When light passes from one medium to another with a different refractive index, it changes its direction of travel. This phenomenon is called refraction. Snell's law relates the angles of incidence and refraction to the refractive indices of the two media:
n1 sin θ1 = n2 sin θ2
where n1 and n2 are the refractive indices of the first and second media, respectively, θ1 is the angle of incidence, and θ2 is the angle of refraction.
In this problem, we can assume that the light is incident vertically on the glass block, so θ1 = 0°. We want to find the actual thickness of the block, which we can call d. We know that the apparent thickness when viewed from above, which we can call h, is 6 cm.
We can use Snell's law to relate the refractive index of the air to that of the glass:
n1 sin θ1 = n2 sin θ2
sin 0° = 1.5 sin θ2
0 = 1.5 sin θ2
This equation tells us that the angle of refraction is also 0°, which means that the light passes straight through the glass block without deviating from its path. Therefore, the apparent thickness h is equal to the actual thickness d:
h = d
So, the actual thickness of the block is 6 cm