# reflection and transmission of a simple 3Dslab issues

I am new to Lumerical, so starting with a simple 3D slab with constant 3.5 index. the simulation region is quite small compared to slab dim. and simulation region is at the interface. Why am I not getting exactly .35 reflection and 0.65 transmission. I am getting 0.25 for reflection and around 0.7 for transmission.
thanks.

Thanks

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@bkhanaliloo
Si-slab.fsp (278.0 KB)
please find the attached file. yes, I am looking at other examples too.
thanks.

Dear @perveen.akhter

From Fresnel equations, shouldn’t you get 30% reflection and thus 70% transmission?

R=( (3.5-1) / (3.5+1) )^2 = 0.3

This is what I got by running your simulations. Please correct me if I am wrong.

Your simulation is set properly. What I can recommend is to use a finer mesh and increase the FDTD region in the z-direction so that PML boundaries are at least half the wavelength away from air-silicon interface (you need to consider max wavelength which is 800 nm in your simulation).

For advanced simulations you can take a look at Brewster angle example.

Thanks

@bkhanaliloo makes total sense. I was wrong in numbers. thanks for other suggestions too.
yes I will look at Brewster angle, as next step I am thinking is using different polarizations and incident angles. I am also looking for any manual or document with different examples, like this Brewster angle example. if you can send me a link that would be good.
thanks.

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The Brewster angle example uses both plane wave with Bloch BC and Gaussian source with PML BCs to study S and P polarized light. You can check this KB example as well that talks about light polarization.

Thanks

Hi @bkhanaliloo
I want to use my own material index file for a simulation.