The light output would be the same because there is still only one light source. But as you note, some of the “light” would be absorbed by the wall, so the room would be brighter if you had a mirror. Not twice as bright though, because the wall also “reflects” light, otherwise you would not see the wall.
So, would it make sense to have mirror walls to maximize room brightness? Maybe not, because what the walls end up doing is actually scattering the light, meaning light is spread out evenly, wheras with the mirrors you would have some bright areas (corresponding to the lamp) and some darker areas.
Good question.
The light output would be the same because there is still only one light source. But as you note, some of the “light” would be absorbed by the wall, so the room would be brighter if you had a mirror. Not twice as bright though, because the wall also “reflects” light, otherwise you would not see the wall.
So, would it make sense to have mirror walls to maximize room brightness? Maybe not, because what the walls end up doing is actually scattering the light, meaning light is spread out evenly, wheras with the mirrors you would have some bright areas (corresponding to the lamp) and some darker areas.