where, $T_{1}$ is the temperature of the source and $T_{2}$ that

of sink. since, $\frac{T_{2}}{T_{1}}=\frac{Q_{2}}{Q_{1}}$

So, $\quad \eta=1-\frac{Q_{2}}{Q_{1}} |$

To obtain $100 \%$ efficiency (i.e., $\eta=1), Q_{2}$ must be zero i.e., if a sink at absolute zero would be available, all the heat taken from the source would have been converted into work. The temperature of sink means a negative temperature on the absolute scale at which the efficiency of engine is greater than unity. This would be a violation of the $2\, nd$ law of thermodynamics. Hence, a negative temperature on the absolute scale is impossible. Hence, we cannot reach absolute zero temperature.