I came across an article comparing the human eye with a digital camera. The eye stood out far above the camera as for now. But we do not know how long it will be before the camera catches up with the eye. While the camera has a maximum capacity of 12 megapixels, the eye is estimated to be equivalent to over 500 megapixels!
The article got me thinking about light and the eye. I read that light is not only wave form of energy, it has a particle aspect too. This makes for the duality in the nature of light. Light when passed through 2 vertical narrow slits creates bright and dark bands on the screen, attributed to interference of the split wave. The bands cannot be explained by the particle theory of light. On the other hand, scattering of reflected light cannot be explained by the wave theory. How can the 2 aspects be together? This duality may be an easy to digest concept for the boffins, but it did get my head spinning.
I learnt that the particle of light, named ‘photon’ by Gilbert N. Lewis, had amazing properties. It always moved at an incredible speed of 300,000 kilometers per second, never stopping. It had no mass. But it had momentum and energy! Even when the photons hit a surface and they would scatter reflecting off it in all directions, but they would not lose any speed after the impact! And it was this scattering of photons from the surface of an object which enabled an eye to see it!
It struck me that the impact of a photon impinging on the retina could register a sensation in the brain resulting in vision! This led me to dwell on how the eye functions. I looked around my well-lit room. I saw a framed painting on the wall. The scattered photons made the painting visible to my eye. Each minute part of the painting was sending a scattered photon to a point on the surface of my retina where a light receptor was receiving its energy and sending message to my brain. When light was reflected from a red portion of the surface, the photon which impinges our eye excited only the red-color-sensing receptor on the retina. The receptors for sensing the blue and the yellow colors were not excited. This indicated that there are different types of photons for different colors. I realized that millions of photons must be involved to create the picture of the frame in my mind. I shook my head a little. I could see the frame without any break as I moved. For every new position, a new set of millions of photons were hitting my retina. How many photons are there? I wondered.
A quick research on the retina revealed that the eye has receptors on the retina which are called rods for sensing dim light and 3 types of cones for sensing color; for red, blue and for green light. These receptors were akin to sensor pixels in a digital camera. The camera comparison had given me a figure of 500 megapixels for the eye! Multiplying that number with 3, one for each color, I arrived at a colossal number of 1500 mega photons. These myriads of photons were impacting on my eye at least 24 times a second for me to see properly without a flicker! And the eye is seeing through my pupil, the small aperture on my cornea which is hardly a few millimeters in diameter. The room is much larger. It must have therefore perhaps a million times more photons beaming about! And what about the garden, and the road, and the city and the country and the World! As my brain started to buzz, my mind rebelled, ‘Stop. Just stop there; no more. I agree there are infinite photons all over in this World.’
I was wonderstruck with the awesome quality of these photons. What are they? And where do they come from. This culminated in the basic question how is light made. Besides Sunlight which is its natural source, I searched for the history of the torch. The first was through combustion. Fire. Igniting wood creates fire and fire results in light. I went through the successive stages of the arc lamp, incandescent light, xenon flash, and the modern light emitting diodes (LED). Initially it appeared to me that light was related to high temperature. It was as if the material was white with rage and crying out loud making a desperate throw of light energy in an effort to cool down. The LED, however, are low temperature light makers, though their light was bluish. They need electricity, not heat, to give out light. In both cases there was expenditure of energy to create light; and more the energy, the more the intensity of light.
What is the measure of intensity? I knew that electric bulbs are rated in watts. But that, I found was the measure of the power consumed by the bulb, but as the bulb was an inefficient converter of electricity to light, it was not a true measure of the light. Light is measured in watts per square meter and also in micromole per sq. m per sec which is the measure of photons. One unit of Einstein is one mole of photons which has 6 x 10^23 photons per sq. m per second and is equivalent to about 2250 watts per sq. m. The amount of solar energy at noon which falls on the surface of the Earth is 168 watts per sq. m. This is equivalent to about 5 x 10^22 photons per sq. m per sec! The number of photons is 5 followed by 22 zeros per second for every square meter of the surface of the Earth! And the Earth’s surface facing the Sun is 250 trillion square meters! It is this vast number of photons coming from the Sun every single second that makes Nature so beautiful. It is these photons which light up the features of the Earth and make them visible to our eye! Thanks to them that we can see the blue sea and the red roses! These very protons catalyze photosynthesis the process by which the plants make food for us. There was no doubt in my mind that without food we could not live and without light we could not see. We owe it all to the photons.
The photons are so completely invisible that we cannot see them at all. We can only experience their effect when our eye comes in their path. So while trillions and trillions of photons are jetting along the whole atmosphere on Earth, we do not know of their existence. They show us objects by reflecting off the surface but they keep themselves behind the scene. It is completely selfless service they are rendering to the life on the planet! Providence has truly blessed us with the Sun and its photons! Man too adds photons to the planet, particularly through the lighting at night. He fills up night-space with the photons from lamps and electric bulbs!
Man does more than that! He fills the atmosphere with tremendous amount of radio waves. There are 3.3 billion users of mobile phones in the World. Satellite communications, GPS systems – there are a myriad different waves created by man filling the space. But they are not particles. They are pure electromagnetic wave forms. Photons are something special. Incessant movement, no loss of speed on reflection, no mass, color quality, silent and subservient, responsible for the vision of insects and animals, responsible for growth of plants for our food, enriching our life every single moment. We take them for granted, but these protons ought to be recognized for their life-sustaining benevolence and invaluable contribution to our indispensable eyesight!