Skip to Content

Where is the color red?

Red is a color that exists in our visual perception. It is associated with the long wavelength end of the visible spectrum, next to orange and opposite violet. The wavelength range for red is approximately 625-740 nanometers. Red is one of the three primary colors, along with blue and green, that can be mixed to produce all other colors. But where does the color red exist? Let’s explore this question further.

The Science of Red

Physically speaking, red is a color that corresponds to a specific wavelength range of visible light. When light with wavelengths in the ~625-740 nanometer range hits our eyes, our visual system interprets it as the color red. This light energy is detected by special photoreceptor cells called cones in our retinas at the back of our eyes. There are three types of cones that are each sensitive to different wavelength ranges – short, medium, and long. Red light stimulates the long wavelength cones.

The sensation and perception of red is produced in our brains. Light entering our eyes triggers chemical and electrical signals that travel through the optic nerve to the visual cortex in the brain. Here the signals are processed to create the experience of seeing the color red. So while red corresponds to physical light, the qualitative sensation we know as “redness” really exists only in our minds.

Where We See Red

We see the color red in objects and light sources that reflect, emit, or transmit light in the ~625-740 nm wavelength range. Here are some common examples of where we encounter the color red:

  • Ruby gemstones – Trace amounts of chromium absorb blue and green light, causing rubies to reflect more red.
  • Ripe tomatoes – Lycopene and other pigments in the tomato skin absorb non-red wavelengths, leaving mainly red to be reflected.
  • Blood – The iron-containing protein hemoglobin reflects red light back.
  • Red flowers – Pigments like anthocyanin absorb non-red light and reflect red back to our eyes.
  • Sunset skies – Shorter wavelength blue/violet light is scattered away, leaving mostly longer wavelength red/orange.
  • Lasers – Some red laser pointers emit light at 633 nm, directly stimulating the red cones in our eyes.
  • Red clothes – Dyes and pigments in the fabric absorb other wavelengths, selectively reflecting the red portion back.

So in summary, we see the color red wherever something absorbs non-red light and reflects, transmits, or emits light predominantly in the long wavelength red region of the visible spectrum.

When Red Appears

Red can appear to us under various conditions that produce light centered around the 625-740 nm range:

  1. Reflection – Objects may selectively reflect back red light while absorbing other wavelengths. Examples are red paint, flowers, gemstones.
  2. Transmission – Materials may transmit mostly red light through them. Rubies appear red due to chromium impurities transmitting red.
  3. Emission – Light sources can directly emit light in the red wavelength range. Examples include red LEDs, neon signs, ruby lasers.
  4. Scattering – Shorter wavelengths are scattered away by particles in the air, leaving mostly red. This gives rise to red sunrises/sunsets.
  5. Shifting – Wavelengths may be shifted into the red range via the Doppler effect or gravitational redshifting. This allows us to see redshift from distant galaxies.

In most everyday cases, we see red due to the reflection or transmission of red wavelengths from pigments, dyes, or selective absorption in materials. But red can also arise from direct emission, scattering effects, or wavelength shifting.

Red in Nature

Red appears commonly across the natural world. Here are some of the many cases where red coloration arises:

  • Plants – Red flowers, leaves, fruits to attract pollinators and seed dispersers.
  • Animals – Red markings as warning colors or mating displays. Rusty red soil-dwellers as camouflage.
  • Minerals – Ruby, garnet, red jasper contain trace elements like chromium that absorb non-red light.
  • Rocks & terrain – Iron oxide rich rocks and soils yield rust red coloration.
  • Water – Red algae blooms, suspended clay particles reflecting red light back.
  • Sky – Red sunrises/sunsets due to Rayleigh scattering filtering out short blue wavelengths.

Red coloration provides evolutionary advantages in nature by attracting, warning, camouflaging, or protecting living things. It also arises innately based on a substance’s chemical composition and the physics of light interaction.

Red’s Cultural Symbolism

The color red holds rich cultural symbolism and meanings. Here are some of the key associations of red in culture:

  • Danger, warnings
  • Heat, fire
  • Love, passion
  • Aggression, intensity
  • Energy, excitement
  • Importance, urgency
  • Celebration, festivity
  • Power, leadership
  • Courage, valor

Red evokes strong emotional reactions and meanings for us. While these vary by culture, red often denotes concepts like danger, energy, passion, and importance across many societies.


In summary, the color red exists in our visual perception when light in the ~625-740 nanometer wavelength range enters our eyes. We see red when objects selectively reflect, transmit, or emit light in this long wavelength red portion of the visible spectrum. Red appears commonly in nature thanks to selective absorption by pigments and the physics of light. Culturally red is rich with symbolism, often representing concepts like danger, love, power, and celebration. So in our minds, nature, and culture, the color red holds an evocative and significant place.