To obtain our N and K Values we will use the page https://refractiveindex.info/

1. Pick a material of your choice. Here we choose Gold (Au)
   
   
   
   
2. You can see the N and K Values for the given Wavelength being displayed
   
   
3. Next we need to get the individiual n,k values for the colors red,green and blue, so we can use it in our node.
   To do that we need to enter three different wavelengths in the wavelength field above the Complex refractive index.
   
   The wavelength field
   
   
   
4. Let's first find the wavelengths for red, green and blue light. We can use google to do so 
   
   
   
   
5. You can see the values we obtained from google are in nm however we need them in µm.
   We can use the Unit Converter Button to convert them
   
   
   
   However in general all you need to do is divide the nm unit by 1000.
   
   For example for Red, we take the maximum wavelength 750 nm and divide it by 1000, giving us a value of 0.75 µm
   
   
6. Let's get the correct N,K values for red, green and blue. 
   We are using the maximum wavelength here, however you can choose a value between its minimum and maximum as required
   
   
   
   
7. Next let's go into our Complex IOR Node in Mari.
   By default the N and K values are represented by a single slider, however we need three separate values.
   
   To expose the individual values, we click on the "S" Button next to the slider
   
   Clicking on the S Buttons will decompose the Slider into individual Values
   
   
   
8. Let's enter our previously determined N and K Values into the Fields. We leave the last entry at 0.0.
   
   
   
   
9. The converted Reflectivity and Edge Tint will now be output on the respective ports of the Node
   
   
   
   
10. If we view one of the ports, when it is not connected to a scalar input (e.g. a scalar Channel), Mari will by default display it color managed in the viewport
    
    For pure viewing purposes we can temporarily deactivate the Color Management by clicking on the Color/Scalar Toggle in the Color Management Toolbar:
    You do not need to do this. It is just illustrated here in order to see correct values directly in the viewport.  Remember to turn it back on after viewing.
    
    We click on the "Color" Wheel in the Color Management Toolbar, so it is greyed out, 
    indicating that Color Management is currently disabled
    
    
    
11. If we now attach a Viewer Node (Hotkey 1) to the Reflectivity or Edge Tint Output Ports, we see the "artistic" representation of the physical N and K Values
    
    
    
    
12. You can now use this node output(s) and feed it into a scalar channel, that you can use in your rendering engine