- 天同Lv 76 years agoFavorite Answer
The 1st film,
The ice pieces are already inside the pot of water. Because the refractive index of ice (1.31) is close to that of water (1.33). The light refracted and reflected by the ice pieces is minimal. The ice pieces are thus not easily visible when viewed by the audience.
The 2nd film,
There is already an explanation given in the film. The liquid used in the beaker is not water, but Wesson oil. The refractive index of Wesson oil is almost the same as that of the glass rod, which is made from Pyrex glass. As such, there is little reflection and refraction of the glass rod that makes it visible.
You may refer to the following web-page for detailed explanation of the experiment: