Answer:
the frequency of this light is [tex]1.5\,\,10^{16}[/tex] Hz (which coincides with the fourth option listed among possible answers)
Explanation:
Recall the relationship between wavelength and frequency of radiation based upon the speed of light in vacuum:
[tex]frequency\,*\,wavelength\,=\, speed\,\,of\,light\\frequency\,*\,wavelength\,=\,3\,\,10^8\,\frac{m}{s} \\frequency\,*\,20 \,\,10^{-9}\,m\,=\,3\,\,10^8\,\frac{m}{s}\\frequency\, =\,\frac{3\,\,10^8}{2\,\,10^{-9}} \,\frac{1}{s} \\frequency\,=1.5\,\,10^{16}\,\,Hz[/tex]
In the process shown above, we have written the speed of light in meters per second, and also converted the 20 nano-meters into meters, in order to reduce SI units properly and have the result shown in units of 1/second (that is hertz's - abbreviate "Hz")
