Chapter 5Light: The Cosmic Messenger

There is a lot of information about distant objects, encoded in light. By analyzing light we can learn what distant objects are made of, how hot they are, how fast are they moving…

Before we understand the messages carried by light we have to learn what light is and how matter and light interact.

Also, in this chapter we will learn more about the telescopes and techniques they are using.

5.1 Basic Properties of Light and Matter

Our goals for learning:

a. What is light?

b. What is matter?

c. How do light and matter interact?

a. What is light?

One of the first things people noticed about light:

When passed through the prism it produces rainbow, or the spectrum of light

Experiments performed by Newton in the 1660s provided first insight into the nature of light.

People new that prism would make a rainbow out of with light, but thought that it is just a property of prism.

Question: Newton put a prism in front of only one of the colors of rainbow, lets say red. What do you think happens when red light goes through the prism? What actually happens when white light passes through the prism?

Newton’s experiment proved that white light is actually a mix of all the colors of rainbow. Later, scientists found that there is light beyond the rainbow, as well – the visible light that splits into the rainbow is only a tiny part of complete spectrum of light, or electromagnetic spectrum.

So, what is light:

1) Light is an electromagnetic wave

What is a wave?

Something that can transmit energy without carrying any material along with it.

Question: if you move your hand up and down on one end of a rope, how does each piece of rope move? How does the piece on the other end of rope move? What moves outward?

How do we describe waves:

•Wavelength•Amplitude•Frequency•Speed

In what sense is light a wave?

If we could set up electrons in a row, when light passes they would move up and down, as the rope does.

Notice: we talk about electrons, because light is electromagnetic wave, it affects electrically charged particles and magnets.

Also, two light waves sum up as two waves on the water, producing ‘silent’ spots.

But, 2) Light is also a particle

It turns out that light behaves both a particle and a wave. In some situations it behaves as a particle, and in some as a wave.

When it behaves as particle, we call them photons: “pieces” of light or pieces of energy, each with precise wavelength, frequency, and energy.

Each photon carries particular amount of energy. The higher the frequency of the photon/wave, the more energy it carries.

Why is light a particle:

It doesn’t need medium to propagate.

Interaction of light with matter can be better described if we assume light is made up of photons, “pieces of energy”. We will see that photons can interact only “one at a time” with matter.

All nature actually exhibits that dual nature, we are all waves as much as particles. The higher the mass, smaller wavelength and the particle nature becomes more apparent.

For example, the wavelength of a baseball ball is about 10–34 m, much smaller than the size of a proton. Which is unobservable.

In 19th century it became clear that

The speed of light is always the same,c=300, 000 km/sfrequency * wavelength= speed of light

Question: when frequency of light increases what happens with its wavelength?

Review question: Do you remember how long does it take to light to arrive to the Earth from Moon? And from the Sun? Make a guess…

Wavelengths of visible light are from 400 to 700 nm.

Each wavelength corresponds to some color/hue:

Shorter wavelengths (higher or lower energies?) correspond to blue light.

Longer wavelengths to red.

Electromagnetic spectrum

Visible light is a tiny portion of electromagnetic spectrumLonger wavelengths:

Wavelengths longer than red are called infrared. Molecules moving around in a warm object emit this light, so we associate it with heat.

Radiowaves are the longest wavelength light. They carry so little energy that they have no noticeable effect on our bodies. But they make electrons move in an antenna, and that is how our radios receive radio waves from radio station.Shorter wavelengths:

Light with wavelength shorter than blue is ultraviolet. Ultraviolet light from the Sun carries enough energy to harm our skin.

Light with even shorter wavelength is X-ray. These photons have enough energy to penetrate our skin, but not teeth and bones.

The shortest wavelength (and highest energy) belongs to gamma rays. This light is typically emitted in radioactive decays.

The Electromagnetic Spectrum

Thought QuestionThe higher the photon energy…

a) the longer its wavelength.

b) the shorter its wavelength.

c) energy is independent of wavelength.

The higher the photon energy…

a) the longer its wavelength.

b) the shorter its wavelength.

c) energy is independent of wavelength.

b. What is matter?

Atomic structure:

All matter is made of atoms. There is more than 100 types of atoms identified

today.

Atom:nucleus (protons + neutrons)+ electrons

Question: if the nucleus would be the size of your fist, how far would electrons be?

b. What is matter?

Atomic structure:

All matter is made of atoms. There is more than 100 types of atoms identified

today.

Atom:nucleus (protons + neutrons)+ electrons

Question: if the nucleus would be the size of your fist, how far would electrons be?In a cloud many miles wide.

