Expert: Courtney Seligman - 8/13/2010

Question
You told in previous question that speed of every colour in vaccum is same but we know that wavelength of each colour is different. So how  can the speed of each colour  is same with varying wavelength

Answer
All light waves (or if light is thought of as particles, all photons) move through empty space at the same speed. If they have different wavelengths, more or fewer waves pass in a given period of time, according to the formula f = c / w , where c is the speed of light, w is the wavelength, and f is the frequency, or number of waves which pass a stationary observer per second. For light in the middle of the visible spectrum, the wavelength is about 5000 Angstroms, or about one twenty-thousandth of an inch, and the frequency is about 235 million million waves per second (or 235 million Megahertz). If the wavelength was only half as much (2500 Angstroms, which is well into the ultraviolet spectrum), the frequency would be twice as much (about 470 million million waves per second); since the waves are only half as long, twice as many pass by in a given period of time, but the overall speed is the same. If the wavelength was twice as much (10000 Angstroms, which is well into the infrared spectrum), the frequency would be half as much (about 117 million million waves per second); since the waves are twice as long, only half as many pass by in a given period of time, but the overall speed is still the same.