If you’re looking in a mirror, and you see the reflection from a light bulb 20 feet behind you, how far away is that reflection?
Ultimately, the light’s reflection is 40 feet away from the bulb.
Why? Because the light bulb’s light does not originate on the mirror. Light travels from the bulb twenty feet to the mirror’s surface, where it’s reflected another twenty feet back to it’s original location.
But what if you’re not standing next to the light bulb? What if you’re standing at the mirror, say, two feet away from its surface? Your own reflection is four feet away from you, by the logic previously stated. But does that mean that the light bulb’s reflection is only 22 feet away from you?
Well, the light travels 20 feet to the mirror, and then is reflected 22 feet back to your eye where your iris catches the light and holds the image. 20+2 does indeed = 22. So the reflection is 22 feet away from you, even though you’re looking at a light bulb that is only 18 feet behind you.
If you see, say, a lipstick smudge or a piece of dust on the mirror’s surface, that does originate on the mirror’s surface, and is only two feet away from you, and twenty feet away from the light bulb. Notice how your vision is forced to change when you look from the smudge back to your reflection (two feet to four feet). Then, in your peripheral vision, you will notice that the smudge is doubled, and just a little bit blurry: It’s only two feet away, but your eyes are focused on your image that’s four feet away.
Looking at your reflection in a mirror (x) feet away, is the same as looking at a twin (2x) feet away.
And, as a light bulb cannot see, your perception of the bulb’s reflection is the only one that matters.
This is also the reason why a mirror facing a window makes a room look larger than it is, but a photo of the room from the same perspective does not: When you’re looking at the mirror, you’re actually looking outdoors, even if you have your back to the world.
But what about the reflection of your eyes in the mirror reflecting the reflection of the light bulb? I.e. The reflection of the light in your eyes? Another four feet (back to your reflection back to your eyes)? 26 feet, then? And do reflections just compound exponentially?
Time to get out of the shower.
The moral of the story is, never trust a mirror as your only perspective.