Every object emits radiation over a range of wavelengths, so when we study the radiation from a given object (e.g. the Sun), we see light from a range of energies, or wavelengths. The distribution of energies (or wavelengths) coming from an object peaks at its average value, or the object's blackbody temperature.
Objects at higher temperatures emit more power (or radiation) at all wavelengths. Hotter objects give off more energy overall than cooler objects.
The higher the temperature of an object, the shorter the wavelength at which the peak radiation is emitted.
The exact relation between peak wavelength and temperature is known as Wien's Law, which states that peak wavelength = 3 x 106 / Temp.
Here the peak wavelength is in units of nm, and the temperature is in units of Kelvins.
The spectrum of an object, where its radiation can be broken up into its individual colors, can be used to determine its temperature and chemical composition. Temperature is determined by observing what wavelength, or color, an object's blackbody curve peaks at. Composition is determined by observing absorption or emission lines that correspond to special wavelengths that are absorbed or emitted by the particular atoms or molecules present in an object.
Here is a sample spectrum of the Sun:
You are probably more familiar with the Doppler shift as seen in sound waves rather than light waves, but the effect is the same. Have you ever listened to the sound of a siren or a train whistle as the ambulance or train approaches you and then moves away from you? The pitch of the siren changes. This is because as the ambulance moves toward you, the sound waves get compressed, or scrunched together. A shorter wavelength (in sound) results in a higher pitch. After the ambulance passes you, the sound waves become more spread apart, which results in a longer wavelength, or lower pitch.
The Doppler effect for light works the same as for sound. An object moving towards us has its light waves scrunched together, so the wavelengths become shorter. The object therefore appears bluer than if it were at rest. An object moving away from us has its light waves more spread apart, so it appears redder than normal.
Here is a good figure that illustrates how it works.
NOTE: Lines in an object's spectrum are shifted towards the red end of the spectrum (if the object is moving away from us) or towards the blue end of the spectrum (if the object is moving towards us). This does not mean that the overall color of the object changes, it just means that the spectral lines corresponding to particular elements shift wavelengths very slightly.
The Doppler Effect has many applications, including:
Telescopes are the instruments used by astronomers to collect light from distant objects. Galileo was the first person to point a telescope at the sky in 1610, and he discovered many wonders of the solar system.
Question: what kind of place would you consider to be a good location to put a telescope, and why?
Telescopes are made with glass, either mirrors or lenses, that focus the light to a point. At that point, you can either put your eye or have an electronic detector to collect the light. The most important characteristic of a telescope is its light-collecting power, i.e. the size of the mirror or lens that is collecting the light. This is known as the aperture size.
There are two basic kinds of telescopes: refractors and reflectors. Refractors form an image of an object by focusing the light using a lens. Reflectors form an image of an object by reflecting the light off of mirrors and into an eyepiece or electronic detector. The following web page, from Nick Strobel's Astronomy Notes (www.astronomynotes.com), gives a good description of how these two kinds of telescopes work.
Here are some pictures of local telescopes at the Apache Point Observatory in Sunspot, NM.
Here is a link to a cool movie from Mauna Kea, Hawaii, the site of the world's largest telescope.