About light in the universe As a light source in the universe (e.g. sun) emits light in different directions, some of the light emitted reaches places like Earth, and some doesn't. So does the light that reaches the Earth disappear or it is reflected in other directions? And for the light that doesn't reach any place, does it keep on going forever? If it does keep on going forever, will the universe become brighter and brighter? Thanks!
 A: As you probably know, the light that you mention is an electromagnetic radiation, so it is part of a large spectrum in which $\textit{visible light}$ makes only a small part. Even the sun emits tons of radiation, but not all of it is visible light. When you say about the brightness of the Universe (referring only to the visible light which is radiated by the cosmic objects), you need to be careful here: the brightness of the Universe can be calculated actually if you know how much light each cosmic object emits. Of course if you know the brightness of say Andromeda galaxy, then you probably know enough details about its components (i.e. nebulas, stars and so on). 
Scientists want to measure the brightness of the Universe ( see here). Probably they will obtain a value which of course will be large, but of course, finite. It can also change in value due to normal cosmic events: supernovae, quasars, death of stars (so it can decrease or increase) but I guess the change in value is NOT noticeable in one day :D
A: When any source emits light it may hit any objects like Earth and it will be reflected back in universe and yes the light which doesn't hit any thing will travel forever but that doesn't mean that the universe is going to be brighter because as the universe expands the light with in it gets red shifted and eventually get out of visible spectrum
There is a good video which will help you to get it 
https://m.youtube.com/watch?v=3tCMd1ytvWg
