Light and Vision

Examining the science behind sight

By Jenean Carlton, ABOC, NCLC

In our first installment of Eyecare Business' exclusive Back to Basics series, we explore light—what it is, the speed at which it travels, and how our eyes and brain use visible light to perceive vision—and why this basic understanding is so important in our careers as optical professionals.

Each Back to Basics article will also offer learning with a twist—a crossword puzzle on the subject (see puzzle on page 57).

The best scientists in the history of the world spent much of their lives studying the complexities of light, and have long been perplexed about its behavior, partly because the results of early scientific experiments were inconsistent. Some of those experiments suggested that light behaved in a wavelike manner, while results from other experiments could only be explained if light were composed of tiny particles.

EARLY THEORIES ABOUT LIGHT

Before the 1800s, there were two camps regarding theories about the nature of light. Some scientists believed in Sir Isaac Newton's particle or corpuscular theory of light: Light consists of tiny particles, or corpuscles, moving in straight lines at the speed of light. Max Planck later proposed the Quantum Theory of Light: Light is radiated and absorbed in a continuous manner in small units of energy called photons.

Others speculated that the wave theory of light, as explained by 17th century Dutch astronomer Christian Huygens, was accurate as it explained diffraction, the tendency of light to bend around corners of objects. A good example of diffraction is a shadow. As a wave of light passes the front edge of an object, parts of the wave will bend around the object, causing edges of the shadow to appear fuzzy.

In 1803, British scientist Thomas Young proved that light propagates as waves in his Double-Slit experiment. After making two cuts on a solid plate and shining a single beam of light through the slits, Young found a pattern of wave-like formations with bright and dark areas appeared on the wall opposite the plate.

Albert Einstein explained that light behaves as a waveform of energy and as tiny particles of electromagnetic (EM) radiation carrying a specific amount of energy called photons. Einstein's theory, the Wave-Particle Duality of Light, is accepted as the definitive nature of light.

When contemplating light as having both particle and wave-like properties in shorter wavelengths, such as x-rays, the particle theory explains most of the characteristics of energy in this range of wavelengths. For longer wavelengths, like radio waves, the wave model explains observed properties in this range of the EM spectrum. The visible light spectrum is in the middle of the electromagnetic spectrum, so both the wave and particle theories of light apply to these wavelengths.

SPEED OF LIGHT

When we flip on a light switch, it may seem that the room immediately becomes illuminated by the energy produced from the light source. However, light travels fast, but it's not immediate. Due to its high rate of speed, our eyes aren't able to perceive light initially radiating from a primary source, like a light bulb, and then spreading throughout a room.

Photo courtesy of Scalemodelguide.com

The speed of light was proposed in 1905 by Albert Einstein, who stated that nothing travels faster than the speed of light. Denoted as the letter ‘c', it is a universal fixed constant, particularly in physics, the science of matter and energy.

All light waves, including radio waves, microwaves, visible light, x-rays, and gamma rays, travel at the same velocity or speed: 186,282 miles per second, or about 300,000 kilometers per second, in a vacuum.

Let's consider that the speed of sound is approximately 1,130 feet per second. The difference between the speed of light and the speed of sound is, for example, why we see lightning before hearing thunder. Light travels at a measured speed, so it takes longer for light to travel to and from far-away distances. The sun, our primary light source and closest star, is 93 million miles away from Earth. It takes approximately eight minutes for light from the sun to reach Earth. The moon is approximately 238,000 miles from Earth, much closer than the sun. It takes only one second for light to travel from the moon to the Earth.

It travels in straight lines or rays in the rectilinear propagation of light. Its rays also travel faster through air than through other substances. When light rays move through air and then strike transparent matter at an angle, they slow down and then speed up again after exiting the object. When rays of light change speed, they bend, and the passage through a transparent medium is what we know as refraction.

Reflection, on the other hand, occurs when light strikes a surface and then bounces off. For example, as light strikes a flat mirror, the mirror reflects light rays at the same angle as they struck the surface.

ANATOMY OF A WAVE

Waves transport energy from one place to another. The crest of a wave is the highest point and the trough is the lowest. One wavelength is the distance from a point on a wave to the same corresponding point on the next wave; a wave's amplitude is the distance from the middle level of a wave to the crest or trough. The amount of energy a wave carries is dependent on its amplitude. High energy waves have a greater degree of amplitude than low energy waves.

