The figure shows a tank holding water of depth $20\ \text{cm}$ and refractive index $\dfrac{4}{3}$, above a horizontal plane mirror on the bottom. A pin hangs in the water with its tip $8.0\ \text{cm}$ below the water surface. An observer looks vertically down into the water, close to the pin, and sees the pin itself and also a second image of it, formed by the mirror. How far below the water surface does the observer see this second image?

Fig. 1: Side view of a tank of water, 20 cm deep with refractive index four thirds. A plane mirror lies on the bottom, drawn as a hatched line. A pin hangs in the water with its tip 8.0 cm below the water surface, with dimension arrows showing the 20 cm depth and the 8.0 cm. An eye above the surface looks straight down at the pin. No image is drawn.