Layman studies Light

It was the very first time. Her optic nerve sparked like a benevolent thunderbolt. The lucky photon came bouncing off the dining room light. It penetrated layers of stretched biological tissue, the amniotic sac, the translucent fluid inside, and reached the photo-sensitive cell at the precise moment when the cell became active. She twitched her still-developing limbs; her first acknowledgement of a visible world beyond.

She was a fortunate child amidst caring people: the mother, father, grandparents, aunt and uncle, a little niece too.

They were eating dinner and talking about light and shadows. She was listening.

"I saw two shadows of myself outside the door today", aunt said.

"When was your last eye checkup?", father asked. He was enamored by science and would lead any conversation into scientific bylanes - regardless of the weather; time of day, or night; and regardless of streetlights and shadows lurking in the corners. "He's like that", they all would now and then affirm and nod to each other in agreement.

"There was only one Sun overhead. How could there be two shadows?", aunt queried ignoring her brother's remark.

"I've seen that before", grandma said, "The second shadow - it'll be dull - is from the light reflected off a glass window. Was there a car nearby?"

"Ah! That's what it is! There was a car nearby", aunt exclaimed.

The photo-sensitive cells were waking up one by one. The world blinked at her. Ambient red light with flashes of white filled her all her senses - a senesthet in the womb.

Aunt and grandma turned to father. Aunt said, "What's a shadow?".

Aunt comes from a long line of fascinated creatures - creatures that sometimes ran, sometimes stood transfixed by light and shadows. The plankton that wished for more light, the boy walking back from school dodging the shadows that followed him, the agitated dalmation trying to catch its own tail ...
Light and shadow: the fascinating, beguiling particles of pure energy wandering in the void with their ghost-selves, quantum messengers with non-existent attendants.

"A shadow is where light decides not to go", father said, "Shadows occur on a surface when an opaque object blocks the path of light", he continued walking blithely into an obscure lane of reason and science, "There are occasions when a shadow does not form, even though there is an opaque object blocking the path of light"

"Yeah?", aunt wondered whether to follow or stay back and finish the dinner.

Mother shifted in the chair. Her head knocked softly on the walls of the amniotic sac. She slowly turned inside the warm fluid; a perceptible change of perspective. She had rudimentory memory: light and sounds now came from a different direction.

Father said, "Yes. Light is the name given to photons of a wide range of energies - the range is called the electromagnetic spectrum. We call the visible portion as Light"

"How would a shadow not form?", grandma was curious.

The sounds brightened her brain. Light was loud. A blob drifted in her field of vision.

"Light is a wave and like all waves, it can bend around obstacles smaller than its wavelength. Consider radiowaves. It is light with wavelength of a few meters. It can easily bend around, say, your enormous head or small arms - and it does"

Aunt kicked father's leg under the table. Father continued, "...ouch.. If the sun emitted only radiowaves, you wont see shadows of yourself at all - assuming you can somehow live and 'see' in radio wavelength"

"Eat your dinner, dear", mother reminded him.

Light, shadows and sounds reached her, caressed her, prodded her to participate in the quantum dance. She did. Reaching out from her fluid reality - an expanding sphere of senses, dimly illuminated by an infant brain still in development - she kicked.

---

Some physics fireworks now. I asked Arunn if it correct to say that we see anything at all because light bends around the atoms and molecules in the air? The air being full of atoms that, if light did not bend around them we'd have a hard time seeing anything.

My question is only useful in that it elicited a clear and illuminating response from Arunn. Many of you would see my question for what it is: stupid and ignorant. I'll let Arunn clarify:

We "see" (say, each other) undisturbed by the air because air has very less density (in a given volume) to influence the light (obstructing its 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 which 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).

To add more: even in air, say, inside a room where the two of us facing 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 than 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. The sunlight using which we see each other in the room, 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.

A clear and concise note that - I am glad and thankful - has penetrated even the concrete folds of the grey matter between my two ears.