The high school physics text books that I have studied usually devote a chapter for each of the different “branches” of physics. Perhaps not to scare the gentle (layman?) students off by using scary jargons, these chapters would be titled in “plain English” as, say, Light, Sound, Heat, Modern Physics (as against Optics, Acoustics etc.). In my home town high school public exam the question paper usually reserves disproportionate marks to the respective branches of physics above – more for Light and very less for Sound, for instance. The PJ before the exam is, “Study Light soundly and Sound lightly”.
There could be a sound point to this that should be apparent before the end of this post, but let me now digress into some thing Light.
Some time back, while preparing his Layman studies Light post Selva asked me by email this question, “Is it correct to say that we are able to see anything at all because light bends around the atoms and molecules in the air? The air is full of atoms that if light did not bend around them, we’d have a hard time seeing anything. Is this thought scientifically tenable or have I gone horribly wrong somewhere in my understanding?”
An explanation I mustered, which I present here with some modifications…
We “see” (say, each other) undisturbed by the air because it has very less density in a given volume to influence the light (so that it could obstruct light’s path by its molecules). So light doesn’t have to “bend around” the atoms. It more or less evades hitting them as there are only a few of them with a relatively large mean free path.
On the other hand, if we have the same light passing through a liquid, the molecules of that liquid do influence the light path and results in refraction. As you know, a measure of how much refraction happens is done through the refractive index of that medium (for air it is one).
On the other hand, even in air, say, inside a room where the two of us face each other and we keep “seeing” each other without any disturbance from the room air and its molecules, it is worth noting we also don’t see the dust that is present in the air (which is much bigger in size that the molecules of air) between us. The dust does actually reflect back the light but since the density of the dust again is relatively small in a room full of air, we still are able to “see” each other unhindered – if we are hindered, we will also see the dust particles.
Again, the density of the dust alone is not the controlling factor, but the intensity of the incident light also matters. As the sunlight using which we see each other in the room, in the first place, is of less intensity it doesn’t trigger the light sensitive part of the eye to discern the weak intensity variation in the reflected light from the dust particles. However, as we know, if we shine a torch (flash) light in the room, the column of the light beam has enough intensity to get scattered from the dust particles and reach our eyes with enough intensity for us to “see” the dust.
Again, in the same room if we use a laser beam, we should be able to see the dust.
There ends the explanation I wrote then. There is a thought that such explanations done mostly in “plain talk” are suited for the “layman (and woman)” and that is the best way to propagate Science. In my opinion there are no “laymen (or women)” in this World.
There are perhaps only (sound) explanations and light/lay-explanations, like the one given above.
UPDATE (addressing comments below):
1) Yes, refractive index of air is not EXACTLY EQUAL to 1, but in the context of this essay it is alright to �assume� it to be one.
2) Sunlight IS of less intensity when it reaches Earth surface. In fact, it IS diffuse radiation, the reason why we do all those solar �collecting� to (re)focus the sunlight.
On the other hand, the energy in sunlight when it comes out of the Sun IS definitely huge. But the intensity (total radiation per unit area) when it hits the Earth�s surface, in the visible spectrum, is about 600 kW/m^2 (again, this number is not exact, but not totally wrong either, I guess). Point is, when we see each other (visible spectrum) in a room lit ONLY by sunlight (from the windows, say), we don�t see the dust in between because sunlight gets reflected not only from the dust (which anyway is minimal) but also from other sources (us, table, chair, walls etc.) as well. The human eye is unable to discern the difference in the intensity of the reflected radiation from the dust over the rest of the reflection (OK, at least in the case of dust, ideally, I should replace reflection by scattering and there are three types of it etc., but I hope readers will allow me to use this terminology to keep the discussion simple).
On the other hand, light from a torch light close to its source (near the torchlight) IS of higher intensity than that of the sunlight that we have on Earth in the visible spectrum. Intensity also follows the inverse square law with distance right? So, in a room when a torch light is shined, I can see the dust particles near to the light source more because, the eye can discern the scattered light from the dust (which is of higher intensity as it is merely reflecting the high intensity light near to the source) more easily than that of the rest of the surrounding light. Of course, this works only with a small volume of space close to the source of the light.
Even in case of sunlight, if we make our high school pin-hole camera like set-up, the sunlight passing through the slit would be more localized to reflect from the dust, to be seen by us (if the rest of the room is assumed to be dark).
To tie this with what Lakshmi writes below in the comments, torch light, close to its source, has more photon density (more photons per unit volume) than at a far away distance. Same with Sunlight. A more intense radiation like that from a LASER would have less diffusion of their photons for relatively a long distance, hence their �intensity� is intact. Hence they will be reflected (scattered) by the dust also, with more intensity (when compared to the other light sources).
