# B How does light travel through semi-opaque surfaces?

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1. Apr 11, 2016

### kasykid

So i was wondering....if you have 2 surfaces of different areas..one bigger and one smaller...and the one with a smaller area disperses light through a lens..does more light travel through the one with the lens or the other or does the same amount of light travel through both?

To get further details into this question and how I came to it. I was thinking if you have 2 cars, one with a big headlight that had no lenses in front of it and one with a lens but smaller headlight and the headlights get covered with the exact same amount of dust...(the same thickness on a square cm) will one car have brighter headlights or just the same?(of course you have the same exact light source in both of them)

2. Apr 11, 2016

### BvU

Hi kasykid,

Apart from the fact that there are no cars without a lens in front or a mirror behind the lamp (*):
Opaqueness is generally the fraction of the incoming light that gets absorbed (or scattered). So it all goes proportional and the ratio between the brightnesses doesn't change if the grime layers are the same thickness.

(*) the inverse square law would mean there isn't enough lighting in the desired direction.

3. Apr 11, 2016

### kasykid

Thanks for the welcome :D

I know all cars have a mirror behind the lamp but some have a lens in front of it and some don't(my car has only the plasticky(or glass) material that has some indents but no actual lens,but it does have the mirror behind it).

I am not sure if you understood my question...in my scenario I imagine that because the area that the light leaves through is smaller for the light with a lens...there might be "more" photons of light that can escape in a smaller area(making it brighter) and on the other side the same amount of light will try to escape through a bigger dirty area making it dimmer?

I am sorry if I can't explain the problem correctly, I am not a native English speaker and I guess I can't explain things as I should.

Maybe if you ignore my scenario completely and could say if you have 2 boxes...that are coated with reflective material on the inside and one has a whole side that is just normal glass that does not change light's path but is dirty and one that has only a small part of a side(50% for example) with a lens that is also dirty and both have a light source of the same power. would more light escape the one with the lens or the one with the glass pane(assuming that all light is concentrated in the lens and is not lost or something.

4. Apr 12, 2016

### Khashishi

To first approximation it would be the same, since the "dirt" absorbs a fixed fraction of the light that passes through it.