The circuit shown below is working  as 6 V dc regulated voltage source when 10 V dc used as input, the power dissipated (in mW) in each diode is ( Consider both zener diodes identical)

 

Current is the circuit = $\frac{V}{R}$

                                          $\begin{array}{l}=\frac{10-6}{100+100}=\frac{1}{50}=0.02\\ =2\times {10}^{-2}A\end{array}$

Potential Drop across each diode = $\frac{6}{2} = 3 V$

Power dissipated in each diode = V I

                                              $\begin{array}{l}=3\times 2\times {10}^{-2}\\ =6\times {10}^{-2}\\ =60\times {10}^{-3}W\\ =60\text{mW}\end{array}$