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The c = v x lambda should be used when you know either know the frequency or wavelength. Because c is a constant, the unknown variable (v or lambda) can easily be solved using algebra by isolating the variable. Both equation are used in the e- ejection problems because the v in the energy equation can be replaced by c/lambda to solve for what the question may ask of you.
It usually depends on what information is given in the question that I use to determine what equation to use. Wavelength c=v x and E=hv for energy of light. Use both when it is the ejection of an electron.
In order to know what equation to use you would have to look at the problem and distinguish what will be needed and in some instances as seen on Friday lecture we will need to rearrange the equations in order to obtain the answer. Always check what is given in order to get to the answer.
It depends on the information given in the problem. Equations can be rearranged or combined to form an equation that will fit the problem. For example, E=hv can be combined with c=lambda x v, to form the equation E=hc/lambda, when you are looking for the wavelength but are not given the frequency.
Actually both can be used at the same time, the main thing is understand what each variable means, after understanding that you'll be able to see the relationships that are crossed over from both equations. The combination of the two depending on what is given in the question, can lead to the finding of frequency, energy, length,
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