[Jimbee (OP)]

Did you ever notice? When someone is looking out from a brightly lit room to some place that is pitch dark, like the outside at night, they really can't see anything at all. But the people outside can see them clearly. Also, shades seem to make a difference. It is much harder to see into a room with shade in the window if it is brighter outside. I think I heard some place that is why they used to use shades on windows.

What is this optic effect called? And how does it work?

[paul cotter]

In a brightly lit room one's pupils are constricted to reduce the incident light and there is not enough light entering the eye to observe dark objects outside. Those outside in the dark have dilated pupils, due to the low ambient light and are well adjusted to see into the brightly lit room. I am not sure if there is name for this but I doubt it as it is just a matter of the ability of the eye to adjust to different light levels.

[alancalverd]

Two different but additive effects:

The outdoor ambient light intensity can be 10,000 times that of indoors. No problem if you are in or out as the pupil and retina can compensate for that range. But

1. Window glass reflects about 5% of the incident light, so, looking in from outside, the input to  your eyes consists mostly of reflected ambient, and the "wanted" signal from inside is swamped by this "noise". In the opposite direction, the "noise" is only 5% of the indoor ambient  and the "signal" is very much stronger.

2. "Shades" doesn't translate unambiguously across the pond, but I think "net curtains" means  the same in Yankish and British. Outside to inside, the total signal is attenuated by one pass of the curtain, to say 50% but the ambient noise is attenuated by two passes - 25%. More to the point, however, white net also reflects another 10% of the exterior ambient whilst attenuating the interior signal by 50%, thus making the interior scene very difficult to interpret from outside.

I had a problem with the curved glass TV screens  we used for x-ray fluoroscopy in operating theaters in the 1970s. The ambient was sometimes brighter than daylight, phosphor brightness and contrast were at best "marginal", and the curved face always seemed to reflect one of the operating lamps, leading to much frustration - and a frustrated orthopedic surgeon with a saw is not a Good Thing.    We solved the problem for a few years by stretching a nurse's black stocking over the screen, which doubled the apparent contrast, then gradually replaced the TVs  with high contrast flat LED screens with a built-in anti-glare mesh. Problem solved, though I got a reputation among theater technicians as "the bloke who asks for your old stockings".

 

[paul cotter]

Not disputing one word of what you have said but I fear you have got the "wrong end of the stick". The OP was talking about an illuminated room versus a dark exterior and your example was a detailed analysis of an illuminated room with daylight outside.

[vhfpmr]

It's not about interior vs exterior, it's lighter side vs darker side.

When you look from the brightest lit side to the darker side, the light reflected off the window/net curtains/ half-silvered mirror is greater than the light transmitted through it from the darker side, so you only see the reflected light because your eye doesn't have sufficient dynamic range to see both.

Move to the darker side, and the light transmitted from the bright side is greater than the light reflected from the dark side, so you see through.

The way to see the "wrong" way through a "one way mirror" is to put your face right up against it, and shade around your eyes with your hands (the same way oscilloscope hoods work).

[alancalverd]

Not disputing one word of what you have said but I fear you have got the "wrong end of the stick". The OP was talking about an illuminated room versus a dark exterior and your example was a detailed analysis of an illuminated room with daylight outside.

Mea culpa, but geometric optics is commutative!

[Petrochemicals]

Regardless of if it is a building or not, in the bright situatiins pupil dilation is smaller so less light from outside enters plus the fact that if you are closer to a light it makes up much more of your field of view, if you sat next to a lamp means the light has diffused little and makes up a 45 degree portion of your view. At 10 metres the light has weakened significantly and only makes up 5 degrees of your vision.

Guards on duty in the army used to stand next to a light until someone realised the reason they where being shot. Another army saying is that you can see a lit cigarette at 200 metres