Light and Matter

Light in Everyday Life

Our goals for learning:• How do we experience light?• How do light and matter interact?

The warmth of sunlight tells us that light is a form of energy.

Energy is measured in joules. We can measure the flow of energy in light in units of watts: 1 watt = 1 joule/s

Interactions of Light

• White light is made up of many different colors

4 process: • Emission• Absorption• Transmission• Transparent objects transmit

(allow to pass) 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. Objects with color (e.g. a red rose) appear that color because they absorb all the other colors and reflect (or scatter) that one.

What is light?

• Light is a form of energy that can act either like a wave or like a particle (with energy and momentum) depending on its interaction with matter– A light wave is a vibration of electric and

magnetic fields – an electromagnetic wave– Particles of light are bundles (quanta) of energy

called photons

Waves

• A wave is a pattern of motion that can carry energy without carrying matter along with it Leave bobs

up and down

Properties of Waves

• Wavelength is the distance between two wave peaks• Frequency is the number of times per second that a

wave vibrates up and down (cycles per second or hertz)wave speed = wave length x frequency

Mathematically: c = f: the Greek letter lambda

Eyes are sensitive to wavelength; we sense wavelength differences as “color”.

Particles of Light

• Particles of light are called photons• Each photon has a wavelength and a

frequency associated with it• The Energy of a photon depends on its

frequency, E = hf• h is a constant of nature called Planck’s

constant

Wavelength, Frequency, and Energy

l x f = cl = wavelength, f = frequency

Visible Light: = 0.5 x 10-6 m, f = 6 x 1014 s-1 (Hz)c = 3.00 x 108 m/s = speed of light

E = h x f = photon energyh = 6.626 x 10-34 joule x s = Planck’s constant

One photon of visible light carries ~4 x 10-19 joules of energyA 100W bulb emits 2.5 x 1020 photons every second!

What have we learned?• What is light?

– Light can behave like either a wave or a particle

– A light wave is a vibration of electric and magnetic fields

– Light waves have a wavelength and a frequency

– Photons are particles of light. • What is the electromagnetic spectrum?

– Human eyes cannot see most forms of light.– The entire range of wavelengths of light is

known as the electromagnetic spectrum.

Properties of Matter

Our goals for learning:• What is the structure of matter?• What are the phases of matter• How is energy stored in atoms?

We need to know this in order to understand the end phase of a star’s life & what’s happening inside a white dwarf or neutron star.

What is the structure of matter?

Atom Nucleus

ElectronCloud

Proton: positive charge

10-15 m

Electrons have negative charge and are almost 2000x less massive than protons

Volume of atom = 1,000 trillion times that of nucleus

Neutron:no charge

Everything is made of atoms

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

• Molecules: consist of two or more atoms (H2O, CO2)

Atomic Terminology

• Isotope: same # of protons but different # of neutrons. Example: (4He, 3He)

What are the phases of matter?

• Phases: – Solid (ice)– Liquid (water)– Gas (water vapor)– Plasma (ionized gas)

• Phases of same material behave differently because of differences in chemical bonds. By chemical bonds we mean the electric forces between atoms.

Phase Changes

• Ionization: Stripping of electrons, changing atoms into plasma

• Dissociation: Breaking of molecules into atoms

• Evaporation: Breaking of flexible chemical bonds, changing liquid into gas

• Melting: Breaking of rigid chemical bonds, changing solid into liquid

Read from bottom to top

What have we learned?• What is the structure of matter?

– Matter is made of atoms, which consist of a nucleus of protons and neutrons surrounded by a cloud of electrons

• What are the phases of matter? – Adding heat to a substance changes its phase by

breaking chemical bonds.– As temperature rises, a substance transforms

from a solid to a liquid to a gas, then the molecules can dissociate into atoms

– Stripping of electrons from atoms (ionization) turns the substance into a plasma

Learning from Light

Our goals for learning:• What are the three basic types of spectra?• How does light tell us what things are made

of?• How does light tell us the temperatures of

planets and stars?• How do we interpret an actual spectrum?

What are the three basic types of spectra?