The properties of an atom depend mainly on the amount of electrical charge of its nucleus.(the charge of a proton is defined as basic positive charge, +1).

Question: What force is keeping nucleus and electrons together?

Question: How come that nucleus stays together despite having only positively charged protons in it?

The properties of an atom depend mainly on the amount of electrical charge of its nucleus.(the charge of a proton is defined as basic positive charge, +1).

Question: What force is keeping nucleus and electrons together?

Question: How come that nucleus stays together despite having only positively charged protons in it?

There is another type of force, strong force, acting in a nucleus, attracting protons and neutrons, which keeps it together.

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

Atomic Terminology • Isotope: same # of protons (same element) but

different # of neutrons. (4He, 3He)

• Molecules: consist of two or more atoms (H2O, CO2)Our world is so diverse because it consists of atoms combined in molecules (there

are many ways atoms can combine…)

Example: adjustable platform vs ladder.

Question: are there any circumstances under which an electron in a hydrogen atom can gain 2.6 eV of energy?

Electrons in atoms have distinct energy levels (they are quantized).

c. How do light and matter interact?

Energy carried by light can interact with matter in four ways, through:

• Emission: material can emit light, for example, light bulb

• Absorption: matter can also absorb light, for example your body on the sun

• Transmission: material can allow light to pass through, for example glass. • Reflection or Scattering: or the material can make photons bounce back,

all in the same direction (reflection, mirror) or in random directions (scattering, snow)

Interactions of light and matter; Examples:

Question: Why trees are green?

Interactions of light and matter; Examples:

Question: Why trees are green?

The color of object is given by the wave length of light which that object reflects back.

Thought QuestionWhy is the rose red?

a) The rose absorbs red light.

b) The rose transmits red light.

c) The rose emits red light.

d) The rose reflects red light.

Why is the rose red?

a) The rose absorbs red light.

b) The rose transmits red light.

c) The rose emits red light.

d) The rose reflects red light.

What have learned?• What is light?• Light is an electromagnetic wave that also can be

seen as if it comes in individual “pieces” called photons. Each photon has a precise wavelength, frequency and energy.

• Forms of light are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays

What have we learned?

• What is matter? Ordinary matter is made of atoms, which are made of protons, neutrons and electrons.

• How do light and matter interact? Matter can emit light, absorb light, transmit light or reflect light

5.2 Learning from Light

Our goals for learninga. What types of light spectra can we observe?

b. How does light tell us what things are made of?

c. How does light tell the temperatures of planets and stars?

d. How does light tell us the speed of a distant object?

a. What types of light spectra can we observe?

Intensity plots: they show the amount of intensity of light coming from each color.

a. What types of light spectra can we observe?

There are three basic types of spectra:

Thermal radiation spectra: continuous rainbow of color, like the one from incandescent light bulb.

Absorption line spectrum: if you observe a cloud of gas which lies between you and the light bulb you will see that some colors are missing from your thermal spectrum. The gas actually absorbs some light and leaves you with a set of dark lines on your rainbow spectrum.

Emission line spectrum: if now you observe the same cloud of gas, but from a side, you will measure spectrum which contains only the lines which were missing from the absorption spectrum against a black background.

Example: Solar Spectrum

b. How does light tell us what things are made of?

•Remember: Electrons in atoms have distinct energy levels.

Electron in an atom can absorb or emit only a photon of precise energy, which will allow electron to change its energy level.

Absorption spectrum

Example: Suppose a light bulb illuminates hydrogen gas, let’s understand what happens:

Light bulb produces white light which contains all colors of rainbow

However, hydrogen atoms will absorb only those photons which have the right energy for energy transition of electrons, resulting in a spectrum which is left without those absorbed photons - absorption spectrum

Once when they are in a higher level, electrons fall back to their previous lower level, and emit back photons they previously absorbed.

Question: Why this reemitted photons do not mask an absorption spectrum we see?

Distinct energy levels lead to distinct emission or absorption lines.

Emission spectrum

Example: Lets analyze spectra of a hot hydrogen cloud:

Atoms in a gas move freely and constantly collide, exchanging energy.

A few of collisions transfer the right amount of energy to bump an electron from lower to higher energy level.

Electrons can not stay in a higher level, they fall back to the lower level and emit the energy they gained, by emitting a photon of light.

Question: Do we expect to see an emission line spectrum from a very cold not illuminated cloud of hydrogen gas?

Question: Why does the absorption spectrum have a rainbow light in its background while the emission spectrum has no light?

Hydrogen Energy Levels

Notice: we can have absorption or emission spectrum only from gases.

Question: Can you guess why is that so?

Notice: we can have absorption or emission spectrum only from gases.

