why is it harder to see outside if you have a light on inside?

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Happens in my house as well as when driving.

In: Physics

It’s because the light on the inside is preventing your pupils from getting larger to better see in the dark. Take a hand mirror with you and step outside on a sunny day. Look at your own pupils, in the mirror. They will be small. Bigger pupils (not exposed to light) are necessary to see better in darkness.

Apart from pupil dilatation mentioned by another commenter, it also has a lot to do with the reflection created on glass surfaces: if you’re in a room at night and light up a lamp you’ll notice that you can clearly see the reflection of the room on the window, but not what’s outside. This is because the light coming from the lamp, which is reflected by the glass, is much more intense than that coming from outside, which in turn gets suppressed by the former.
The result is that your eye perceives only the reflected light, making it hard to see outside.

In addition to what others have posted, remember that if you have trouble seeing OUTSIDE because it’s darker there, those people will have a very easy time seeing INSIDE. This is why we have shades, curtains, shutters, blinds, and other adjustable things to block the view through a window.