Properties of Light

The textbook web resources for Chapter 3 are here.


Light... radiant energy - it can travel through space in waves. electromagnetic radiation - it has characteristics of both
   electric and magnetic energy.

Light has properties of both a wave and a particle.  This is called having a wave-particle duality.  When it acts like a wave, we refer to is as an electromagnetic wave.  When it acts like a particle, we talk about photons (particles of energy).

Two major characteristics of light are:
  - brightness/intensity (the amount of energy in the light) - this
     refers to the height of the wave (wave model) or the number of
     photons (particle model).
  - color - this is determined by the wavelength of the light.

Wavelength is the distance between the crests (tops) of two adjacent waves.  For visible light, this is usually measured in nanometers (nm - 1 billionth of a meter), but can also be measured in micrometers (mcm - 1 millionth of a meter) or angstroms (1 ten-billionth of a meter).
    Red Light has a wavelength of about 700nm.
    Violet Light has a wavelength of about 400nm.
Wavelength is usually denote by the Greek letter lambda (λ), which looks like an upside-down "y".

Frequency is the number of wave crests that pass per unit of time.  It is generally measured in Hertz (Hz) and is denoted by the Greek letter nu (which looks like a lower-case "v" in italics).

The speed of light in empty space is a constant: 300,000km/s.  This is denoted by the letter c.  The speed of light is the universal speed limit - nothing travels faster than c.

Because c is constant, the relationship between wavelength and frequency is c = wavelength x frequncy.

White light is light that contains a mixture of all the colors.  There is no dominant color, so they all add together to make white.  The spectrum of visible light that we see contains red, orange, yellow, green, blue, indigo, and violet.  The red light has the longest wavelength of these colors, violet has the shortest.  As a general rule shorter wavelengths are more energetic, longer ones are less energetic.

The electromagnetic spectrum contains the visible light that we see, but also many other types of radiant energy.  These differ by their wavelength.  These include (from longest wavelength to shortest)...
  Radio Waves
    - AM
    - Short-wave
    -TV/FM radio
  Radar/Cell Phones
  Infrared (heat)
  Visible Light
  Gamma Rays

Short-wavelength radiation carries proportionally more energy than long-wavelength radiation.  This means that Gamma Ray end of the spectrum can carry more energy than the Radio Wave end.  So, the energy of a photon (E) at a given wavelength is equal to Planck's constant (h) (6.63x10^-34 joule-seconds) times the speed of light (c), divided by the wavelength (lambda).  This means that hc = 1.99x10^-25 joule-meters.

Where this is applicable to everyday life is that ultrviolet light has enough energy to break atomic and molecular bonds (which is why it can cause sunburns and damage your DNA - causing skin cancer) but infrared does not (so it can't cause sunburns or skin cancer).

As an object's temperature increases, the object radiates light more strongly at shorter wavelengths.  In other words, if an object is heated up enough to give off light, it will start off glowing red and progress across the spectrum towards blue as it gets hotter.

Wien's (/Veenz/) Law states that the wavelength at which an object radiated light most strongly is inversely proportional to  its temperature.  
  Temp = 12,000K, Wavelength =    250nm (blue)
  Temp =   6,000K, Wavelength =    500nm (yellow)
  Temp =   1,000K, Wavelength = 1,000nm (red)

A blackbody is an object if the radiation it emits is dependant only on its temperature - not on other factors, such as its composition.  Stars are a good example of blackbodies because the wavelength (color) of light they emit depends on their temperature.  The peak wavelength emitted is calculated using Wein's Law and this formula:
      max. wavelength = 2.9x10^6 / temperature in Kelvin
Check out the interactive here - it explains this concept well.

