I think so too...if its a square,then it easily works out to rt 2.

See,in the given figure, the black rays are the path of light.Only if the light gets totally internally reflected will the print be invisible.

The red line is the normal.

You know refractive index =sin i/sin r..

sin r =1 as r= 90..

sin i, as seen in figure(angle @ is angle i there..), is h/rt.(h²+l²)..

and we're stuck there..

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But if its a square, h=l

and sin i = refractive index = rt 2.

so please check the question..