# What do you understand by term reaction ? Define degree of reaction and derive the follwing expression for maximum efficiency in 50% reaction turbine.

$\eta =\frac{2{\mathrm{cos}}^{2}\left({\alpha}_{1}\right)}{1+{\mathrm{cos}}^{2}\left({\alpha}_{1}\right)}$

Where $\left({\alpha}_{1}\right)$ is nozzle angle at inlet.