Let the bubble be at a distance x from face I , then distance from face II be (210 – x).
As μ = real depth /apparent depth
Therefore, for face I, μ = x / 100 and
for face II, μ = (210 − x ) / 40
x / 100 = (210 − x ) / 40
x = 150 m m
∴ Actual distance of bubble from face II = 210 − x = 210 − 150 = 60 mm .
μ = x / 100
= 150 /100 = 1.5