Continuous Spectrum

Emission Line SpectrumAbsorption Line Spectrum

Spectra of astrophysical objects are usually combinations of these three basic types. We can take a picture of a spectrum (lower bar) or we can plot a graph of intensity versus wavelength (upper).

Continuous Spectrum

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

Slit in screen

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

Each colored “line” is an image of the entrance slit.

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

Chemical Fingerprints• Electrons in atoms

can only occupy certain energy states or levels

• The lowest energy state (level 1) is the Ground State

• Downward transitions between energy states produce a unique pattern of emission lines (E = hf = hc/)

Chemical Fingerprints• Because those atoms

can absorb photons with those exact same energies, upward transitions produce a pattern of absorption lines at the same wavelengths

Chemical Fingerprints

• Each type of atom has a unique spectral fingerprint

Chemical Fingerprints

• Observing the fingerprints in a spectrum tells us which kinds of atoms are present

Energy Levels of Molecules

• Molecules have additional energy levels because they can vibrate and rotate

• The “spring” just represents the electrical bond between the atoms of the molecule

Energy Levels of Molecules

• The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated

• Many of the energy transitions due to vibration and rotation of molecules occur in the infrared part of the spectrum

Thermal Radiation• Nearly all large or dense objects emit thermal

radiation, including stars, planets, you…• Collisions between atoms in a hot object causes

electrons to jump to higher energy levels for a while and then drop down again to emit light. As a result, the photons produced are intimately linked with the temperature (average kinetic energy) in the collisions. Radiation produced this way is called thermal.

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

Properties of Thermal Radiation1. Hotter objects emit more light at all frequencies per

unit area. Power per sq. meter = σT4 (Stefan’s Law)

2. Hotter objects emit photons with a higher average energy. maxT ~ 3000 (for in m)

(Wien’s Law)

Larger objects can emit more total light even if they are cooler. For a sphere (star), luminosity is L = 4πR2σT4

Thought QuestionWhy don’t we glow in the dark?

a) People do not emit any kind of light.b) People only emit light that is invisible to our

eyes.c) People are too small to emit enough light for us

to see. d) People do not contain enough radioactive

material.

Thought QuestionWhy don’t we glow in the dark?

a) People do not emit any kind of light.b) People only emit light that is invisible to our

eyes. We glow in the infrared.c) People are too small to emit enough light for us

to see. d) People do not contain enough radioactive

material.

What have we learned?• What are the three basic type of spectra?

– Continuous spectrum, emission line spectrum, absorption line spectrum

• How does light tell us what things are made of?– Each atom has a unique fingerprint.– We can determine which atoms something is

made of by looking for their fingerprints in the spectrum.

What have we learned?• How does light tell us the temperatures of

planets and stars?– Nearly all large or dense objects emit a

continuous spectrum that depends on temperature.

– The spectrum of that thermal radiation tells us the object’s temperature.

• How do we interpret an actual spectrum?– By carefully studying the features in a

spectrum, we can learn a great deal about the object that created it.

The Doppler Effect

Our goals for learning:• How does light tell us the speed of a distant

object?• How does light tell us the rotation rate of an

object?

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

The Doppler Effect

Waves are compressed in the direction of motion wavelength is decreased frequency is higher

Same thing happens for light.

Measuring the Shift

• We generally measure the Doppler Effect from shifts in the wavelengths of spectral lines

• The fractional shift is: ( - 0)/0 where 0 is the undisturbed wavelength; this number is equal to the speed of the object relative to that of light (V/c)

Stationary

Moving Away

Away Faster

Moving Toward

Toward Faster

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

Pure radial motion – maximum Doppler shift

Transverse motion – no Doppler shift

Part radial, part transverse – Doppler shift gives Vr = Vcosθ (less than V)

θ Vr

Thought QuestionI measure a spectral 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 toward me.

c) It has unusually long spectral lines.

Thought QuestionI measure a spectral 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. This is a REDSHIFT

b) It is moving toward me.

c) It has unusually long spectral lines.

And redshift = (502.8 – 500.7)/500.7 = 0.004194

Therefore the radial component of velocity = 0.004194 x c

= 1,258 km/s

Measuring Redshift

How does light tell us the rotation rate of an object?

