A ray of light is incident at an angle $i$ on a glass slab of refractive index $\mu$. The angle between reflected and refracted light is $90^o$. Then, the relationship between i and $\mu$ is :

As situation can be diagrammatically as below (image)
From law of reflection $i=\theta$
Now, $\theta +r+90^o=180^o$
$\Rightarrow i+r+90^o=180^o$
$r=90^o-i$
Also, from Snell's law :
$\frac{sini}{sinr}=\mu \Rightarrow \frac{sini}{sin(90^o-i)}=\frac{sini}{cosi}=\mu$
$\Rightarrow tani=\mu$