A small ink mark is made on the bottom face of a glass slab of refractive index 1.5. Seen from above, near the normal, the mark appears 4 cm below the top face. The thickness of the slab is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 6 cm
Seen from above, the mark appears raised. The apparent depth \( h_1 \) is the real depth \( h_2 \) divided by the refractive index. Here the real depth is the thickness of the slab.
\( h_1=4\ \mathrm{cm} \)
\( n=1.5 \)
\( \begin{aligned} h_2 &= n\,h_1 \\ &= 1.5\times4 \\ &= 6\ \mathrm{cm} \end{aligned} \)
Hence, the thickness of the slab is 6 cm.
NCERT (2026-27):
Refraction – “… For viewing near the normal direction, it can be shown that the apparent depth (h1) is real depth (h2) divided by the refractive index of the medium (water).”