Spectrum of a Rotating Object

Spectral lines are wider when an object rotates faster

Faster

Slower

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

• How does light tell us the rotation rate of an object?– The width of an object’s spectral lines can tell us how

fast it is rotating

Telescopes: Portals of Discovery

A Selection of Major Telescopes

Mauna Kea, Hawaii4 x 8m

Chile

2 x 10m

New Mexico

VLA:27 radio telescopes

Hubble: 2.5m in spaceX-ray telescope in space

Eyes and Cameras: Everyday Light Sensors

Our goals for learning:• How does your eye form an image?• How do we record images?

How does your eye form an image?

The lens bends (refracts) light rays and brings to a focus on retina.

Refraction

• Refraction is the bending of light when it passes from one substance into another

• Your eye uses refraction to focus light

Focusing Light

• Refraction can cause parallel light rays to converge to a focus

Image Formation

• The focal plane is where light from different directions comes into focus

• The image behind a single (convex) lens is actually upside-down!

Focusing Light

• A camera focuses light like an eye and captures the image with a “detector”

• The electronic detectors in digital cameras are similar to those used in modern telescopes

Digital cameras detect light with charge-coupled devices (CCDs)

What have we learned?

• How does your eye form an image?– It uses refraction to bend parallel light rays so that they

form an image.– The image is in focus if the focal plane is at the retina.

• How do we record images?– Cameras focus light like your eye and record the image

with a detector. – The detectors (CCDs or charge-coupled devices) in

digital cameras are like those used on modern telescopes; these devices convert photons into electrons and then into an electronic image that gets stored in a computer

Telescopes: Giant Eyes

Our goals for learning:• What are the two most important properties

of a telescope?• What are the two basic designs of

telescopes?• What do astronomers do with telescopes?

What are the two most important properties of a telescope?

1. Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time.

2. Angular resolution: Telescopes that are larger are capable of taking images with greater detail.

Light Collecting Area

• A telescope’s diameter tells us its light-collecting area: Area = π(diameter/2)2

• The largest telescopes currently in use have a diameter of about 10 meters: these are the twin 10m telescopes of the W. M. Keck Observatory which are owned and operated by the California Association for Research in Astronomy (UC and Caltech).

Bigger is better

Thought QuestionHow does the collecting area of a 10-meter telescope compare with that of a 2-meter

telescope?

a) It’s 5 times greater.

b) It’s 10 times greater.

c) It’s 25 times greater.

Thought QuestionHow does the collecting area of a 10-meter telescope compare with that of a 2-meter

telescope?

a) It’s 5 times greater.

b) It’s 10 times greater.

c) It’s 25 times greater. Fives times larger in diameter means 25 times more area.

Angular Resolution• The minimum

angular separation that the telescope can distinguish.

• At a great distance the pair of lights will look like one rather than two; the light will still be visible but we will not be able to distinguish that it is from two sources.

Angular Resolution• Ultimate limit to

resolution comes from interference of light waves within a telescope.

• Larger telescopes are capable of greater resolution because there’s less interference

Angular Resolution• The rings in this

image of a star come from interference of light wave.

• This limit on angular resolution is known as the diffraction limit

• Angular resolution scales as /D

Close-up of a star from the HubbleSpace Telescope

= wavelength; D = telescope diameter

What are the two basic designs of telescopes?

• Refracting telescope: Focuses light with lenses

• Reflecting telescope: Focuses light with mirrors

Refracting Telescope

• Refracting telescopes need to be very long, with large, heavy lenses

Largest lens is ~1m diameter

Reflecting Telescope

• Reflecting telescopes can have much greater diameters; invented by Isaac Newton

• Most modern telescopes are reflectors

Designs for Reflecting Telescopes

Mirrors in Reflecting Telescopes

Twin Keck telescopes on Mauna Kea in Hawaii

Segmented 10-meter mirror of a Keck telescope

Hard to make really big mirrors by grinding glass. The Keck telescopes have 36 hexagonal segments computer-controlled to act like one large curved mirror.

What do astronomers do with telescopes?

