Chapter 4: Light and It's Properties

Astronomical Information

  • To understand objects beyond Earth, we need information from those objects.
  • Meteorites.
  • Moon rocks.
  • Cosmic rays.
  • Very little information on all these 3.

Learning from Light

  • We study the properties of light on Earth to understand the light from beyond Earth.
  • Newton discovered white light is the mixture of all colors.
  • Sorting white light into the different colors produces the spectrum of light.
  • Studying the spectrum of light is how we learn light’s information.

Properties of Light

  • Light is a form of energy - sunlight warms Earth.
  • Light is a form of energy that travels through the vacuum of empty space.
  • Light’s speed in a vacuum is constant for all colors.
  • Speed of light symbol is “c”.
  • c is about 300,000 km/s - The fastest possible speed.
  • Light’s speed is slower in transparent material (air or glass or water or plastic or…) than in a vacuum, and different colors have different speeds.


What is Light?

  • Answering this question took centuries of study.
  • Experiments found that light is a wave of electric energy and magnetic energy.
  • The “wave” is the pattern of how the electric energy and the magnetic energy increase and decrease with location.

Properties of Light Waves

  • Light waves have a wavelength that is the distance from crest to crest of the wave.
  • The symbol for wavelength is “ƛ”, which is the Greek letter L for Length.
  • The wavelength of the light we see, such as red, is very tiny, ƛ = 0.0000007 m.
  • Light waves arrive at a location with a certain frequency.
  • Frequency is expressed as (number of waves/second) and is represented by the symbol 𝒗, which the “n” in the Greek alphabet, for “number”.
  • The unit of frequency is named “Hertz” abbreviated - “Hz”, named for Heinrich Hertz who studied light’s properties.
  • The frequency of light is too fast to measure.
  • However, there is a simple relation between the speed of light, the wavelength, and the frequency: ƛ𝒗 = c.
  • So the frequency can always be found from measuring the wavelength and doing a simple division.

Wavelength and Color

  • Each of the colors we see in the spectrum of light has a different wavelength.

Wavelength Units

  • More convenient units of wavelength.
  • Micrometer = μm = 0.000001.
  • Ex. Red light = 0.7 μm.
  • Nanometre = nm = 0.0000000001.
  • Ex. Red light = 700 nm.

Calculating Frequency

  • 𝒗 = c/ƛ.
  • Units are “Hz” for 𝒗.

“Particles” of Light

  • Many experiments show us the wave properties of light.
  • But other experiments show us that light is a “particle” of energy - the “photon”.
  • The photons of light travel through space at the speed of light.
  • Experiments show that light is both a wave and a photon at the same time.
  • The energy of the light’s photon depends on the frequency of the light’s wave.
  • Photon’s energy depends on wave’s frequency.
  • We will use whichever description is easiest for a particular need.

Brightness of Light

  • The brightness of the light can be described in waves
  • The amplitude of the light wave arriving at our eye determines the brightness.
  • The number of photons arriving at our eye each second.

The Full Electromagnetic Spectrum

  • Our eye sees “visible” white light, violet-red, with ƛ = 400-700 nm.
  • In 1800 the astronomer Sir William Herschel discovered radiation in the Sun’s spectrum beyond the red - infrared.
  • In 1801 the chemist Johann Ritter discovered radiation below the violet = ultraviolet.
  • From 1800 to about 1925, we discovered many other forms of electromagnetic radiation, usually by accident, using many different methods.
  • The different kinds of radiation are all light, but different names are used because of themethod of detection.
  • Radio Waves - ƛ > 1m
  • Microwaves - 1mm < ƛ < 1m
  • Infrared - 0.7 µm < ƛ < 1mm
  • Visible - 400 nm < ƛ < 0.7µ
  • Ultraviolet - 10 nm < ƛ < 400 nm
  • X-rays - 0.01 nm < ƛ < 10 nm
  • Gamma rays - ƛ < 0.01 nm

Atmospheric “Windows”

  • Earth’s atmosphere is transparent to only a few kinds of radiation: visible, radio, and some of the infrared = “windows”.
  • The other forms of radiation are absorbed by the air.
  • To observe all forms of radiation we use satellites in space above the atmosphere.


Temperature

  • Temperature is used to measure energy.
  • Higher energy \rightarrow Higher temperature.
  • Lower energy \rightarrow Lower temperature.
  • Celsius temperature scale has 0 degrees Celsius = water freezes, 100 degrees Celsius = water boils.
  • Water was an arbitrary choice.
  • Kelvin temperature scale.
  • More fundamental because it is based on the energy content of an object.
  • Set 0 K = “absolute zero” where there is no energy and everything is frozen.
  • Kelvin scale just shifts the Celsius scale down to absolute zero, 0K = -273C.
  • Water freezes at +273K and boils at +373K.

