In a vacuum light travels at the maximum velocity which is 299,792,458 m/s. Light will travel slower than this speed and will decelerate when it travels through gases (such as the earth's atmosphere), liquids (such as the earth's ocean) and land. The denser the object and the longer light has to travel through the object, the more light will slow down and absorb into the object. If the substance is dense enough such as land or very deep waters it will absorb the energy completely. Light has a couple of important properties which are frequency and wavelength. Light travels as both a particle and a wave. Waves have a frequency (number of waves passing a fixed point through time) and a wavelength (length of one complete wave). The frequency is measured in Hertz (waves passing per second) and the wavelength is measured in meters or fractions of a meter. While the maximum speed that light travels is not a function of relativity, the motion of an object compared to the motion of surrounding objects will cause the wavelength and frequency to be different from different perspectives. Suppose an object emits radiation with a wavelength of 7 micrometers and this object is moving quickly away from an observer. The observer will discover the wavelength as being greater than 7 micrometers when it reaches the observer. This is called a red shift since the wavelength is observed as being longer than what the emitter radiates. The light from many galaxies is red shifted and this is used as evidence that the universe is expanding since many galaxies are moving away from each other. Imagine a very long rope with waves traveling through the rope. Suppose you are stationary and notice 100 waves passing you per minutes. Now suppose you are set in motion in the same direction the waves are moving through the rope. From this new observational perspective you will notice less than 100 waves passing you per minute. The frequency has decreased. This is a red shift. When the frequency of light decreases the wavelength increases. When an emitter of radiation is getting closer to the observer over time the observer will notice a higher frequency and shorter wavelength than what the emitter is radiating. This is known as a blue shift. Suppose you are stationary and notice 100 waves within a very long rope passing you per minutes. Now suppose you are set in motion in the opposite direction the waves are moving through the rope. From this new observational perspective you will notice more than 100 waves passing you per minute. The frequency has increased. When the frequency of light increases the wavelength decreases. See this link for information on universe expansion and evidence for it: http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/ExpandUni.html |