Light is composed of electromagnetic waves that travel through space in a harmonic pattern, with each wave containing differing amounts of energy. The energy increases as the frequency increases, meaning higher frequency light waves contain more energy than lower frequency waves. When light enters our eyes, we perceive different frequencies as different colors.
The Visible Spectrum
The full range of frequencies of electromagnetic radiation is referred to as the electromagnetic spectrum. The very small portion of the spectrum that is visible to the human eye is known as the visible spectrum. The visible spectrum is composed of the colors that are produced by the different frequencies of visible light. The visible spectrum can be remembered by the acronym ROYGBIV:
- Red
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet
These colors are listed in order of increasing frequency, with red having the lowest frequency and violet having the highest. This ordering corresponds with the amount of energy within each light wave – higher frequency violet light contains more energy than lower frequency red light.
Wavelength and Frequency
The color of visible light is determined by its wavelength, which is related to the frequency of the light wave. Wavelength refers to the distance between successive wave peaks, while frequency refers to how many wave cycles pass a given point per unit of time. Shorter wavelengths correspond to higher frequencies, while longer wavelengths correspond to lower frequencies.
The wavelengths of visible light range from about 380 nanometers (violet) to about 740 nanometers (red). Frequency is measured in hertz (Hz), or cycles per second. Violet light has a frequency of around 790 terahertz (THz), while red light has a frequency of around 405 THz.
| Color | Wavelength (nm) | Frequency (THz) |
|---|---|---|
| Violet | 380-450 | 668-789 |
| Blue | 450-495 | 606-668 |
| Green | 495-570 | 526-606 |
| Yellow | 570-590 | 508-526 |
| Orange | 590-620 | 484-508 |
| Red | 620-750 | 400-484 |
This table shows the wavelengths and corresponding frequencies of the visible spectrum colors in order. As wavelength increases and frequency decreases, the amount of energy decreases, going from violet to red.
Dispersion of Light
When white light passes through a prism, the different wavelength components are refracted at slightly different angles, causing the white light to be dispersed into the visible spectrum. This occurs because the index of refraction in the prism differs slightly for the various colors. The longer red wavelengths are bent the least while the shorter violet wavelengths are bent the most. This dispersion of light into the color spectrum reveals the order of colors from longest wavelength/lowest frequency (red) to shortest wavelength/highest frequency (violet).
Isaac Newton was the first to demonstrate that white light is composed of the full spectrum of colors using prism experiments in the late 1600s. By recombining the dispersed colors, he showed that they make up ordinary sunlight.
Order of Energy
The ordering of colors by increasing frequency corresponds directly to an increase in the energy carried by the light. Radiation with higher frequencies/shorter wavelengths packs more energy. Violet light, with the highest frequency in the visible spectrum, is the most energetic, while red light with the lowest frequency is the least energetic.
When light interacts with matter, the energy it transfers depends on its frequency. Higher frequency light can initiate chemical reactions, cause fluorescence, and produce vision through more energetic photon absorption. Lower frequency light provides less energy to interact with matter. The energy order of visible light is as follows:
- Red – Lowest energy
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet – Highest energy
Heat and Color
When an object is heated, it begins to emit radiation starting with lower frequency infrared light, which is invisible to our eyes. As the temperature increases further, it starts emitting visible red wavelengths. With more heat, orange and then progressively higher frequency colors are emitted. Very hot objects appear bluish-white as they give off light across the entire visible spectrum.
The order of color appearance indicates the object’s temperature. Red is emitted at lower temperatures around 1000°F. Yellow-white light is given off by hotter objects from 3000-5000°F. Even hotter objects like the sun at 10,000°F appear white with a hint of blue due to the addition of higher frequency light.
Color Vision
The cones in our retinas contain photopigments that are sensitive to different wavelength ranges, allowing us to distinguish various colors. S cones detect shorter wavelengths in the blue-violet end of the spectrum. M cones are sensitive to middle wavelengths corresponding to green light. L cones are stimulated by longer red wavelengths.
Signals from the three cone types are processed by the visual cortex to produce our color vision. The cone sensitivity ranges match the order of the visible spectrum, with S cones responding to the most energetic violet light and L cones to the least energetic red light.
Rainbows
When sunlight enters a water droplet, dispersion occurs that separates the light into the full spectrum of colors. The different wavelengths are reflected at slightly different angles, producing the sequence of rainbow colors. Longer wavelengths like red are reflected at a shallower angle than shorter violet wavelengths.
In a rainbow, red appears on the outer band while violet is on the inner band, corresponding to their differing angles of reflection. The ordering of colors from the inside to outside of a rainbow also follows the increasing frequency/energy of the visible spectrum.
Absorption and Transmission
Materials absorb and transmit different wavelengths preferentially based on their molecular structure. This selective absorption determines what color we perceive an object or substance to be.
For example, a red apple absorbs higher frequencies and reflects longer red wavelengths to our eyes. A blue sky transmits shorter wavelengths but scatters out longer red and orange frequencies of sunlight.
In general, the transmitted or reflected color matches the frequency range that was not absorbed by the material. Following the visible spectrum order, materials transmit/reflect lower frequencies and absorb higher frequencies.
Conclusion
The visible spectrum colors are ordered from longest wavelength and lowest frequency red light to shortest wavelength and highest frequency violet light. This ordering corresponds from low to high energy in the light waves. Knowing the color sequence is useful in understanding rainbow dispersion, heat emission, color vision, and light interactions with matter, which all follow the same increasing frequency pattern.
Remembering the acronym ROYGBIV provides a mnemonic for recalling the proper sequence of visible colors from red, through the rainbow, to violet.
