When light enters a medium with a different refractive index, it undergoes refraction, which means the light changes direction. This bending of light occurs at the interface between two mediums with different refractive indices, such as air and glass.
In the case of a semi-circular glass block, when light is shined at the center of the straight side and enters the block through the curved side, the light does indeed refract, but it appears not to bend or change direction. This effect is known as "no apparent refraction."
The reason for this phenomenon is that the incident light is entering the curved side perpendicularly to the surface. When light enters a medium perpendicular to the surface, it does not experience any change in direction or refraction. Instead, it continues to propagate straight through the medium without bending.
In this specific case, since the light is entering the curved surface perpendicularly, it does not undergo refraction at that interface. However, when the light exits the curved side and reaches the straight side of the glass block, it will again undergo refraction as it leaves the block and enters the surrounding medium, such as air.
It's important to note that if the light were incident at an angle on the curved side, it would refract as expected, following the laws of refraction at the curved surface.