An air bubble in a glass slab (µ = 1.5) is 6 cm deep when viewed from one face and 4 cm deep when viewed from the opposite face. The thickness of the glass plate is

15 cm

Let the thickness of slab be t and distance of air bubble from one side is x

When viewed from side (1): 6 = $\frac{x}{1.5}$ ⇒ x = 9 cm

When viewed from side (2): 4 = $\frac{(t-x)}{1.5}$ ⇒ 4 = $\frac{(t-9)}{1.5}$ ⇒ t = 15 cm

Formula used: $D_{\text {apparent }}=\frac{D_{\text {actual }}}{\mu_{\text {rel }}}$