Tuesday, October 9 , 2012
AST101H1 – Astronomy
Introduction to Light and Spectroscopy
Textbook (pg. 142)
Chapter 5 – Light and Matter
5.1 Light in Everyday Life
Light is a cosmic messenger, carrying information from distant objects to Earth
How do we experience light?
Light is a form of energy of which there are three types: radiative (energy carried by light), kinetic, and potential
Light is measured in joules (J) and in science, the rate of energy flow is called power
- Power = The rate of energy usage, usually measured in watts.
- Watt = The standard unit of power in science; defined as 1 watt = 1 joule/s.
Light comes in a spectrum of colours (rainbow: ROYGBV). Light from the Sun or a light bulb is called white light
because it contains all the colours of the rainbow. Black is what we see when there is no light and no colour. All
colours come from mixing a variety of other colours in different proportions together. The three primary colours
are red, green (yellow on paper), and blue.
Diffraction Grating = A finely etched surface that can split light into a spectrum.
How do light and matter interact?
Light can interact with matter in four ways:
- Emission = The process by which matter emits energy in the form of light (e.g. light bulb)
- Absorption = The process by which matter absorbs radiative energy (e.g. warming your hand)
- Transmission = The process in which light passes through matter without being absorbed (e.g. glass)
- Reflection/Scattering = The process by which matter changes the direction of light (e.g. mirror)
Materials that transmit light are transparent and those that absorb it are opaque
We see things as a certain colour because they transmit that to us and absorb other colours
All information that light brings to Earth from the universe was coded by one of the four basic interactions
between light and matter common to our everyday experience
5.2 Properties of Light
What is light?
Light behaves more like waves, as opposed to particles. A particle is a thing, while a wave is a pattern revealed
by its interaction with particles.
Waves have three basic properties:
- Wavelength = The distance between adjacent peaks (or troughs) of a wave.
- Frequency = The rate at which peaks of a wave pass by a point, measured un units of 1/s, often called cycles
per second or hertz (the number of times it bounces up and down).
o Cycles per Second = Units of frequency for a wave; describes the number of peaks (or troughs) of a wave
that pass by a given point of a second. Equivalent to hertz (Hz), or units of 1/s.
- Speed = The rate at which an object moves. Its units are distance divided by time, such as m/s or km/hr.
General rule: wavelength × frequency = speed
- The shorter the wavelength, the higher the frequency and energy, and vice versa.
Field = An abstract concept used to describe how a particle would interact with a force. For example, the idea of
a gravitational field describes how a particle would react to the local strength of gravity (e.g. Earth’s
gravitational field), and the idea of an electromagnetic field describes how a charged particle would respond to
forces from other charged particles.
Electromagnetic Wave = A synonym for light, which consists of waves of electric and magnetic fields.
All light travels at the same speed, which is about 300,000 km/s.
Light comes in individual pieces called photons, which have properties of both waves and particles 2
- Photon = An individual particle of light, characterized by a wavelength and a frequency.
- Each photon carries a specific amount of radiative energy. The shorter the wavelength of the light and
higher the frequency, the higher the energy of the photons. Blue photons carry more than red photons.
What is the electromagnetic spectrum?
There is light that our eyes cannot see, as the spectrum of visible light that split into the rainbow is only a small
part of the complete range of light’s wavelengths.
Electromagnetic Spectrum = The complete spectrum of light, including radio waves, infrared light, visible light,
ultraviolet light, X rays, and gamma rays. Electromagnetic radiation is another name for light of all types.
- Radio Waves = Light with very long wavelengths (and hence low frequencies), longer than infrared light.
They carry so little energy that they have no effect on our bodies.
o Microwaves = The region near the border between infrared and radio waves, where wavelengths range
from micrometres to millimetres.
- Infrared Light = Light with wavelengths that fall in the portion of the electromagnetic spectrum between
radio waves and visible light (lies beyond red end of rainbow)
- Visible Light = The light our eyes can see, ranging in wavelength from about 400 to 700 nm.
- Ultraviolet Light = Light with wavelengths that fall in the portion between visible light and X rays (lies
beyond blue end of rainbow).
- X Rays = Light with wavelengths that fall in the potion between ultraviolet light and gamma rays.
- Gamma Rays = Light with very short wavelengths (and hence high frequencies), shorter than X rays.
In general, certain types of mater interact more strongly with certain types of light, so each type of light carries
different information about distant objects in the universe 3
5.3 Properties of Matter
What is the structure of matter?
The Greek philosopher Democritus believed all materials were made from four basic elements: earth, air, water,
and fire. Today, we know that all matter is composed of atoms, which come in different types and correspond
with different elements, of which there are over 100.
- Atoms = Consist of a nucleus made from positively charged protons and neutrons, which have no charge,
surrounded by a cloud of negatively charged electrons. An atom’s mass is based on its nucleus
- Elements = A substance made from individual atoms of a particular atomic number.
Electrical Charge = A fundamental property of matter that is described by its amount and as either positive or
negative. It also describes how strongly an object will interact with electromagnetic fields. A proton has a charge
of (+1) and an electron has a charge of (-1)
- Opposites attract and likes repel (this attraction holds atoms together)
- The distribution of electrons gives an atom its size
Atomic terminology includes:
- Atomic Number = The number of protons in an atom.
- Atomic Mass Number = The number of protons and neutrons in an atom.
- Isotope = Versions of an element with varying numbers of neutrons (the number of protons never changes)
Molecules = Combinations of two or more atoms held together by chemical bonds (e.g. O ) 2
- Compounds = A substance made from molecules consisting of two or more atoms with different atomic
numbers (e.g. H 2)
Chemical Bond = The interactions between electrons that hold a molecule together (e.g. bond between H O)
What are the phases of matter?
Phase changes occur when chemical bonds are broken and replaced by others, which are caused by changes in
either pressure or temperature. As a substance is heated, the average kinetic energy of its particles increases,
enabling particles to break their bonds. Basically, as you increase temperature, matter changes phases.
1. Solid – Atoms or molecules are rigidly together (then melting point)
2. Liquid – Atoms or molecules remain together but move relatively freely (then boiling point)
3. Gas – Atoms or molecules move essentially unconstrained
4. Molecular Dissociation – Molecules and chemical bonds break apart into component atoms
5. Plasma Phase – Free electrons move among positively charged ions
-Follows ionization, the process by which electrons are stripped from atoms, making them charged ions
-Plasma is often called the fourth phase of matter. The Sun and most stars are made from it, “hot gas”.
6. Fully Ionized Plasma – Atoms in plasma become increasingly ionized
-The degree of ionization in plasma depends on its temperature and composition
Liquid becomes gas through evaporation and solid becomes gas through sublimation
Pressure = The force (per unit area) pushing on an object. In astronomy, we are generally interested in pressure
applied by surrounding gas (or plasma). Ordinarily, such pressure is related to the temperature of the gas
- The greater the pressur