School of PE Blog

Look around you right now and take in the vibrant colors of the world. The bright blue sky, the lush green grass, the warm tan of wooden furniture. We often take the rich visual experience of color for granted, but our ability to see these hues is thanks to the unique properties of visible light.

The portion of the electromagnetic spectrum that our eyes can detect is known as the optical spectrum. This range of wavelengths from violet to red, comprising all the colors of the rainbow, allows us to make sense of the world. In this article, we’ll explore what exactly the optical spectrum is, how it works, and why we can see the colors we do.

To understand the optical spectrum, we first need to know a bit about light itself. Visible light is a form of electromagnetic radiation, the same kind of energy that radio waves, microwaves, and X-rays are made of. Light travels in waves, vibrating electric and magnetic fields.

The wavelength of light determines its properties and color. Wavelength refers to the distance between consecutive peaks of a light wave. The entire electromagnetic spectrum is categorized into ranges based on wavelength and frequency. Radio waves have long wavelengths and low frequencies, while gamma rays have short wavelengths and high frequencies.

Visible light wavelengths span from roughly 380 to 700 nanometers (trillionths of a meter) (National Aeronautics and Space Administration, 2010). A nanometer is incredibly tiny—over 100,000 times thinner than a human hair! But this infinitesimally small sliver of the spectrum enables us to visualize the world around us in all its multi-hued glory.

The optical spectrum is commonly organized into named colors, going from the shortest to longest visible wavelengths. You probably remember learning these as ROYGBIV (National Aeronautics and Space Administration, 2010):

Red - 620-750 nm

Orange - 590-620 nm

Yellow - 570-590 nm

Green - 495-570 nm

Blue - 450-495 nm

Indigo - 445-450 nm

Violet - 380-445 nm

Red light has the longest wavelengths that humans can see, while violet has the shortest. But within this narrow span, our eyes can detect millions of distinct colors! Pretty impressive for a biological system that evolved through natural selection.

The visible spectrum represents a tiny sliver - less than 0.0035% - of the entire electromagnetic spectrum (National Nuclear Security Administration, 2018). However, the properties of visible light make it perfectly suited for organisms on Earth to develop visual systems to navigate their environments. Shorter wavelengths like ultraviolet can damage cells, while longer wavelengths like infrared are absorbed by the atmosphere.

Rods allow us to see in low light, like at night. But cones are responsible for our color vision. The retina contains three types of cones that are each sensitive to different wavelengths of light. According to the trichromatic theory, these cone cells are stimulated by red, green, or blue light. All the colors we see are combinations of signals from these three cone types.

So why do cones detect red, blue, and green, corresponding with specific portions of the optical spectrum? The peak sensitivities of our cones seem to be an evolutionary adaptation. Early organisms developed cones that responded most strongly to dominant wavelengths in their environment.

Having three separate cone types, rather than just one or two, allows an immense range of color discernment. With blue, green, and red receptors, the brain can approximate almost any shade visible to humans through color mixing. Pretty nifty trick!

Fun fact: A very small percentage of women may have four cone types instead of three, allowing them to see millions more colors. This condition is known as tetrachromacy.

Here’s an interesting question: Are the colors we see accurate representations of reality, or are they just how our minds interpret different wavelengths?

The truth is that color is inherently subjective. Wavelengths along the optical spectrum exist as an objective reality. But how those wavelengths translate into color perception depends on the biological mechanisms in an observer's visual system.

Human color vision relies on comparing levels of stimulation between different cone cells. Our retina assigns color labels to wavelengths based on relative cone responses. Different animal species have evolved alternate color vision matching the needs of their niche. Butterflies see ultraviolet patterns on flowers that are invisible to us. Reef mantis shrimp perceive twelve primary colors using different photoreceptors (Gardner et al., 2018).

So far, we’ve primarily discussed color as it relates to wavelength. But light can also be modeled using other properties like energy, frequency, and photon behavior. And things get...complicated.

Albert Einstein explained light as both a wave and a particle. Photons are discrete packets of light that act like particles. The energy of a photon is directly proportional to its frequency but inversely proportional to its wavelength (Halpern, 2020). Even though our visual system evolved to see a range of wavelengths as color, physicists think about light differently.

Here are some mind-bending facts about photons:

- Blue photons are more energetic than red photons (UCAR, n.d.).

- Gamma rays, x-rays, and ultraviolet lights are different wavelengths along the electromagnetic spectrum. But they can all give you cancer through photon ionization of DNA (National Cancer Institute, 2019).

- Photosynthesis involves plants absorbing photons and using their energy to power chemical reactions (National Geographic, 2022).

- WiFi, cell phones, TV remotes, and more technologies rely on photons and electromagnetic fields to transmit information (World Health Organization, 2014).

So, the next time you think about the vivid red of a rose or clear blue sky, remember the intricate physics underlying the spectrum of visible light. Our limited human eyesight gives us but a small glimpse into the mysteries of the universe.

