It seems like you're describing a situation involving the refraction of light at the bottom surface of a block of material in air.
Let's break down the scenario and explain what happens:
Angle of Incidence (i): The angle of incidence (i) is the angle between the incident ray (the incoming light ray) and the normal (a line perpendicular to the surface) at the point of incidence (where the light ray hits the surface). You mentioned that the angle of incidence (i) at the bottom surface is equal to something, but you didn't specify what it's equal to. Please provide that information for a more accurate explanation.
Critical Angle: The critical angle is the angle of incidence in the optically denser medium (in this case, the block of material) beyond which total internal reflection occurs at the boundary between the two media. It's the angle at which the refracted ray would emerge parallel to the boundary instead of passing into the less dense medium (in this case, air). You mentioned that the critical angle for the material of the block in air is 39º. This means that if the angle of incidence is greater than 39º, total internal reflection will occur, and no light will pass through the boundary.
Assuming that the angle of incidence (i) at the bottom surface is less than the critical angle of 39º:
When light passes from a denser medium (the block of material) to a less dense medium (air) at an angle of incidence less than the critical angle, refraction occurs. Here's what happens:
The incident ray approaches the bottom surface of the block at an angle.
At the boundary, the light ray is partially transmitted into the air and partially refracted (bent) within the material of the block due to the change in its speed.
The refracted ray bends away from the normal line, moving towards the surface normal of the block.
The refracted ray exits the bottom surface of the block and continues its path through the air.
In summary, when the angle of incidence is less than the critical angle, the light ray undergoes refraction and continues to pass through the material-air boundary, changing direction but not undergoing total internal reflection. If the angle of incidence were equal to or greater than the critical angle, total internal reflection would occur, and the light would stay within the material without passing into the air.