A beam of light is made of photons, which simply travel in a line from point $\text{A}$ to point $\text{B}$. But we can only see things when photons hit our retina, so doesn't this mean that the photons of the beam ought to travel to our eyes? How is it possible to see the beam?
[Physics] How to see a beam of light
everyday-lifephotonsvision
Related Solutions
Short answer:
- Proteins or photopigments involved in vision have different sensitivity to light
- The transduction paths are different or the paths to produce/mantain the proteins/photopigments.
Generic answer:
The proteins or photo pigments responsible for sensing light usually undergo a conformation change/chemical change induced by light, but this change depends on the wavelength of the incident light. There are many different proteins that are light sensitive and the reasons why this reaction happens on a specific wavelength interval and what kind of reaction happens depends on the protein. However, not only different proteins react differently to light, but also what happens next may be different. After the protein suffers a change because of light (whatever that is) a long chain of chemical reaction begins which will end up as an electrical signal. Also, usually proteins or photopigments have to be recycled after being activated/bleached, which is done by parallel biochemical pathways. Mistakes on these paths will also after the ability of seeing a specific colour.
Example:
There are many kinds of vision mechanisms in the animal kingdom. The colours (sets of wavelength intervals) that each animal sees highly depends on the set of proteins that these animals have and the transduction path after the proteins are activated. We humans, for example, are good at seeing green and other colours that help us to identify food that we usually eat [1]. Interestingly, even small changes on the aminoacid sequence of the proteins change its sensitivity for light. For example, some people have cones (some of the cells responsible for vision in humans) with photo pigments that are sensitive at 530 nm and some people are sensitive at 562 nm. This difference is caused by only 3 aminoacids substitutions in a protein [2]. Another example involving the recycling pathway of a photopigment can be found at [3]. It was observed that some mutant Drosophila flies had their vision impaired because they could not keep the concentrations of the visual pigment Rhodopsin.
[1] Surridge, Alison K., Daniel Osorio, and Nicholas I. Mundy. "Evolution and selection of trichromatic vision in primates." Trends in Ecology & Evolution 18.4 (2003): 198-205.
[2] Neitz, Maureen, Jay Neitz, and Gerald H. Jacobs. "Spectral tuning of pigments underlying red-green color vision." Science 252.5008 (1991): 971-974.
[3] Ostroy, SANFORD E. "Characteristics of Drosophila rhodopsin in wild-type and norpA vision transduction mutants." The Journal of general physiology 72.5 (1978): 717-732.
As a beam of light travels, it spreads out so that for each doubling of the distance, the intensity of the light (defined as the number of photons passing through a unit area per second) is decreased by a factor of four. This is a basic consequence of the way that light beams propagate through 3-dimensional space and has nothing to do with the speed of the beam- which does not vary with distance.
So at a point B that is distant from a light source at A, not all of the photons leaving A will hit B. The result is that fewer and fewer photons will arrive at B as the distance between A and B gets larger and larger, causing the intensity of light measured at B to decrease with increasing distance.
Best Answer
You're not seeing the photons in the beam that are traveling from A to B (beam starting point to beam destination), you are seeing photons that are scattering off of dust particles that are in the path of the beam.
This is the reason why you see lasers in a night club more clearly when there is a smoke machine, and why cat burglars blow dust onto security beams, to expose them ;-)