About the "Seven Fairies" wine glass

The magical glass wine glass called "Seven Fairies". When the cup is empty, nothing can be seen. When the wine is poured in, a lifelike picture of a fairy appears on the bottom of the cup.

In the discussion of the cause of the formation of the fairy image, the correct answer is that the focal length of the convex lens at the bottom of the glass becomes larger after pouring the wine, so that the picture becomes an enlarged virtual image within one focal length.