Question

A rectangular glass block of refractive index 1.5 has an air bubble trapped inside it as shown in the diagram. When seen from the surface AB, it appears to be 4 cm deep.

1. Calculate the actual depth of the air bubble from the surface AB.
2. For which colour of light, blue or yellow, the apparent depth will be greater?
3. Turning the glass block upside down, DOES NOT change the apparent depth of the air bubble. State True or False.

Answer 1

a. Refractive index of glass w.r.t. air = `"Real depth"/"Apparent"`

Real depth of air bubble = Refractive index × Apparent depth

= 1.5 × 4

= 6 cm

b. Since the wavelength of the yellow colour of light is greater than the wavelength of the blue colour of light. Therefore, yellow has departed less than blue. Yellow would therefore appear to have more depth.

c. Air bubble, 15 − 6 = 9 cm from surface CD

Now, Apparent depth from CD = `9/1.5` = 6 cm

Turning the block upside down would change the apparent depth of the air bubble, as it is not located at the geometric centre. Hence, the given statement is false.