Published 2 years ago
- To understand objects beyond Earth, we need information from those objects.
- 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.
- More convenient units of wavelength.
- Micrometer = μm = 0.000001.
- Ex. Red light = 0.7 μm.
- Nanometre = nm = 0.0000000001.
- Ex. Red light = 700 nm.
- 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
- 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 is used to measure energy.
- Higher energy Higher temperature.
- Lower energy 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.
- 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 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.
- 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.