Eli5 : how does light from a light source spread?


There are rules like the the inverse square law, where it it often visualized that light travels in rays.
At first the rays are a lot and really dense, the further away the object it hits is, the lesser rays will hit it because it spreads harder than the object is big.

Now with the logic of a watering can. If the water streams would always spread the same amount, there should be places that never get hit by water depending on the distance of the object.

Can this happen with light? Are there blind spots of light even without any direct obstacle? Or does light travel like a wave hitting the shore, without gaps or something? I always wondered

Edit :This is super informative and interesting thank you alot Champs!

In: 1

Using the analogies mentioned, it’s like a watering can that you keep shaking back and forth.

Photons (the “water droplets” that make up light) are emitted based on the movement and energy present in electrons (high energy particles that orbit atoms, the building blocks of everything). These particles aren’t released from the same place every time, and certainly aren’t released at the same angle.

Much like shaking a watering can, that means when you’re super close to the object in question, it’s almost guaranteed that you’ll hit every spot. This gets less and less likely the farther away you get.

Rays of light are simplifaction, it is a way to think of how they move through space. It is a good tool to look at how to change direction in lenses, mirror, and when if something block light from in some location. But if you consider how the intensity drop it is not the best thing to consider

Light is made of individual photons. If you would count the number of phtons that is a square meter perpendicular to the sunlight you get numbers like 3.6*10^21 photons/second which is 3.6 thousand billion billion photons. There is a good reason not to think about individual photons but to idealize it with a few light rays in most situations


The inverse square law is a result of geometry. It is only valid for a point source that emits light equally in all directions. It is a good approximation for most light sources if you are far enough away from them so their angular size is negligible.

The explanation is that the surface area of a sphere is proportional to the squares to the radius. So the same amount of radiation that clovers a smaller area closer to the sun.

Could some area on earth not be high by any photons from the sun for some moment of time even if nothing is in the way to block or redirect light? If you take shore enough time interval and small enough area then is certainly could be the case. But you need a short time interval. an area 400×400 nanometer, that is sides with the wavelength of blue light, will be high by 576 million photons per second. But if you talk about coming to light a human eye for a duration you can spot it then the answer is no, or more exactly it is unlikely there even on during the life of earth been a place to put an eye for that to be observed.

Byt lest look at when the light from the sun is 1 photon per square centimeter meter and second to approximate a human eye area. That is a distance of 600 million times longer or 600 million AU. A light-year is 63241 AU. So we talk of a distance of 9487 light-years from the sun. That said you will not spot a star that dim in the sky, a normal naked eye could spot the sun at a distance of around 50 lightyears.

The number above assume earth surface that is around 2/3 of the light level above the atmosphere but I skipped that in the calculation,

So at earth’s distance from the sun at sizes and time scale humans can observe there are enough photons so the sun would always be visible. But if we talk about interstellar distance then you certainly reach a points where the number of photons is so low that we can say there is the time where none from the sun is an area.

[This neat diagram](https://en.wikipedia.org/wiki/File:Inverse_square_law.svg) from Wikipedia might help.

Our light source is sending out individual packets of light (photons) at random, in random directions. If we just look at those hitting the square at distance “r”, each photon will hit the target square. However when we get to distance “2r” there is only a 1 in 4 chance that our photon will hit any one square. By “3r” there is a 1 in 9 chance that our photon will hit any one of the squares.

So it isn’t that there are blind spots, more that the chance of a photon hitting our target area gets lower the further out we go.

Remember that the “rays” or lines in that diagram are for illustrating things only. We are really looking at a steady stream of individual photons shooting out from the light source in random directions. So it isn’t like our watering can where there are certain ‘fixed’ streams of water coming out of fixed holes. More like a watering can where the entire nozzle was open, but each droplet of water picked its own random direction to head out in.

Light is made of particles, and not lines.

These particles (from an undirected light source) spread out in all directions. If you get so far away that the particles are scarce, then the time between getting hit by them can become significant. At this point things aren’t really “illuminated” in the traditional sense, since even a single particle of light strikes fairly rarely.

This is connected to the wavelike nature of light. Where the waves are stronger, the odds of encountering a particle are higher.

The quantum physics explanation is that a photon takes all possible paths. Literally. The sum of all these paths is represented as a wave. The wave spreads out on all possible directions. But that wave can only resolve into a single interaction, such as hitting a receptor in your eye.

Why a wave that goes in all possible directions suddenly becomes a single point that has taken one particular path instead of another is a major unresolved problem called the measurement problem.