JEE Advanced 2020 · Paper 2, Physics question 1

This question was given marks to all.

Setting up. Put the corner at the origin, the line of sight along $+x$ towards the eye, and the wire at $W=(-12,0)$ cm. Each wall through the corner makes $45^\circ$ with the line of sight, so a ray that leaves a wall travelling along $+x$ meets it at $i=45^\circ$.

Refraction at one wall.
$$\sin45^\circ=\tfrac43\sin r\implies\sin r=\frac{3}{4\sqrt2}\approx0.530,\qquad r\approx32.0^\circ.$$
Inside the water the ray makes $45^\circ-32.0^\circ=13.0^\circ$ with the line of sight. It runs from $W$ to the wall $y=-x$ and meets it about $2.25$ cm from the line of sight. It then leaves parallel to the line of sight, so this image appears $2.25$ cm off-axis. By symmetry the other wall gives an image $2.25$ cm on the other side, about $4.5$ cm apart.

Why no single answer works. The pencils of rays refract obliquely at $45^\circ$, so the images are astigmatic and their positions depend on how the eye accommodates. The paraxial (near-normal) formula does not apply, and different reasonable models give different separations. That is presumably why marks were given to all.