after our introduction of wave, we will explore the concept of how wave travel through space.
which will also help find some evidence to support the Big Bang theory.
watch video...
remember the lab#3 on spectroscopy ---
http://coolcosmos.ipac.caltech.edu/cosmic_games/spectra/index.html
Let's say you are sending a spacecraft to Mars. You'd like your spacecraft to help you figure out what the rocks on Mars are made of. Or, let's say you'd like to know what gases are in the planet Jupiter's atmosphere. Or, maybe a strange gas has entered your school building and you'd like to figure out if it's dangerous or not. A spectrometer will help you in all these cases. It turns out that different substances absorb or emit light at different wavelengths in the Electro Magnetic (EM) Spectrum. If you shine the light you get from burning sodium into a prism or diffraction grating or other spectrometer, you'll find that the light comes out in bands of color at different places on the spectrum.
The amazing thing is, every time you see burning sodium, you'll see this same pattern of light if you send it through a spectrometer. You will
always see those same bright aqua and green lines by themselves in the middle of the spectrum. These lines are called
spectral lines, and they are related to the way the atoms in the material are arranged.
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Variations in Air Pressure and Corresponding Waveform
Suppose that there is a happy bug in the center of a circular water puddle. The bug is periodically shaking its
legs in order to produce disturbances that travel through the water. If these disturbances originate at a point, then they would travel outward from that point in all directions. Since each disturbance is traveling in the same medium, they would all travel in every direction at the same speed. The pattern produced by the bug's
shaking would be a series of concentric circles as shown in the diagram at the right. These circles would reach the edges of the water puddle at the same frequency. An observer at point A (the left edge of the puddle) would observe the disturbances to strike the puddle's edge at the same frequency that would be observed by an observer at point B (at the right edge of the puddle). In fact, the frequency at which disturbances reach the edge of the puddle would be the same as the frequency at which the bug produces the disturbances. If the bug produces disturbances at a frequency of 2 per second, then each observer would observe them approaching at a frequency of 2 per second.
Now suppose that our bug is moving to the right across the puddle of water and producing disturbances at
the same frequency of 2 disturbances per second. Since the bug is moving towards the right, each consecutive disturbance originates from a position that is closer to observer B and farther from observer A. Subsequently, each consecutive disturbance has a shorter distance to travel before reaching observer B and thus takes less time to reach observer B. Thus, observer B observes that the frequency of arrival of the disturbances is higher than the frequency at which disturbances are produced. On the other hand, each consecutive disturbance has a further distance to travel before reaching observer A. For this reason, observer A observes a frequency of arrival that is less than the frequency at which the disturbances are produced. The net effect of the motion of the bug (the source of waves) is that the observer towards whom the bug is moving observes a frequency that is higher than 2 disturbances/second; and the observer away from whom the bug is moving observes a frequency that is less than 2 disturbances/second. This effect is known as the Doppler effect.
The Doppler effect is observed whenever the source of waves is moving with respect to an observer. The
Doppler effect can be described as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding. It is important to note that the effect does not result because of an
actual change in the frequency of the source. Using the example above, the bug is still producing disturbances at a rate of 2 disturbances per second; it just appears to the observer whom the bug is approaching that the disturbances are being produced at a frequency greater than 2 disturbances/second. The effect is only observed because the distance between observer B and the bug is decreasing and the distance between observer A and the bug is increasing.
The Doppler effect can be observed for any type of wave - water wave, sound wave, light wave, etc. We are most familiar with the Doppler effect because of our experiences with sound waves. Perhaps you recall an instance in which a police car or emergency vehicle was traveling towards you on the highway. As the car approached with its siren blasting, the pitch of the siren sound (a measure of the siren's frequency) was high; and then suddenly after the car passed by, the pitch of the siren sound was low. That was the Doppler effect - an apparent shift in frequency for a sound wave produced by a moving source.
The Doppler effect is of intense interest to astronomers who use the information about the shift in frequency of electromagnetic waves produced by moving stars in our galaxy and beyond in order to derive information about those stars and galaxies. The belief that the universe is expanding is based in part upon observations of electromagnetic waves emitted by stars in distant galaxies. Furthermore, specific information about stars within galaxies can be determined by application of the Doppler effect. Galaxies are clusters of stars that typically rotate about some center of mass point. Electromagnetic radiation emitted by such stars in a distant galaxy would appear to be shifted downward in frequency (a
red shift) if the star is rotating in its cluster in a direction that is away from the Earth. On the other hand, there is an upward shift in frequency (a
blue shift) of such observed radiation if the star is rotating in a direction that is towards the Earth.
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/redshift.html
Redshift
| Astronomers often use the term redshift when describing how far away a distant object is. To understand what a redshift is, think of how the sound of a siren changes as it moves toward and then away from you. As the sound waves from the siren move toward you, they are compressed into higher frequency sound waves. As the siren moves away from you, its sound waves are stretched into lower frequencies. This shifting of frequencies is called the Doppler effect. |
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| A similar thing happens to light waves. When an object in space moves toward us it light waves are compressed into higher frequencies or shorter wavelengths, and we say that the light is blueshifted. When an object moves away from us, its light waves are stretched into lower frequencies or longer wavelengths, and we say that the light is redshifted. |
In the visible portion of the
electromagnetic spectrum, blue light has the highest frequency and red light has the lowest. The term blueshift is used when visible light is shifted toward higher frequencies or toward the blue end of the spectrum, and the term redshift is used when light is shifted toward lower frequencies or toward the red end of the spectrum. Today, we can observe light in many other parts of the electromagnetic spectrum such as radio, infrared, ultraviolet, X-rays and gamma rays. However, the terms redshift and blueshift are still used to describe a Doppler shift in any part of the spectrum. For example, if radio waves are shifted into the ultraviolet part of the spectrum, we still say that the light is redshifted - shifted toward lower frequencies.
The light from most objects in the Universe is redshifted as seen from the Earth. Only a few objects, mainly local objects like planets and some nearby stars, are blueshifted. This is because our Universe is expanding. The redshift of an object can be measured by examining the
absorption or emission lines in its spectrum. These sets of lines are unique for each atomic element and always have the same spacing. When an object in space moves toward or away from us, the absorption or emission lines will be found at different wavelengths than where they would be if the object was not moving (relative to us).
The change in wavelength of these lines is used to calculate the objects redshift. Redshift is defined as the change in the wavelength of the light divided by the wavelength that the light would have if its source was not moving (called the rest wavelength).
Redshift = (Observed wavelength - Rest wavelength)/(Rest wavelength)
Cosmological Redshift
The cosmological redshift is a redshift caused by the expansion of space. As a result of the Big Bang (the tremendous explosion which marked the beginning of our Universe), the Universe is expanding and most of the galaxies within it are moving away from each other. Astronomers have discovered that all distant galaxies are moving away from us and that the farther away they are, the faster they are moving. This recession of galaxies away from us causes the light from these galaxies to be redshifted. As a result of this, at very large redshifts, much of the ultraviolet and visible light from distant sources is shifted into the infrared part of the spectrum. This means that infrared studies can give us much information about the ultraviolet and visible spectra of very young, distant galaxies.
