# 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.

• 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?

• 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.