Temperature and Radiation

  • An object’s temperature (in K) determines the radiation it emits - example: light bulb.
  • Wein’s law derives the temperature from the spectrum of the object’s emission.
  • Measure the wavelength where the object emits radiation most strongly, ƛmax.

How Hot is the Sun?

  • Wavelength of peak radiation is about 500 nm
  • Temp = (2,9000,000 K*nm)/(500 nm) in Kelvin

Connecting light and matter

  • Understanding the properties of light is important.
  • But our goal is to learn about objects in space from the light they emit and/or reflect.
  • Therefore, we need to understand the properties of matter.


Structure of Matter

  • All matter is made of atoms - this idea was first developed by the Greeks 2000 years ago although they could not prove it.
  • Over the last century this idea has been confirmed, identifying about 100 different kinds of atoms which are called chemical elements.
  • An atom’s diameter is about 10-10 m, much too small to see directly.

Structure of Atoms

  • Further research found that every atom is built from just three very tiny particle.
  • Proton.
  • Neutron.
  • Electron.
  • The proton and the neutron
  • Have the same mass.
  • Are located in the nucleus at the centre of the atom, which is only 10-15 m in diameter.
  • The mass of the electron is much less than the mass of a proton or neutron, so the electron does not contribute to the atom’s mass.
  • The electron fill out the atom’s volume.
  • Atoms are held together by the electric force between the protons and the electrons.
  • Gravity is not important inside atoms.
  • The number of protons defines the chemical element.
  • Sum of protons and neutrons equals the atomic mass number.
  • Because neutrons do not have an electric charge, the number of neutrons does not have to equal the number of protons.
  • Isotopes of a certain element have different masses, despite being of the same element.
  • The atomic model described before has a problem - the electrons with negative charge are on the outside and the protons with positive charge are in the nucleus.
  • Therefore the electric force should pull the electrons into the nucleus, causing the atom to collapse, but it doesn’t.
  • Additional research found that the electrons orbit the atom’s nucleus.
  • But because of atomic laws, only certain orbits with specific sizes and energies are possible.
  • These are quantum rules that apply because electrons behave like a wave inside the atom.
  • For an electron to move from an orbit close to the nucleus to a higher orbit, energy is required to lift the electron away from the electric force of the nucleus.
  • Because of the specific orbits, there are specific amounts of energy required.
  • Similar to walking up stairs.
  • An atom with an electron in a higher orbit is said to be “excited”.
  • One way to supply the specific energy to lift the electron between orbits is for the atom to absorb a photon of light with a specific frequency or wavelength.


Molecules

  • Molecules are made by bonding atoms together.
  • Ex. N + N = N2

Light and Atoms

  • Light interacts with atoms (and molecules)
  • Therefore, observing light can provide information about the atoms on Earth and on objects beyond Earth.
  • To understand the information present in light about atoms, we need to explore atoms in more detail.

Conservation of Energy

  • Energy is an indestructible quantity - it cannot be created or destroyed, only transferred (the same is true of matter).
  • If the electron in a higher orbit returns to a lower orbit it must return the specific amount of energy that lifted it up.
  • The returned energy is emitted as a photon of light with a specific wavelength or frequency.

Identifying Atoms (or Molecules)

  • Each element has a unique number of protons.
  • Each atom has equal numbers of electrons and protons to balance + and - charges.
  • Each element has a unique number of electrons.
  • Each element has a unique number of electron orbits.
  • Each element has a unique number of wavelengths it can absorb/emit.
  • The unique pattern or absorption and emission serves as a “fingerprint” for each chemical element.
  • We can use the spectrum of light to do a chemical analysis of objects beyond Earth.

Types of Spectra

  • If we observe a glowing object, we see an “emission-line spectrum”.
  • If we observe an object that is scattering light from another object, such as a planet scattering sunlight, we see an “absorption-line spectrum” where the planet and its atmosphere have taken energy from the sunlight.

Doppler Shift

  • Light can also be used to measure the motion of objects toward or away from us.
  • This was discovered by the scientist Christian Doppler, first for sound and then for light - the Doppler effect or shift.
  • However, if we and the light are coming together or moving apart - radial motion, we observe a different wavelength.
  • Coming together compresses the wavelengths shifting them toward the blue - “blueshift”.
  • Moving apart stretches the wavelengths shifting them toward the red - “redshift”.
  • We experience the same effect on sound waves.
  • We can calculate the speed from the change of wavelength:
  • V = c ((observed wavelength-true wavelength)/true wavelength).
  • V is positive when it is a “redshift”.
  • V is negative when it is a “blueshift”.
  • The Doppler shift measures only radical motion toward or away - it cannot measure sideways - transverse motion.
  • In the solar system, we can measure radical speeds as small as cm/s.
  • When we observe a source of light and we are not moving toward each other or moving away from each other, we measure.


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