At one time, I wondered why the human eye was limited by its inability to see very small objects.

I subsequently learned for that to occur would require a microscope, an invention by Antoni van Leeuwenhoek (1632-1723), a Dutch cloth merchant and improved upon by Robert Hooke (1635-1703), who discovered the existence of cells by examining a slice of cork .   (A microscope by virtue of its precisely made glass lenses takes minutely separated light rays coming from a tiny object and spreads them apart so they appear to be coming from a much bigger object.)  

I then learned that the naked eye can see objects of any size if those objects either emit or scatter enough light to trigger the detector cells in the eye.  Viruses, bacteria, fungi and other microbes are too small to scatter such light and there may be as many as 100 trillion of them inhabiting in and on the outside of your body.  Imagine the confusion and terror that would result from observing  them or the one million microbes contained in or on each gram of our food.

Light

 Light is complicated and fascinating. It is made of photons; which have no mass, but carry momentum and travel at the speed of light.  The light waves are not deflected by either electric or magnetic fields.  

Photons are elementary particles; which constitute electromagnetic radiation and carry electromagnetic energy, proportional to the radiation frequency. For this reason, a photon is considered an energy packet. Photons are emitted by charged particles; as well as radioactive decay. Since they are extremely small particles, their wave-like characteristics are more significant. Each photon has a wavelength and a frequency — the same as electromagnetic waves.

Electromagnetism

The components of electromagnetic (EM) waves are electric and magnetic fields, at right-angles to each other. Vibrations of these two fields generate EM waves. Hence, oscillating magnetic and electric fields produce EM waves. Thus, light — visible or invisible — occurs as the particles move in a wave-form.

In order of decreasing wavelength and increasing frequency and energy, the EM spectrum is divided into seven regions:  radio waves, microwaves,  infrared (IR), visible light,  ultraviolet (UV),  x-rays and  gamma-rays.

The electromagnetic spectrum comprises the entire range of radiation — visible and invisible — from radio (communication) waves to Gamma rays (γ-rays). However, just a tiny portion of the EM Spectrum (0.0035%), is visible to us.

Electromagnetic radiation travels in the form of transverse waves, with their wavelength ranging from very long radio waves (communication) to very short gamma rays (γ-rays).

The Human Eye

Human vision is a complex and wonderful process that is not completely understood even today.   It involves the eye, which consists of an outer shell, pupil, lens and retina, nerves, and ultimately, perhaps most importantly, the brain.  

As mentioned above, detector cells found in the human retina (and other mammalian eyes)  consist of three known types of photoreceptor cells: rods, cones and intrinsically photosensitive retinal ganglion cells, that are triggered by light.  

The two classic photoreceptor cells are rods and cones, each contributing information used by the visual system to form a representation of the visual world, sight. ach human retina (and you have two, one in each eye) contains 125 million rods and about 6 million cones. The rods handle low light level (or night vision) and cones are responsible for high-light-level, high resolution, color vision.

The  very small region of the electromagnetic spectrum is visible light and the frequencies are comparable to the resonance frequencies of the receptors of your eyes.  Typically, the human eye can detect light from 380 nm to 700 nanometer (nm) Wavelength.  

All visible waves are quite small on a human scale, a nanometer is a billionth of a meter.  Different colors of the spectrum correspond to different wave lengths, the shortest possible visible wave lengths produce the perception of violet and the longest, the perception of red. We cannot detect light beyond violet (UV, X-rays and γ-rays) or before red (infra-red and the longer radio waves), on the other side of the spectrum.  

Light that enters one of the rods or cones first causes a chemical change in a substance inside called a visual pigment.  The rods have one kind of pigment, while each cone has one of three different kinds to distinguish colors.  The first step is a bleaching of the visual pigment from a darker state to a clear state by the light.  

This change becomes transformed into an output in the form of a nerve impulse from the outer end of the cell. The information from the receptors is converted from light to electricity and transmitted along one million nerve fibers to the 1% of the cortex of the brain.  As little as one photon can trigger a photoelectric cell.

Final Thoughts

There are a number of books that fully describe the physics of light, vision and color. One is “The Introduction of Light” written by Gary Waldman.  Another is “Seeing the Light” by David Falk, Dieter Drill and David Stork.

Max Sherman is a medical writer and pharmacist retired from the medical device industry.  His new book “Science Snippets” is available from Amazon and other book sellers. It contains a number of previously published columns.  He can be reached by email at  maxsherman339@gmail.com.