A ray of light of intensity $I$ is incident on a parallel glass-slab at the point $A$ and it undergoes partial reflection and refraction as shown in the figure. At each reflection, $25\%$of the incident energy is reflected and the rest is transmitted. If the rays $AB$ and $A\prime B\prime$undergo interference, then the ratio $\frac{I_{\max }}{I_{\min }}$ is