Matter is made up of atoms.  Atoms have three parts:
1. Proton - Has a positive (+) charge, and is found in the nucleus
    of the atom.  Has a mass of 1 amu (atomic mass unit).
2. Neutron - Has a neutral (o) or no charge, and is found in the
    nucleus of the atom.  Has a mass of 1 amu.
3. Electron - Has a negative (-) charge.  Electrons orbit the
    nucleus in electron shells (sometimes called electron clouds).
    Has a mass of 1/2,000 amu.

An element is a substance that contains only one type of atom.  Elements are organized into the Period Table, which sorts them by size.  You can download a Periodic Table here.

The Periodic Table is set up in such a way that it tells you about the elements and their atoms.  Each period (horizontal row) represents an electron shell.  The first period has two elements in it - which tells you that the first electron shell can only hold two electrons.

Each element in the Periodic Table has a chemical symbol (a letter or two that represents that element), an atomic number (which tells you the number of protons in an element's atom), and an atomic mass number (which tells you the number of protons and neutrons combined in the atom).

Electrons orbit the atom's nucleus at certain distances.  The distance from the center of the nucleus is calculated by the formula:
                          orbital radius = 0.053nm x n^2
In this case, n is the level of elevtron orbit (1, 2, 3, etc.).  Therefore, the first orbit is 0.053nm from the center of the nucleus.  The second is 0.21nm (or 0.053 x 2^2).  The third is 0.48nm.  You cannot have an electron orbit in between  two of these levels - it just doesn't happen.  This is what we mean when we say that they orbots are quantized.

As an atom gains energy, its electrons can move from a lower orbit to a higher one.  We say that the atom is excited.  A common way that atoms become excited is by absorption of light.  Only certain wavelengths of energy can be absorbed and stored in an atom, thus raising an electron to a higher level.

The reverse process, where an atoms can lose energy as a photon of light, is called emission.  In this case, an electron drops from a higher orbit to a lower one.

Spectroscopy is the process of capturing and analyzing the spectrum of electromagnetic radiation (light) given off by an astronomical body.  Because the wavelengths of light absorbed and emitted depend on the kind of atoms absorbing and emitting them, this can tell us about the bodies we are looking at.

If an electron in a hydrogen atom drops from orbit 3 to orbit 2, it gives off a photon of light that has a wavelength of 656nm - bright red light.  If that same electron were to drop from orbit 4 to orbit 2, the photon would have a wavelength of 486nm and be a turquoise blue color.  If we were to look at a spectrum of light given off by excited H atoms, we would only see light at these two wavelengths.  This is hydrogen's emission spectrum - it shows the colors given off by hydrogen.

On the other hand, if we were to shine some white light through a cloud of hydrogen and look at the colors that came out the other side, we'd see a dark band at the places in the spectrum that correspond to those two wavelengths: 656nm and 486nm.  This is hydrogen's absorption spectrum.

Astronomers use light emitted from atoms, compounds, or even reflected off of solid surfaces to tell them something about the distant objects they study.

Types of Spectra
1. Continuous Spectrum - all colors present, brightness changes smoothly with wavelength.  Typical of solid or dense objects.
2. Emission-Line Spectrum - light is emitted at only a few specific wavelengths - the rest is dark.  Typically produced by hot, tenuous gas - i.e.: flourescent bulbs, aurora, and interstellar gas clouds.
3. Absorption Spectrum (aka - Dark-Line Spectrum) - Similar to a continuous spectrum, but with dark lines at certain wavelengths which were absorbed.  Typically produced when the light from a hot, dense object passes through cooler gas and some of the wavelengths get absorbed.

The Doppler Effect (Click here.)

When a star is  moving towards us, the light waves get compressed - so the wavelengths get shorter.  We say the light is blueshifted.

When a star is moving aways form us, the light waves get stretched out - so they get longer.  We say the light is redshifted.

We can use redshift and blueshift to tell whether a star is moving towards or away from us, and even to calculate how fast it is moving.

The Doppler Effect


Homework from the Text:

  • Read Chapter 3 (pg. 92-116). 
  • Answer Review Questions #1-9 (pg. 116).