Frequency, measured in hertz (Hz), (equivalent to one full wave passing by per second), is the number of vibrations of a given wavelength in one second. A wave's speed is dependent on the type of wave and the material through which it travels. Light travels in a vacuum at 186,282 miles per second, through water at 140,000 miles per second, and in CR39 at 124,000 miles per second.

The equation Wavelength x Frequency = Speed shows how wavelength, frequency, and speed are associated. Determining the speed for a given wavelength is simple if the frequency and wavelength are known values. Multiply the wavelength (in meters) by the frequency (in hertz). The result is the speed of the wave in meters per second.

Photo courtesy of Colour Therapy Healing

ELECTROMAGNETIC SPECTRUM

The electromagnetic spectrum is an arrangement of all known radiant energy—from low-energy radio waves that have long wavelengths and low frequencies to high-energy gamma rays that have extremely short wavelengths, high frequencies, and that produce a dangerous level of energy.

“Electromagnetic” means that there are two types of energy forces working together: an electrical force and a magnetic force. In the 19th century, James Maxell noted that electromagnetic waves consist of electrical and magnetic fields that vibrate perpendicular to each other. All electromagnetic waves, including visible light, travel at the speed of light.

The EM spectrum is arranged in the order of increasing wavelength/lower energy radiation to the left of the spectrum and decreasing wavelength/higher energy radiation to the right. It shows that visible light accounts for just a small range in the spectrum, from 760 nanometers to 380 nanometers. Our eyes are only able to perceive those wavelengths that fall in that window of the EM spectrum known as the visible spectrum.

VISIBLE SPECTRUM

Newton discovered that white light is a mixture of seven colors: red, orange, yellow, green, blue, indigo, and violet (the mnemonic for remembering this is ROY G. BIV). Each has a particular wavelength of light that makes up the visible spectrum. By directing a beam of light through a prism, he saw that the process of splitting white light into its color components, known as dispersion, occurs.

Since light travels in a wave-like manner, colors of the visible spectrum equate to wavelengths increasing from one end of the visible spectrum to the other. Violet has the shortest wavelength, 380 nanometers, and red has the longest, 760 nanometers.

Our eyes perceive different colors because objects absorb or reflect different color wavelengths. Grass appears green because it reflects green light and absorbs all other colors of the spectrum. Items appear to be white when the objects reflect all colors of the spectrum and absorb none, while objects that appear black absorb all colors of the spectrum without reflecting color wavelengths.

The sun emits radiation across most of the EM spectrum, and we have vision because our eyes perceive the sun's energy that falls in the range of the visible light spectrum. EB

This fun crossword puzzle pertains to light theory and how the human vision system functions. The answers to this puzzle will be listed in the July issue of Eyecare Business. We’ll be posting weekly hints at our Facebook page, facebook.com/eyecarebusiness and our website, EyecareBusiness.com.

into the light

across

1. The scientist who first proposed the speed of light.
3. Each color of the visible spectrum equates to a different _______________ of light.
7. Newton thought that light consisted of tiny particles or ____________.
8. The scientist who proved that light propagates with wave-like properties.
11. A mnemonic for the colors of the visible spectrum.
13. The bending of light as it passes through a transparent medium.
14. He explained that light has two distinct sets of behavior.
15. The number of vibrations of a given wavelength in one second.
18. To transport energy from one place to another.
21. The tendency of light to bend around corners of objects.
22. The highest point on a wave.
23. Sometimes light behaves as packets of energy called ____________.
24. The __________ light spectrum lies in the middle of the electromagnetic spectrum.
26. ________________ propagation of light. Begins with an 'R'.
27. The amount of energy a wave carries is dependent on this.
28. The speed of _____________ moves at 186,282 miles per second.

down

1. An orderly arrangement of all known radiant energy.
2. Our primary light source.
4. The speed of _____________ is 1,130 feet per second.
5. ______________has the shortest wavelength at 380 nanometers.
6. Light moves through this medium at 140,000 miles per second.
9. Refers to the science of matter and energy.
10. Our eyes are only able to perceive __________ in the visible spectrum range.
12. The process of splitting white light into its color components.
13. This occurs when light strikes a surface and then bounces off.
16. A ____________ is one-billionth of a meter.
17. Light travels in straight lines or __________.
19. Frequency is measured in _____________.
20. Objects either ________ or reflect different color wavelengths.
23. He proposed the Quantum Theory of Light.
25. The lowest point on a wave