Question: Can you guess why is that so?In a gas molecules are far away and move freely. Photons easily pass through. They interact (if so) only once with electrons inside the atom/molecule, and then easily escape from the cloud to our telescopes.

Chemical Fingerprints

• Every atom, ion, and molecule has a unique spectral “fingerprint”

• We can identify the chemicals in gas by their fingerprints in the spectrum.

• With additional physics, we can figure out abundances of the chemicals, and much more.

Thought Question

Which letter(s) labels absorption lines?

A B C D E

Which letter(s) labels absorption lines?

A B C D E

Thought Question

Which letter(s) labels the peak (greatest intensity) of infrared light?

A B C D E

Which letter(s) labels the peak (greatest intensity) of infrared light?

A B C D E

Thought Question

Which letter(s) labels emission lines?

A B C D E

Which letter(s) labels emission lines?

A B C D E

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

Thermal Radiation Spectrum: also the continuous thermal spectrum can tell us something…

• Nearly all large or dense objects emit thermal radiation, including stars, planets, you…

• An object’s thermal radiation spectrum depends on only one property: its temperature!

How does thermal radiation happen:

In a gas atoms/molecules move freely, and photons can easily escape. They usually interact only once while inside of gas, and give us information about their absorption/emission.

But, in solid/liquid objects photons get trapped. Think about a rock, for example. Once caught, photons bounce from many molecules inside of it. When they exit, their energy is randomized (it can have wide range of values), and that is why we see escaping photons having rainbow like spectra.

Question: can you guess why thermal spectrum depends only on temperature of a body? (hint: what is a temperature measure of?)

How does thermal radiation happen:

In a gas atoms/molecules move freely, and photons can easily escape. They usually interact only once while inside of gas, and give us information about their absorption/emission.

But, in solid/liquid objects photons get trapped. Think about a rock, for example. Once caught, photons bounce from many molecules inside of it. When they exit, their energy is randomized (it can have wide range of values), and that is why we see escaping photons having rainbow like spectra.

Question: can you guess why thermal spectrum depends only on temperature of a body? (hint: what is a temperature measure of?)

The many bounces of the photons inside the object mean that the photons end up with energies that match the kinetic energies of the object’s molecules.

Two Properties of Thermal Radiation:

1. Hotter objects emit more light at all frequencies per unit area.

2. Hotter objects emit photons with a higher average energy.

Example: a light bulb with a dimmer switch. As you turn switch gradually on, the bulb emits first infrared, then also red light, and when switch is fully on, all visible spectra – white light.

Star appears reddish

Star appears bluish

Maximal radiation in green, but the Sun appears white.

Why we do not glow in the dark?

Spectra

Thought QuestionWhich is hotter?

a) A blue star.

b) A red star.

c) A planet that emits only infrared light.

Which is hotter?

a) A blue star.

b) A red star.

c) A planet that emits only infrared light.

d. How does light tell us the speed of a distant

object? The Doppler Effect: affects all waves.

Sound waves Light

When a train passes by you can hear the dramatic change

from high to low pitch.

What happens: Each pulse of a sound wave is emitted a little closer to you and the waves got bunched up between you and the train, giving them a shorter wavelength and higher frequency.

The Doppler Effect

The amount of blue or red shift tells us an object’s speed toward or away from us:

Suppose we recognize the pattern of hydrogen lines in the spectrum of a distant object. We know the wavelengths of a stationary cloud. From a difference between position of stationary and measured lines we can calculate the speed of that object.

Doppler shift tells us ONLY about the part of an object’s motion toward or away from us:

Thought QuestionI 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?

a) It is moving away from me.

b) It is moving towards me.

c) 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?

a) It is moving away from me.

b) It is moving towards me.

c) It has unusually long spectral lines.

Measuring Redshift

Measuring Redshift

Measuring Velocity

Measuring Velocity

Doppler Effect Summary

Motion toward or away from an observer causes a shift

in the observed wavelength of light:

• blueshift (shorter wavelength) motion toward you

• redshift (longer wavelength) motion away from you

• greater shift greater speed

Question: when we measure the motions of other galaxies in the Universe, are

they generally blueshifted or redshifted with respect to us. Why?

What have we learned?• What types of light spectra

can we observe? • Thermal radiation spectrum

– looks like rainbow of light• Absorption line spectrum –

specific colors are missing from the rainbow

• Emission line spectrum– see light only of a specific color

•

What have we learned?• How does light tell us

what things are made of?• Every kind of atom, ion,

and molecule produces a unique set of spectral lines.

• How does light tell use the temperatures of planets and stars?

• We can determine temperature from the spectrum of thermal radiation

What have we learned?• How does light tell us

the speed of a distant object?

