Answer
Nine lenses.
Work Step by Step
In Section 24–8, page 697, it is stated that at an (uncoated) interface between glass and air, about $4\%$ of the light is reflected. That means that $96\%$ of the light is transmitted into the lens when the light enters the glass, and the same factor applies when the light exits the lens back into air.
Light entering and then exiting a lens will experience two of these reductions, and therefore transmits only $(0.96)^2$ of the light that originally reached it. Find the number of lenses, N, that reduces the initial amount of light to $50\%$ or less.
$$0.50=[(0.96)^2]^N=(0.96)^{2N}$$
$$ln(0.50)=2Nln(0.96)$$
$$N=\frac{ln(0.50)}{2ln(0.96)}=8.49$$
Nine consecutive uncoated lenses would reduce the amount of light to $50\%$ or less.
