$A$ group of students took an old shoe box and covered it with black paper from all sides. They fixed a source of light (a torch) at one end of the box by making a hole in it and made another hole on the other side to view the light. They placed a milk sample contained in a beaker/tumbler in the box as shown in the figure. They were amazed to see that the milk taken in the tumbler was illuminated. They tried the same activity by taking a salt solution but found that light simply passed through it.
$(a)$ Explain why the milk sample was illuminated. Name the phenomenon involved.
$(b)$ Same results were not observed with a salt solution. Explain.
$(c)$ Can you suggest two more solutions which would show the same effect as shown by the milk solution?