Chapter 5 Light: The Cosmic Messenger 5.1 Basic Properties of Light - - PowerPoint PPT Presentation

Chapter 5 Light: The Cosmic Messenger

5.1 Basic Properties of Light and Matter

Our goals for learning:

• What is light?
• What is matter?
• How do light and matter interact?

What is light?

Light is an electromagnetic wave.

Anatomy of a Wave

Wavelength and Frequency

wavelength × frequency = speed of light = constant

The Electromagnetic Spectrum

Electromagnetic Spectrum

No photons

• The book talks about photons
• Looks this way because of matter

Which type of electromagnetic radiation has the longest wave- length?

1. Ultraviolet 2. Visible 3. X-ray 4. Infrared 5. Radio

violet colored light has a wavelength of 400 nm and red has a wavelength of 700 nm. How do their frequencies compare? How do their speeds compare?

• 1. Violet light is faster
• 2. Red light is faster
• 3. They have the same speed
• 4. You can not compare them

Radio waves have a frequency of about 100 Mhz. X-rays have a frequency of about 10^17 Hz. Which is faster?

• 1. Radio waves are faster
• 2. X-rays are faster
• 3. They have the same speed
• 4. You can not compare them

Atoms

• Greeks - indivisible

Dalton (1800)

• Every element is made of atoms
• All atoms are the same size
• Different elements = different atoms
• Compound = combo of elements

Which of Dalton’s ideas below do we no longer believe?

• 1. Elements contain only one type of atom
• 2. Atoms rearrange in chemical reactions
• 3. Atoms are solid masses that can’t be split

into smaller particles

Which of the following are elements?

1. Plastic 2. Oxygen 3. Carbon 4. Gold 5. Water 6. 1, 2, 3, 4 7. 2, 3, 4, 5 8. 1, 3, 4 9. 2, 3, 4

• 10. All of the above

• J. Thomson (~1900)
• Electron
• Smaller than an atom

Which statement about Thomson’s model of the atom is true?

• 1. The charge on the electrons is far greater

than the positive charge in the atom.

• 2. The total charge carried by the electrons

equals the positive charge in the atom.

Rutherford ~1910

• Alpha particles shot at gold foil

In which two ways did Rutherford change Thomson’s model of the atom?

1. He said the electrons were concentrated in the center of the atom (nucleus) 2. He said that the positive charge was concentrated in the center of the atom (nucleus) 3. He said that the electrons were orbiting the center of the atom (nucleus) 4. 1 and 2 5. 1 and 3 6. 2 and 3

Niels Bohr (1914)

• Light from atoms had specific frequencies
• Bohr model has electrons at certain energies

How did Bohr change Rutherford’s model of the atom?

• 1. Electrons were at the center of the atom
• 2. Electrons could only occupy specific

energy levels

• 3. Electrons could orbit in a complete

random way

Schrodinger and Heisenburg (~1920)

Who proposed the “plum pudding” model of the atom?

• 1. Dalton
• 2. Thomson
• 3. Rutherford
• 4. Bohr

Which statement is correct?

• 1. Most of the volume of an atom is taken up

by a large nucleus

• 2. Atoms are mostly space

What is matter?

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.

How do we see

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

Three Types of Spectra

Illustrating Kirchhof's Laws

Continuous Spectrum

Emission Line Spectrum

Absorption Line Spectrum

Chemical Fingerprints

• Downward

transitions produce a unique pattern of emission lines.

Production of Emission Lines

Chemical Fingerprints

• Because those

atoms can absorb photons with those same energies, upward transitions produce a pattern

• f absorption lines

at the same wavelengths.

Production of Absorption Lines

Production of Emission Lines

Composition of a Mystery Gas

Thought Question

Which letter(s) labels absorption lines?

A B C D E

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

Thought Question

Which letter(s) labels emission lines?

A B C D E

Properties of Thermal Radiation

• 1. Hotter objects emit more light at all frequencies per

unit area.

• 2. Hotter objects have shorter wavelength of highest

intensity

Wien’s Law

Wien’s Laws

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.

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

• f a distant object?

The Doppler Effect

The Doppler Effect

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

• We generally measure the Doppler effect from shifts

in the wavelengths of spectral lines.

Stationary Moving Away Away Faster Moving Toward Toward Faster

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

• bject’s motion toward or away from us.

How a Star's Motion Causes the Doppler Effect

Thought Question

• It is moving away from me.
• It is moving toward me.
• It has unusually long spectral lines.

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?

Measuring Redshift

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?

• light-collecting area
• angular resolution
• Other frequencies (e.g., infrared, ultraviolet)

Bigger is better

• 1. Larger light-collecting area
• 2. Better angular resolution

Bigger is better

Light Collecting Area of a Reflector

Angular Resolution

• The minimum

angular separation that the telescope can distinguish

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

Reflecting telescope Gemini North 8-m

Mauna Kea, Hawaii

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

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.

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

Observing problems due to Earth’s atmosphere

• 1. Light Pollution

Star viewed with ground-based telescope

• 2. Turbulence causes twinkling ⇒ blurs images.

View from Hubble Space Telescope

• 3. Atmosphere absorbs most of EM spectrum

Adaptive optics

• Rapid changes in mirror shape compensate for

atmospheric turbulence.

How is technology revolutionizing astronomy?

Without adaptive optics With adaptive optics

Interferometry

• Increase angular resolution

Very Large Array (VLA), New Mexico

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

What happens to thermal radiation (a continuous spectrum) if you make the source hotter?

• 1. More energy comes out at all wavelengths
• 2. The peak of the spectrum-energy curve

(the wavelength at which most energy is emitted) shifts redward

• 3. The peak of the spectrum-energy curve

shifts blueward

• 4. 1 and 2
• 5. 1 and 3

What is found in the nucleus of atoms?

• 1. Protons with a + charge
• 2. Neutrons with no charge
• 3. Electrons with a – charge
• 4. All of the above
• 5. 1 and 2

When an electron in an atom jumps from a high-energy orbital to a lower-energy one, what happens?

• 1. light is emitted
• 2. light is absorbed
• 3. The atom’s temperature changes
• 4. The atom changes color
• 5. None of the above

What kind of spectrum does a hot gas produce?

• 1. Emission (bright lines)
• 2. Absorption (dark lines)
• 3. Continuous (all the colors of the rainbow)
• 4. Infrared
• 5. Ultraviolet

What kind of spectrum does a hot solid produce?

• 1. Emission (bright lines)
• 2. Absorption (dark lines)
• 3. Continuous (all the colors of the rainbow)
• 4. Infrared
• 5. Ultraviolet

What controls the color of a shirt, a planet, or anything that shines by reflecting light?

• 1. Its temperature
• 2. How well it reflects light of different

colors

• 3. The color of the light hitting it
• 4. 2 and 3
• 5. 1, 2, and 3

By looking at the light of a hot, solid

• bject, you can tell:
• 1. Its temperature
• 2. What it is made of
• 3. Both 1 and 2
• 4. Neither 1 nor 2, without some additional

information

If a source of light is moving away from you, all the wavelengths are:

• 1. Shifted to shorter wavelengths (Doppler

shifted)

• 2. Shifted to longer wavelengths (Doppler

shifted)

• 3. Red shifted
• 4. 2 and 3
• 5. None of the above

What advantages come from putting a telescope in space?

• 1. All wavelengths can be seen, even those

that don’t penetrate the atmosphere

• 2. The image may be sharper, without

moving air to blur it

• 3. You are closer to the stars, for a better

view

• 4. All of the above
• 5. 1 and 2

If you had X-ray vision, then you could read an entire book without turning any pages.

1. Yes, but you would not be able to differentiate between different optical colors. 2. Yes, but all the pages would merge into one. 3. No, a book doesn’t emit X-rays so you wouldn’t see anything. 4. No, the X-rays would be absorbed by the book and you would not be able to read past the cover. 5. No, the words would not stand out so you would just see blank pages.

New technologies will soon allow astronomers to use X-ray telescopes on Earth’s surface.

1. Yes, from the highest mountain tops such as Mauna Kea, Hawaii. 2. Yes, but the resolution will be lower than from space. 3. No, X-rays cannot be focused because of the blurring effect of the atmosphere. 4. No, X-rays are absorbed by the atmosphere and don’t reach the Earth’s surface. 5. No, no such technology exists.