# Red or white

1. Nov 8, 2005

### nzahra_ghasemi

there is two rooms in one of them ,i lit a red bulb and in the other room a white bulb when i'm in the red room i see the wall that is between these two room,red,but when i'm in the room with white light i see this wall white.
why?

2. Nov 8, 2005

### Staff: Mentor

A wall painted "white" reflects a broad spectrum of colors. When white light (a mix of all wavelengths) illuminates it, it appears white; but when red light illuminates it, it appears red. The apparent color of the wall depends on the light used to illuminate it.

Imagine this. Paint the wall "blue", meaning use paint that reflects blue light and absorbs other colors. When illuminated with white light, the "blue" wall appears blue. But shine red light on it and that red light will be absorbed. The "blue" wall will appear black.

3. Nov 8, 2005

### IHEPer

key words: wavelength, reflection

4. Nov 8, 2005

### nzahra_ghasemi

pay attention ,what i said,there is two room and an open door between them,when i'm in the room whith red light i see this wall (painted white)between two rooms red and when i'm in the room with white light i see it white.i say why i don't see this wall still red be cause of the open door.

5. Nov 9, 2005

### Hooloovoo

you didn't mention a door when you posed the question.

as posed, your question is essentially "why does a wall look red when illuminated with red light, but look white when illuminated with white light"

the reason, of course, is that the wall is reflecting whatever light you're shining on it.

but now you say there's a door in the wall. presumably it's open. so now you're asking how come the side of the wall facing the red light is still red.

the reason, of course, is that the white light isn't passing through the door only to make a sudden u-turn. in other words, that white light is not shining on the side of the wall in the red-lit room. so that side of the wall is only illuminated by the red light, and that's all that's getting reflected.

6. Nov 9, 2005

### Staff: Mentor

No, I think the question is regarding an adjacent side wall, not the center wall separating the rooms. S/he is saying that the side wall is illuminated from both rooms (because the center partition is open in that area), but you see the same section of wall as having two different colors, depending on where you look from.

The reason would have to do with the reflective properties of the wall. Texturing might cause it to reflect low-angle of incidence light rays back toward their source instead of into the next room. It may be easier to consider the effect of texturing if you make the texture big - say shoebox sized bumps on the wall. If you look at that wall at a low angle and shine a light from the same angle, the bumps will reflect light back to you, but they will also cast shadows. but since the bumps themselves block you from seeing the shadows, you'll only see the reflected light.

Last edited: Nov 9, 2005
7. Nov 9, 2005

### -Job-

Ambiguity really gets to me. It breaks my brain.

8. Nov 10, 2005

### nzahra_ghasemi

ah cheghadr shomaha khengid,so I repeat my question

**********************************************************
*(red light bulb)*
((room1))
wall(1)
-------------|wall(3) <---the wall between these two rooms |---------
-------------| |---------
wall(2)
((room2)) *(white light bulb)*

**********************************************************
I light a red bulb in room 1 and a white one in room 2.
when I'm in room 1,I see wall 1 ,red,and wall 2,white,and wall 3,red.
when I'm in room 2,I see wall 1 ,red,and wall 2,white,and wall 3,white.
I want to know why the color of wall 3 change when i chang my position?

9. Nov 10, 2005

### nzahra_ghasemi

I repeat my question:
**********************************************************
|((room 1))*red light bulb*
|
| wall 1
| v
|-----------------| < wall 3 |-------------------------------------------|
| ^
| wall 2
|((room 2)*white light bulb*
*********************************************************
I light a red bulb in room 1 ,and a white bulb in room 2.
when I'm in room 1,I see wall 1 ,red,and wall 2,white,and wall 3,red.
when I'm in room 1,I see wall 1 ,red,and wall 2,white,and wall 3,white.
I want to know why i see wall 3 in diffrent colors when i change my position?