Today when i was travelling in train(from my study center to home) i found a common but unexplained thing that is a little reflection can be seen on Glass window of train when it is underground or there is any wall behind the windows BUT when there is no window(ex when it is in station there is a lot of light) then i can't see any kind of reflection in window (or very very little reflection). I had read very little on Optics like Lens maker formula etc. I try to explain this during my 30 minute journey but i can't so i want to know it's answer from you why there is reflection in glass window.why this glass window was reflecting when it was underground. This glass window was double(one plate of glass after other as i see) i think because they(manufacturer of train) do this to don't make it hot due to sun in summer/ keep train cool with it's AC.One thing i also want to say that there a lot of lights inside train as a source of light for reflection. How I explain it. first explanation::-> This reflection happen due to reflection backward wall. Limitation wall did not reflect. second explanation ::-> Double glass window of train reflect light rays every time but i can see them only at that time when there is any obstacle that does not overcome the intensity of light of reflected rays by glass window in case of stations there is light entering inside train which overcome the intensity of reflected light by glass window but when it is underground no light enter externally so i can see reflection of persons inside train. this idea has recently came in mind is it correct? Or there is any other explanation for this? there is no double glass window in my house or nearby to experiment so you tell me.