Enrique has bought some sunglasses. If you put them indoors, you need to light two lamps to see as clearly as you do without your glasses on and with a single lamp.
How many lamps do you need to turn on to look at your eyes in the mirror with your glasses on if you want to see them as clearly as you would see them without glasses and with a single lamp?
If you need to light two lamps to see with the same sharpness as with a single lamp and without glasses, it is because the glasses only transmit half of the light they receive.
So to look at your eyes in a mirror, the light that comes out of the lamps will pass through the lenses, illuminating the eyes with half the intensity, will pass through the lenses for the second time, reaching the mirror with a quarter of the intensity, where it will bounce to pass through the lenses for the third time and the image will form in the retina with an eighth of the initial intensity. So You need to light eight lamps.