The optical spectrum we’ve discussed underpins naked eye color vision. But scientists have also harnessed the properties of light to study matter through spectroscopy. Spectroscopy utilizes the interaction of electromagnetic radiation and matter to analyze a material’s characteristics.

Different techniques like absorption and emission spectroscopy all provide valuable insights:

- Emission spectroscopy looks at wavelengths emitted by atoms and molecules as their electrons transition between energy levels. The emission spectrum forms a fingerprint to identify the atom (Worden, 2005, 64-65).

Some common examples of spectroscopy include:

- Optical Coherence Tomography - Uses near-infrared light to capture micron-resolution, 3D images of tissue for medical imaging (Fujimoto et al., 2000, 9).

What an amazing coincidence that this narrow band of visible light not only sustains life but also allows us to appreciate the beauty of our surroundings! The next time you look up at a rainbow arcing through the sky or observe the kaleidoscope of autumn leaves, I hope you’ll appreciate the wondrous science underlying nature’s color and light.

The electromagnetic spectrum represents one of the great mysteries and gifts of our physical universe. Our eyes detect a slim range of optical wavelengths focused on environmental adaptation. Yet, within this tiny sliver, we can experience the sheer aesthetic splendor of color. What we see belongs just as much to biology as physics.

Light also exhibits complex quantum properties beyond the visible spectrum our ancestors evolved to perceive. As scientists reveal more about photon behavior, we continue uncovering the hidden machinations of reality. Color science intrigues those who study eyes and evolution, neurons and the mind, particles, and waves.

So, remember to pause and soak in the colors around you. We’re all wearing perception goggles that filter objective reality into subjective experience. Our human paradigms shape the colors we see, but physics carries on regardless. Keep gazing at the light in wonder.

If you enjoyed learning about the intricacies of light, you'll love diving deeper into engineering principles! Prepare for your FE or PE exam with confidence and unlock your potential by exploring School of PE's comprehensive exam review courses.

Esmonde-White, K. A., Cuellar, M., & Lewis, I. R. (2021, October 20). The role of Raman spectroscopy in biopharmaceuticals from development to manufacturing. 10.1007/s00216-021-03727-4

Fujimoto, J. G., Pitris, C., Boppart, S. A., & Brezinski, M. E. (2000). Neoplasia. Optical Coherence Tomography: An Emerging Technology for Biomedical Imaging and Optical Biopsy, 2(1-2), 9-25. ISSN 1476-5586. https://doi.org/10.1038/sj.neo.7900071

Gardner, J., O'Neill, L. G., & Vasudeva, R. (2018, January 9). The More, the Better? A Butterfly with 15 Kinds of Light Sensors in Its Eye. Frontiers. Retrieved October 10, 2023, from https://kids.frontiersin.org/articles/10.3389/frym.2017.00070

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National Aeronautics and Space Administration. (2010). Visible Light. NASA Science. Retrieved October 10, 2023, from https://science.nasa.gov/ems/09_visiblelight/

National Cancer Institute. (2019, March 7). Risk Factors: Radiation - NCI. National Cancer Institute. Retrieved October 10, 2023, from https://www.cancer.gov/about-cancer/causes-prevention/risk/radiation

National Geographic. (2022, July 15). Photosynthesis. National Geographic Society. Retrieved October 10, 2023, from https://education.nationalgeographic.org/resource/photosynthesis/

National Nuclear Security Administration. (2018, October 17). Visible Light: Eye-opening research at NNSA. Department of Energy. Retrieved October 10, 2023, from https://www.energy.gov/nnsa/articles/visible-light-eye-opening-research-nnsa

Rakshit, A., Meena, V. S., Abhilash, P.C., Sarma, B.K., Singh, H. B., Fraceto, L., Parihar, M., & Singh, A. K. (Eds.). (2021). Biopesticides: Volume 2: Advances in Bio-inoculants (Vol. 2). Elsevier Science. https://doi.org/10.1016/B978-0-12-823355-9.00008-0

UCAR. (n.d.). Wavelength of Blue and Red Light | Center for Science Education. UCAR Center for Science Education. Retrieved October 10, 2023, from https://scied.ucar.edu/image/wavelength-blue-and-red-light-image

University of Washington. (n.d.). Retina. Neuroscience For Kids. Retrieved October 10, 2023, from https://faculty.washington.edu/chudler/retina.html

Worden, R.H. (2005). Encyclopedia of Geology (R. C. Selley, L. R. M. Cocks, & I. R. Pilmer, Eds.). ANALYTICAL METHODS | Geochemical Analysis (Including X-ray), 54-76. https://doi.org/10.1016/B0-12-369396-9/00096-4

World Health Organization. (2014, October 8). Electromagnetic fields and public health: mobile phones. World Health Organization (WHO). Retrieved October 10, 2023, from https://www.who.int/news-room/fact-sheets/detail/electromagnetic-fields-and-public-health-mobile-phones