• Imaging: Taking pictures of the sky• Spectroscopy: Breaking light into spectra• Timing: Measuring how light output varies

with time

Imaging• Astronomical

detectors generally record only one color of light at a time

• Several images must be combined to make full-color pictures

Imaging• Astronomical

detectors can record formsof light oureyes can’t see

• False Color is sometimes used to represent different energies of non-visible light

Spectroscopy• A spectrograph

separates the different wavelengths of light before they hit the detector

Diffractiongrating breakslight intospectrum

Detectorrecordsspectrum

Light from only one starenters

Spectroscopy• Graphing

relative brightness of light at each wavelength shows the details in a spectrum

Timing

• A light curve represents a series of brightness measurements made over a period of time

What have we learned?

• What are the two most important properties of a telescope?– Collecting area determines how much light a

telescope can gather– Angular resolution is the minimum angular

separation a telescope can distinguish• What are the two basic designs of telescopes?

– Refracting telescopes focus light with lenses– Reflecting telescopes focus light with mirrors– The vast majority of professional telescopes

are reflectors

What have we learned?

• What do astronomers do with telescopes?– Imaging– Spectroscopy– Timing

Telescopes and the Atmosphere

Our goals for learning:• How does Earth’s atmosphere affect

ground-based observations?• Why do we put telescopes into space?

How does Earth’s atmosphere affect ground-based observations?

• The best ground-based sites for astronomical observing are– Calm (not too windy)– High (less atmosphere to see through)– Dark (far from city lights)– Dry (few cloudy nights)

Light Pollution

• Scattering of human-made light in the atmosphere is a growing problem for astronomy

Twinkling and Turbulence

Turbulent air flow in Earth’s atmosphere distorts our view, causing stars to twinkle and images to blur

Star viewed with ground-based telescope

Same star viewed with Hubble Space Telescope

Adaptive OpticsA new technology that corrects for atmospheric turbulence

How is it done? Rapidly changing the shape of a telescope’s mirror can compensate for some of the effects

of turbulence

Without adaptive optics With adaptive optics

Calm, High, Dark, Dry• The best

observing sites are atop remote mountains

Summit of Mauna Kea, Hawaii

This is where many astronomers work.

Why do we put telescopes into space?

Transmission in Atmosphere

• Only radio and visible electromagnetic waves pass easily through Earth’s atmosphere

• We need telescopes in space to observe other forms

What have learned?• How does Earth’s atmosphere affect ground-

based observations?– Telescope sites are chosen to minimize the

problems of light pollution, atmospheric turbulence, and bad weather.

• Why do we put telescopes into space?– Forms of light other than radio and visible do

not pass through Earth’s atmosphere.– Also, much sharper images are possible

because there is no turbulence.

Telescopes and Technology

Our goals for learning:• How can we observe non-visible light?• How can multiple telescopes work

together?

How can we observe non-visible light?• A standard

satellite dish is essentially a telescope for observing radio waves

Radio Telescopes

• A radio telescope is like a giant mirror that reflects radio waves to a focus

Arecibo: 300m

Infrared Telescopes

• Infrared (and ultraviolet-light) telescopes operate like visible-light telescopes but need to be above atmosphere to see all IR (and UV wavelengths); examples of a UV telescope – Hubble, GALEX

SOFIA Spitzer

X-Ray Telescopes

• X-ray telescopes also need to be above the atmosphere

Chandra

X-rays are harder to focus …

X-Ray Telescopes

• Focusing of X-rays requires special mirrors• Mirrors are arranged to focus X-ray photons through

grazing bounces off the surface

Gamma Ray Telescopes• Gamma ray

telescopes also need to be in space

• Focusing gamma rays is extremely difficult

Compton ObservatoryCurrent Missions: SWIFT, FERMI

How can multiple telescopes work together?

Interferometry

• Interferometry is a technique for linking two or more telescopes so that they have the angular resolution of a single large one

Interferometry

• Easiest to do with radio telescopes

• Now becoming possible with infrared and visible-light telescopes also

Very Large Array (VLA), Socorro, New Mexico

Concept for a 30-meter Giant Segmented Mirror

Telescope