The ice is not really transparent, but translucent . This means that the photons of light do not cross the material directly , as they interact and collide with the atoms present in the ice.
This is so because the distance between some atoms in the molecular structure of ice is of the order of the wavelength of the light that passes through it, and that is why its trajectory is modified. The case is the same as if we kick a soccer ball on a field in which there are posts equally distributed.
If the space between the posts is much larger than the size of the ball, it is very easy for the ball to cross the field without hitting any of them; But when the distance is significantly reduced, the probability that the ball will hit someone increases.
On the other hand, we must bear in mind that spring or tap water is not pure , since it contains suspended particles and dissolved mineral salts. These impurities also interfere with the passage of light, which gives the ice a poorly diaphanous appearance. If we want to make it as transparent as possible, we must use distilled water.
