I am trying to relate the equation for reflection coefficient in oblique mediums to the frequency but can't figure out how the frequency affects the reflection of light.
$n_1$= intrinsic impedance of medium 1
$n_2$= intrinsic impedance of medium 2
$\theta_1$ = incident angle
$\theta_2$= transmited angle
$$\Gamma= \frac{n_2 \cos(\theta_1) -n_1 \cos(\theta_2)}{n_2 \cos(\theta_1) + n_1 \cos(\theta_2)}$$
Can somebody explain to me how we can make this equation a function of frequency?
The frequency (wavelength) dependence comes in through the refractive index, and it is specific to the medium. In the lab, the manufacturer of your optics will generally provide this dependence, as in e.g. this resource. Calculating this theoretically is generally a hard problem, though.
The reflexion co-efficient equation you have given is part of the Fresnel Equations. There are two ways wherein this equation varies with frequency: ultimately both down to the variation of the refractive indices $n_1$ and $n_2$ with frequency. Clearly the equation you have written can vary with frequency if $n_1$ and $n_2$ do so: also $\theta_2$ does because this is governed by Snell's law $n_2\,\sin\theta_2 = n_1\,\sin\theta_1$. So you're full variation is:
$$\Gamma(f,\,\theta_1) = \frac{n_2(f) \cos(\theta_1) -n_1(f) \cos\left(\arcsin\left(\frac{n_1(f)}{n_2(f)}\sin\theta_1\right)\right)}{n_2(f) \cos(\theta_1) + n_1(f) \cos\left(\arcsin\left(\frac{n_1(f)}{n_2(f)}\sin\theta_1\right)\right)}=\frac{n_2(f)^2 \cos(\theta_1) -n_1(f) \sqrt{n_2(f)^2-n_1(f)^2\sin\theta_1^2}}{n_2(f)^2 \cos(\theta_1) +n_1(f) \sqrt{n_2(f)^2-n_1(f)^2\sin\theta_1^2}}$$
Note that $\Gamma$ is a function of both $f$ and $\theta_1$: if there is only one interface, then of course $\theta_1$ will be fixed with frequency (it's set by how you aim the beam at the interface. But if there are many interfaces, the incidence angle on each will be set by the refracted transmission angle from the former interface in the stack, so it will then also vary with frequency.
First, for oblique incidence one has to distinguish the transverse electrical mode (TE) and the transverse magnetical (TM) polarisation. You give the Fresnel equation for the TE reflectivity. However you denote the impedamdanve by $n$ in place of, for example, $Z$. $n$ commonly denotes the refractive index and this is confusing. If $n$ would denote the refractive index then your equation would give the reflectivity if the TM mode instead.
The refractive index depends on the frequency. It originates from the polarisability of the medium. The polarisability is stronger for frequencies closer to an optical resonance. Multiple optical resonances are common in materials and they result in a complex behaviour of the RI with frequency. You can check out many examples of this at refractiveindex.info.
