Why is it that when you look in the mirror left and right directions appear flipped, but not the up and down?
Mirror Images – Why Flipped Horizontally but Not Vertically?
everyday-lifegeometric-opticsreflectionvisible-light
Related Solutions
Any translucent surface both reflects and refracts light. By refraction, I mean that it bends the light a bit, but lets it through to the other side. Now, reflection for such surfaces is much less than refraction (unless there's total internal reflection, but thats irrelevant for glass+air). Edit: According to @JohnRennie (see comments), only 5% of the light is reflected
During the day, you have light from your room being largely refracted out, and reflected back inwards a tiny bit. The outside light does someing similar. It is largely refracted into your room, and reflected back outside a tiny bit. So, the majority of the light you see coming from the window is due to the outside light. You will see a reflection if you look carefully (exacly how carefully depends upon the lighting of your room)
Now, during the night, there is little or no light coming from the outside. So the majority/all of the light you see is due to reflection. So you see the reflected image.
Now an interesting question is, if the reflected image has the same intensity in both cases, why do you see it in one case and not see it in another? The answer lies in the working of the eye. The eye does not have a constant sensitivity to light. Whenever there is a lot of light, your irises contract, admitting less light into your eyes. This means that you can perceive bright light but dim light becomes invisible. When it is darker, they expand, and the reverse effect happens. That's why you feel blinded by bright light when you leave a dark room, and also why it takes time to adjust to a dark room. (You can actually see your irises contracting; go to a well lit room with a mirror, stare at your eyes, close them for a few seconds, then reopen.. Takes a few tries, but you can see them contracting). Edit: (Credit @BenjaminFranz for pointing this out) The regulatory mechanism does not consist of only the iris/pupil. The retina also does a lot of regulation, which is why it takes half a minute or more to get used to a dark room, whereas our irises can dilate within a few seconds.
So, during the day, the profusion of light refracted from the outside makes your irises contract, thus making the reflected light nearly invisible. During the night, your pupil is dilated, so you can clearly see a reflection.
You are getting reflections from the front (glass surface) and back (mirrored) surface, including (multiple) internal reflections:
It should be obvious from this diagram that the spots will be further apart as you move to a more glancing angle of incidence. Depending on the polarization of the laser pointer, there is an angle (the Brewster angle) where you can make the front (glass) surface reflection disappear completely. This takes some experimenting.
The exact details of the intensity as a function of angle of incidence are described by the Fresnel Equations. From that Wikipedia article, here is a diagram showing how the intensity of the (front) reflection changes with angle of incidence and polarization:
This effect is independent of wavelength (except inasmuch as the refractive index is a weak function of wavelength... So different colors of light will have a slightly different Brewster angle); the only way in which laser light is different from "ordinary" light in this case is the fact that laser light is typically linearly polarized, so that the reflection coefficient for a particular angle can be changed simply by rotating the laser pointer.
As Rainer P pointed out in a comment, if there is a coefficient of reflection $c$ at the front face, then $(1-c)$ of the intensity makes it to the back; and if the coefficient of reflection at the inside of the glass/air interface is $r$, then the successive reflected beams will have intensities that decrease geometrically:
$$c, (1-c)(1-r), (1-c)(1-r)r, (1-c)(1-r)r^2, (1-c)(1-r)r^3, ...$$
Of course the reciprocity theorem tells us that when we reverse the direction of a beam, we get the same reflectivity, so $r=c$ . This means the above can be simplified; but I left it in this form to show better what interactions the rays undergo. The above also assumes perfect reflection at the silvered (back) face: it should be easy to see how you could add that term...
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Best Answer
Here's a video of physicist Richard Feynman discussing this question.
Imagine a blue dot and a red dot. They are in front of you, and the blue dot is on the right. Behind them is a mirror, and you can see their image in the mirror. The image of the blue dot is still on the right in the mirror.
What's different is that in the mirror, there's also a reflection of you. From that reflection's point of view, the blue dot is on the left.
What the mirror really does is flip the order of things in the direction perpendicular to its surface. Going on a line from behind you to in front of you, the order in real space is
The order in the image space is
Although left and right are not reversed, the blue dot, which in reality is lined up with your right eye, is lined up with your left eye in the image.
The key is that you are roughly left/right symmetric. The eye the blue dot is lined up with is still your right eye, even in the image. Imagine instead that Two-Face was looking in the mirror. (This is a fictional character whose left and right side of his face look different. His image on Wikipedia looks like this:)
If two-face looked in the mirror, he would instantly see that it was not himself looking back! If he had an identical twin and looked right at the identical twin, the "normal" sides of their face would be opposite each other. Two-face's good side is the right. When he looked at his twin, the twin's good side would be to the original two-face's left.
Instead, the mirror Two-face's good side is also to the right. Here is an illustration:
So two-face would not be confused by the dots. If the blue dot is lined up with Two-Face's good side, it is still lined up with his good side in the mirror. Here it is with the dots:
Two-face would recognize that left and right haven't been flipped so much as forward and backward, creating a different version of himself that cannot be rotated around to fit on top the original.