Chapter 5Light and Matter

Astronomy 2014

Light: Radiative Energy• Light is a form of energy• Energy and Power– Energy is measured in the unit joules (J)– Energy of light:• “rate at which light carries energy in a direction”

– Rate of energy flow is a measurement of power, measured in the unit watt• 1 watt = energy flow of 1 joule / second

Example Question:

How many joules of energy are used by a 70 watt light bulb in a lamp over a minute time period?

1 watt = 1 joule/second >>> 70 watts = 70 joules/second

60 seconds per minute >>> 70 joules/second x 60 seconds

= 4200 J used in 1 minute

• Light and Color– The various forms of light occur across a spectrum– Colors are the small area of the spectrum that can be

seen by the human eye: Visible Spectrum

Light, Color, and Our Sight:Why do we see a basketball as orange?

A) The basketball absorbs orange light.

B) The basketball emits orange light.

C) The basketball transmits orange light.

D) The basketball reflects orange light.

Properties of Light

• Light is both particles and waves– Photons are mass-less particles of light– Light is an electromagnetic wave• Light travels via vibrations of particles in the electric and

magnetic fields• Waves of light carry energy, not matter

• All light travels through empty space at the same speed– Speed of light (c) = ~300,000 km/second• Speed of a certain bullet = 0.9 km/second

Properties of Light, cont.• The speed of any wave is determined by the wavelength times

the frequency– hWavelength = iFrequency– iWavelength = hFrequency

Page 143 in textbook

Properties of Matter• Light carries information about matter– We must understand the properties of matter in

order to interpret the messages sent in light from these objects

Structure of Matter, cont.• Matter is made up of

atoms– Atoms are made of

protons, neutrons, and electrons

– An element is the most basic unit of pure matter• Same atoms of a specific

atomic number

Structure of an Atom

Page 145 in Textbook

Properties of Matter, cont.

Atomic Terminology– Atomic Number

Number of Protons (# of positively charged particles)– Atomic Mass

Number of Protons + Neutrons (# of total particles)– Isotope

Variant of an element with the same number of protons but a different number of neutrons

Phases of Matter• Matter exists in solids, liquids,

and gases. • Phase of a substance

determined by – Substance’s physical properties

(melting point, freezing point) – Conditions of the environment

(temperature)Example:• The planet Jupiter is made of various

elements including hydrogen and helium.

• The nature of Jupiter’s environment is determined by • physical properties of these

elements • temperature of the planet’s

atmosphere.

How do Atoms Store Energy?

• Atoms possess energy in three components:1. Mass-Energy in amount of mc2

2. Kinetic Energy determined by their motion3. Electrical Potential Energy: dependent on the

arrangement of electrons around the atomic nuclei

• Electrical Potential Energy is stored within the energy levels occupied by an atom’s electrons– This energy is released as electrons move within these

energy levels.

How do Atoms Store Energy, cont.

Energy levels for an electron of a H atom, Page 149 in Textbook

Learning from Light: Spectroscopy• Spectroscopy is the process of obtaining a spectrum

and interpreting the information it contains.• A spectrum for studying an astrological object is

usually a graph that plots each wavelength

Continuous SpectrumContinuous Spectrum: A spectrum of light that spans a broad range of wavelengths without interruption by emission or absorption lines.

Emission Line SpectrumEmission Line Spectrum: A spectrum that contains emission lines (includes specific bands from a source emitting light at different, varied wavelengths)

Emission Line Spectrum, cont.Emission Line Spectra can be thought of as “fingerprints”– Each element has a unique band presence– Analysis can reveal what elements an astrological

body is composed of

Emission Line Spectrum, cont.

Emission Line Spectrum showing the chemical composition of our Sun

Absorption Line SpectrumAbsorption Line Spectrum: A spectrum that contains absorption lines (includes bands showing where specific wavelengths of light are being excluded from an otherwise continuous spectrum)

Thermal Radiation

• Thermal Radiation is the spectrum of energy radiation produced by an object based on temperature– Wavelengths can span the ultraviolet to infrared spectra

• Two Laws of Thermal Radiation– Law 1: Stefan-Boltzmann Law

Each square meter of a hotter object’s surface emits more light at all wavelengths

– Law 2: Wien’s LawHotter objects emit photons with a higher average energy, and therefore a shorter average wavelength

Thermal Radiation, cont.

Thermal Radiation of Living Things:From the previous graph, we see that a human emits wavelengths in the infrared. Why is it then that a human does not glow in the dark?

A) Humans do not emit any kind of light.

B) Humans only emit light that is outsight of the visible spectrum and therefore is invisible to us.

C) Humans emit quantities of light too small for us to see.

D) Humans do not contain enough radioactive material to produce thermal radiation.

The Doppler Effect• Doppler Effect: Effect that

shifts the wavelengths of objects that are moving toward or away from the observer

• Movement distorts waves– For example, the shifting sound

heard as a horn sounds on a car moving past a stationary person

– This change is also observed in waves of light

The Doppler Effect: Shift• BLUESHIFT - Object moving toward

an observer – Light waves bunch up between the

object and observer• Gives the appearance of shorter

wavelengths• Shorter wavelengths are bluer

• REDSHIFT - Object moving away from an observer– Light waves stretch out between the

object and the observer• Gives the appearance of longer

wavelengths• Longer wavelengths are redder

Redshift

Blueshift

The Doppler Effect, cont.

Measuring the Effects of the Doppler Shift

Chapter 5 Key Vocabulary1. Power2. Watts3. Continuous Spectrum4. Emission Line Spectrum5. Absorption Line Spectrum6. Wavelength7. Frequency8. Electromagnetic Wave9. Electromagnetic Spectrum10. Speed of Light11. Photons11. Visible Light12. Infrared13. Atom14. Proton15. Neutron

16. Electron17. Atomic Number18. Atomic Mass19. Isotope20. Electrical Potential Energy21. Spectroscopy22. Thermal Radiation23. Doppler Effect24. Blueshift25. Redshift