• The Doppler effect tells us how fast an object is moving toward or away from us. – Blueshift:objects

moving toward us

– Redshift: objects moving away from us

5.3 Collecting Light with Telescopes

Our goals for learning

• How do telescopes help us learn about the universe?

• Why do we put telescopes into space?

• How is technology revolutionizing astronomy?

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)

• connect to scientific instruments which analyze data (i.e spectrographs)

Bigger is better

Bigger the telescope, more light (more information) it collects.

1. Larger light-collecting area

2. Better angular resolution

Bigger is better

10-meter telescope has light collecting area more than million times that of a human eye.

Angular Resolution

• The minimum angular separation that the telescope can distinguish.

Angular resolution: smaller is better

Human eye has angular resolution of 1 arcminute. Hubble telescope has resolution of 0.05 arcseconds (could read a book from a half a mile distance).

Basic Telescope Design

Refracting telescope Yerkes 1-m refractor

1. Refracting: use lenses(operate much like an eye)

Basic Telescope Design

Reflecting telescopeGemini North 8-m

2. Reflecting: mirrors• Most research telescopes today

are reflecting, it is much easier to make large mirrors than lenses

Keck I and Keck IIMauna Kea, HI

But, visible spectrum is only a small part of the story…

Planets emit light in infrared wavelengths, the hot upper layers of stars emit ultraviolet and x-ray light, some violent events even produce gamma rays – today, there are telescopes which study the entire spectrum.

Every wavelength range poses its own unique challenges in building telescopes.

Different designs for different wavelengths of light

Radio telescope (Arecibo, Puerto Rico)

The long wavelengths of radio waves mean that large telescopes are needed. This telescope stretches 1000 feet (305 m) across a valley in Puerto Rico (notice, reflecting surface is not glass!).

X-ray telescope: “grazing incidence” optics

Trying to focus X-rays is like trying to focus a stream of bullets (what is the energy of X-rays?). The bullets would pass through the metal if fired directly, if they barely graze metal surface, they can be slightly deflected.The mirrors are designed to deflect X-rays in much the same way. The Chandra X-Ray observatory, on a satellite

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 ball point, 15 m from Sun• Nearest stars 4,000 km away• Hubble orbit microscopically above ball- point size Earth

Observing problems due to Earth’s atmosphere

1. Light Pollution: our atmosphere scatters the bright light of cities, that obscures view of even the best telescopes

Star viewed with ground-based telescope

2. Turbulence (the ever changing motion of air) causes twinkling blurs images.

View from Hubble Space Telescope

How to overcome these obstacles:Mauna Kea, Big Island,HawaiiHome of many of the worlds best observatories:

It is dark (limiting light pollution)

Dry (limiting clouds)

Calm (limiting turbulence)

High ( above at least some of the atmosphere)

But,3. Atmosphere absorbs most of EM spectrum, including all UV and X-ray, most infrared.This problem can not be solved by the choice of telescope site or the improvement in technology.

Only visible light, some infrared and radio light reach the ground!

Telescopes in space solve all 3 problems.

• Location/technology can help overcome light pollution and turbulence.• Nothing short of going to space can solve problem of atmospheric absorption of light.

Chandra X-ray Observatory

adaptive optics: one of the most amazing new technology• Rapid changes in mirror shape compensate for atmospheric turbulence. Computer calculates necessary changes by monitoring distortions in an image of a nearby star.

How is technology revolutionizing astronomy?

Without adaptive optics With adaptive optics

interferometry• Allows two or more small telescopes to work together to obtain the angular resolution of a larger telescope.

Very Large Array (VLA), New Mexico

Astronomers some times link radio telescopes around the world for interferometry achieving an angular resolution equivalent of a telescope the size of the earth.

Interferometry is harder to use with shorter wavelenght, but astronomers are hoping is just a matter of time when those techniques will become available. That is why they are building telescopes in groups on a single mountain…

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

What have we learned?• How do telescopes help

us learn about the universe?

• We can see fainter objects and more detail than we can see by eye. Specialized telescopes allow us to learn more than we could from visible light alone.

• Why do we put telescopes in space?

• They are above earth’s atmosphere and therefore not subject to light pollution, atmospheric distortion, or atmospheric absorption of light.

What have we learned?• How is technology

revolutionizing astronomy?

It makes possible more powerful and more capable telescopes

• Adaptive optics• Interferometry

Want to buy your own telescope?

• Buy binoculars first (e.g. 7x35) - you get much more for the same money.

• Ignore magnification (sales pitch!)

• Notice: aperture size, optical quality, portability.

• Consumer research: Astronomy, Sky & Tel, Mercury. Astronomy clubs.