption lines? D E

Thought Question Which letter(s) labels absorption lines? D E

Thought Question Which letter(s) labels the peak (greatest intensity) of infrared light? A B C D E

Thought Question Which letter(s) labels the peak (greatest intensity) of infrared light? A B C D E

Thought Question Which letter(s) labels emission lines? D E

Thought Question Which letter(s) labels emission lines? D E

How does light tell us the temperatures of planets and stars?

Thermal Radiation Nearly all large or dense objects emit thermal radiation, including stars, planets, and you. An object’s thermal radiation spectrum depends on only one property: its temperature.

Properties of Thermal Radiation Hotter objects emit more light at all frequencies per unit area. Hotter objects emit photons with a higher average energy. Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler.

Wien’s Law Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler. Wien’s Laws

Thought Question Which is hotter? A blue star A red star A planet that emits only infrared light

Thought Question Which is hotter? A blue star A red star A planet that emits only infrared light

Thought Question Why don’t we glow in the dark? People do not emit any kind of light. People only emit light that is invisible to our eyes. People are too small to emit enough light for us to see. People do not contain enough radioactive material.

Thought Question Why don’t we glow in the dark? People do not emit any kind of light. People only emit light that is invisible to our eyes. People are too small to emit enough light for us to see. People do not contain enough radioactive material.

Interpreting an Actual Spectrum By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.

What is this object? Reflected Sunlight: Continuous spectrum of visible light is like the Sun’s except that some of the blue light has been absorbed—object must look red

What is this object? Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K

What is this object? Carbon Dioxide: Absorption lines are the fingerprint of CO2 in the atmosphere

What is this object? Ultraviolet Emission Lines: Indicate a hot upper atmosphere

What is this object? Mars!

How does light tell us the speed of a distant object? This figure from the book can give an introduction to the Doppler effect. The Doppler Effect

The Doppler Effect Hearing the Doppler Effect as a Car Passes This and the following slides are tools from the Doppler Effect tutorial. Hearing the Doppler Effect as a Car Passes

Explaining the Doppler Effect Understanding the Cause of the Doppler Effect

Same for light The Doppler Effect for Visible Light

Measuring the Shift Stationary Moving Away Away Faster Moving Toward Toward Faster We generally measure the Doppler effect from shifts in the wavelengths of spectral lines.

The amount of blue or red shift tells us an object’s speed toward or away from us: The Doppler Shift of an Emission-Line Spectrum

Doppler shift tells us ONLY about the part of an object’s motion toward or away from us. How a Star's Motion Causes the Doppler Effect

Thought Question I measure a line in the lab at 500.7 nm. The same line in a star has wavelength 502.8 nm. What can I say about this star? It is moving away from me. It is moving toward me. It has unusually long spectral lines.

Thought Question I measure a line in the lab at 500.7 nm. The same line in a star has wavelength 502.8 nm. What can I say about this star? It is moving away from me. It is moving toward me. It has unusually long spectral lines.

Measuring Redshift The Doppler Shift of an Emission-Line Spectrum More tools from the Doppler shift tutorial. The Doppler Shift of an Emission-Line Spectrum

Measuring Redshift Doppler Shift of Absorption Lines

Measuring Velocity Determining the Velocity of a Gas Cloud

Measuring Velocity Determining the Velocity of a Cold Cloud of Hydrogen Gas

How do telescopes help us learn about the universe? Telescopes collect more light than our eyes  light-collecting area Telescopes can see more detail than our eyes  angular resolution Telescopes/instruments can detect light that is invisible to our eyes (e.g., infrared, ultraviolet)

Bigger is better Larger light-collecting area Better angular resolution

Bigger is better Light Collecting Area of a Reflector Tool from the Telescopes tutorial. Light Collecting Area of a Reflector

Angular Resolution The minimum angular separation that the telescope can distinguish Emphasize that at a great distance the lights would look like one light rather than two --- but we could still see them. (Students sometimes think that below a particular angular resolution we see nothing at all.) Angular Resolution Explained using Approaching Car Lights

Angular resolution: smaller is better Effect of Mirror Size on Angular Resolution

Basic Telescope Design Refracting: lenses Refracting telescope Yerkes 1-m refractor

Basic Telescope Design Reflecting: mirrors Most research telescopes today are reflecting Show students the correspondence between the diagram and the photograph (I.e., point out the primary and secondary mirrors). Reflecting telescope Gemini North 8-m

Keck I and Keck II Mauna Kea, Hawaii A few cool telescope slides…

Mauna Kea, Hawaii

Different designs for different wavelengths of light Radio telescope (Arecibo, Puerto Rico)

X-ray telescope: “grazing incidence” optics

Want to buy your own telescope? Buy binoculars first (e.g., 7  35) — you get much more for the same money. Ignore magnification (sales pitch!) Notice: aperture size, optical quality, portability Consumer research: Astronomy, Sky & Telescope, Mercury magazines; Astronomy clubs. Optional if you wish to discuss buying your own telescope; also point students to Special Topic box on p. 124.

Why do we put telescopes into space? It is NOT because they are closer to the stars! Recall our 1-to-10 billion scale: Sun size of grapefruit Earth size of a tip of a ball point pen,15 m from Sun Nearest stars 4,000 km away Hubble orbit microscopically above tip of a ball-point-pen-size Earth Closer to the stars is a very common student misconception.

Observing problems due to Earth’s atmosphere Light Pollution

2. Turbulence causes twinkling  blurs images. Star viewed with ground-based telescope View from Hubble Space Telescope

3. Atmosphere absorbs most of EM spectrum, including all UV and X ray and most infrared.

Telescopes in space solve all 3 problems. Location/technology can help overcome light pollution and turbulence. Nothing short of going to space can solve the problem of atmospheric absorption of light. Chandra X-ray Observatory

How is technology revolutionizing astronomy? Adaptive optics Rapid changes in mirror shape compensate for atmospheric turbulence. Without adaptive optics With adaptive optics

Another example of adaptive optics.

Very Large Array (VLA), New Mexico Interferometry This technique allows two or more small telescopes to work together to obtain the angular resolution of a larger telescope. This is a radio interferometer; interferometry now being used in other wavelengths as well. E.g., two Keck telescopes… Very Large Array (VLA), New Mexico

Aerial view of VLA

The Moon would be a great spot for an observatory (but at what price?).