Chapter 42: Problem 2

Question: How many complete vibrations are contained in the wave train of light of wavelength $\lambda~=~520~{\rm nm}$ emitted by an atom for a time of $\Delta t~=~430~{\rm ps}$.

Solution: The distance d traveled by the wavefont in $\Delta t~=~430~{\rm ps}$ is $d~=~c\Delta t~=~ 2.998\times 10^8\times 430\times 10^{-12}~=~0.129~{\rm m}$. The number of wavelengths in this distance is the nearest and least integer to this distance d divided by the wavelength of the light $\lambda$, $n~=~d/\lambda~=~2.4\times 10^5$.