Why does light from a fireplace in the dark seem to dissipate abruptly? Why does it not dissipate smoothly over a larger area?


Goes for any light source really, just using a fireplace/bonfire as an example. If you were to look at a bonfire from a distance in the dark, the light seems to stop rather suddenly. But the only way you’re seeing it in the first place is from the light hitting your eyes.

In: 3

The intensity of light, as one gets further away from the light source, dissipates according to something called the ‘inverse square law’. In general, if the distance from the light source is x, then the intensity of the light reaching you is proportional to 1/x^2 . In other words, if you move twice as far away from the light, you’re a *quarter* as illuminated as you were before. The geometric reasons why this must be so, are best explained with [diagrams like this one.](https://lh3.googleusercontent.com/proxy/2-oOBXjC-vGDwN3BkkNNyBi-LTwpha-UW6a48KOdbDi1iImSFtQ7UjHAcV2DMjsg3dRAS5ULTllaFZKuyQDX3CtcdEE66rEeFrpsrAg113nLmQfi6QoT2w)

This inverse square law, if graphed out as a function, has a [much steeper dropoff](http://www.math-mate.com/chapter48_1_files/image011.gif) than an inverse, or 1/x function. So that’s a reason why the transition from light to dark might seem so abrupt.

There’s probably also lots to talk about regarding human vision and the dynamic range that we perceive, because what subjectively feels ‘twice as bright’ to us, isn’t necessarily the same in all conditions. Our eyes do all kinds of stuff like pupil dilation/contraction and retinal cell exhaustion/satiation which can mess with our sense of brightness.

Illumination from a light source decreases proportionately to the square of distance. If you look at the graph of 1/x^2 , it really looks like a rather abrupt drop.

In addition remember that campfires or bonfires tend to be in wide open spaces with no surfaces for the light to reflect off of. Indoor lights seem to “carry” more because they reflect off walls or ceilings, etc.

Even indoor fireplaces are designed to dissipate heat/light as quickly as possible to avoid overheating and combusting any nearby surfaces.

There are two reasons for this. One has already been mentioned by the other posters, which is the inverse square law.

The second, which has more to do with the appearance of a sudden drop off than anything else, is that the fire itself is an open light source, and as long as the fire is within your field of view your eyes are going to adjust to its brightness, which dims the hell out of everything else by comparison. You get the same effect if you take a lampshade off of a lightbulb. If you stand with your back to fire fora couple of minutes you’ll see that the rest of the area is lit up a lot more than it seemed before, because your eyes have adjusted back to the relative darkness.