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Why red is the longest wavelength in visible light?

Light is a form of electromagnetic radiation that is visible to the human eye. The wavelengths of visible light range from about 380 nanometers (violet) to about 740 nm (red). Red light has the longest wavelength within this range, which is why it sits at the long-wavelength end of the visible spectrum.

The Electromagnetic Spectrum

To understand why red is the longest visible wavelength, we first need to understand a bit about the electromagnetic spectrum. The electromagnetic spectrum encompasses all types of electromagnetic radiation, from radio waves to gamma rays. It can be categorized into different regions based on wavelength and frequency, including (from long wavelength to short):

  • Radio waves
  • Microwaves
  • Infrared radiation
  • Visible light
  • Ultraviolet radiation
  • X-rays
  • Gamma rays

Visible light makes up just a small portion of the full electromagnetic spectrum. Its wavelengths range from about 380-740 nm. The visible spectrum looks something like this:

Color Wavelength Range (nm)
Violet 380-450
Blue 450-495
Green 495-570
Yellow 570-590
Orange 590-620
Red 620-740

As you can see, red light has the longest wavelength range within the visible spectrum, sitting at 620-740 nm. Violet and blue light have shorter wavelengths, while green, yellow, and orange fall in the middle.

The Relationship Between Wavelength and Frequency

To understand why red has the longest wavelength, we need to talk about the relationship between wavelength and frequency. Wavelength is the distance between consecutive peaks or troughs in a light wave. Frequency measures how many waves pass a given point per unit of time. Wavelength and frequency are inversely related to each other through the speed of light:

Speed of Light = Wavelength x Frequency

Since the speed of light remains constant at around 3 x 108 m/s, as wavelength increases, frequency must decrease and vice versa. Longer wavelength light like red has a lower frequency than shorter wavelength light like violet or blue.

Dispersion of Light

When white light passes through a prism, the different wavelengths separate into a rainbow spectrum. This separation occurs because the different wavelengths bend slightly different amounts as they pass through the prism, a phenomenon called dispersion. Violet light bends the most while red light bends the least. The result is the visible light spectrum with red at one end and violet at the other.

The order of colors in the rainbow gives us another way to see that red has the longest wavelength. If we remember the acronym ROYGBIV, the colors range from long wavelength (Red) to short wavelength (Violet). Red sits at the long wavelength, low frequency end of the visible spectrum while violet sits at the short wavelength, high frequency end.

Why Longer Wavelengths Bend Less

You might be wondering why longer wavelength light bends less than shorter wavelength light as it passes through a prism. This relationship results from the wave nature of light. As light waves travel through the prism, they interact with the atoms in the material. Higher frequency light has more oscillations per unit time, allowing it to interact more strongly. This stronger interaction results in more bending of the wave. Lower frequency, longer wavelength light oscilates less frequently so it interacts less and bends less.

Real World Significance

The fact that red light has a longer wavelength than other colors has some real-world implications:

  • Sunsets: When the sun is low in the sky at sunset, sunlight must pass through more atmosphere to reach your eyes. Shorter wavelengths like blue and violet scatter more strongly, leaving mostly longer wavelengths like red and orange to reach your eyes, creating a red sunset.
  • Plant growth: Plants primarily absorb red and blue light. Red light sits at the low energy end of the visible spectrum and is important for photosynthesis and leaf development.
  • Blood: The red color of blood results from the large amount of hemoglobin it contains. Hemoglobin absorbs all colors of visible light except red, which it reflects, causing blood to appear red.
  • Ruby lasers: Ruby lasers produce laser light with a wavelength of 694 nm, which sits in the long wavelength red portion of the visible spectrum.

Properties of Longer vs. Shorter Wavelengths

In general, longer wavelength light tends to have different properties than shorter wavelength light:

Longer Wavelength Shorter Wavelength
– Lower frequency – Higher frequency
– Less energy – More energy
– Bends less – Bends more
– Scatters less – Scatters more
– Less damaging to living tissue – More damaging to living tissue

These differing properties result from the inverse relationship between wavelength and frequency. Shorter wavelengths pack energy into more waves per second, giving them higher frequency and energy. Longer wavelengths spread energy over fewer waves, resulting in lower frequency and energy.


In summary, red sits at the long wavelength end of the visible light spectrum, ranging from about 620-740 nm. Its long wavelength translates to a lower frequency than other colors. When white light disperses into a rainbow spectrum, red bends the least due to its longer wavelength. Longer wavelength red also tends to scatter less, have lower energy, and cause less damage than high frequency, short wavelength violet and blue light. So the next time you see a red sunset, think about why red sits at the long, low energy end of the visible rainbow.