Optical system with a medium different from air

[Frank-95]

Hi all!

I have an optical system made up like this:

Circular led source -> smaller circular diaphragm -> circular even smaller lens -> cmos sensor at the posterior focal plane

This is the in scale model:

The image is optically after the focal plane, but practically it will lie on it due to the sensor.

Now what happens is this.
Firstly I insert an empty cuvette between the led and the diaphragm. The cuvette fills completely the gap, and is 12mm and it is made by 1 mm of plastic per side.

What I see upon the sensor is the same light spot, attenuated though. This means that the real part of the plastic refraction index, and/or the small plastic width light has to pass through, is not high enough to see size changes in the spot. The imaginary part is, though, since I can see an attenuation.

Then I fill the cuvette with water, so that I have 1 mm plastic, 10 mm water, 1 mm plastic. What I see in the sensor is a visible widening of the spot.
At first I thought it was because of double refraction rays movement, but then I realize that since water index is greater than air one, rays should focus better than air so the spot should be smaller.

Why does the light spot broads when the light passes though water?

Thank you very much.

 

[Khashishi]

Why do you say that greater index of refraction should focus better? It depends on the focal point of the lens.