A solar cell generates a potential difference of $0\mathrm{.}10$ V when a $500$ resistor is connected across it$,$ and a potential difference of $0\mathrm{.}15$ V when a $1000$ resistor is substituted. What are the $($a$)$ internal resistance and $($b$)$ emf of the solar cell? $($c$)$ The area of the cell is $5\mathrm{.}0$ cm$2,$ and the rate per unit area at which it receives energy from light is $2\mathrm{.}0$ mW$/$cm$2.$ What is the efficiency of the cell for converting light energy to thermal energy in the $1000$ external resistor?