Understanding the Wavelength of Visible Light: Clarifying the Confusion

There is a considerable amount of confusion regarding the specific range of wavelengths that define visible light. Some sources state that visible light spans from 390 to 770 nanometers (nm), while others put the range between 380 and 740 nm, or even suggest 400 to 700 nm. This discrepancy can be quite confusing for anyone trying to understand the fundamentals of light perception and its applications. In this article, we will clarify the true range of visible light and explain why there are variations in the stated wavelengths.

The General Accepted Range for Visible Light

Despite the variations, the generally accepted range for visible light is typically around 380 to 750 nm. This range accounts for the most commonly recognized spectrum. However, different sources may define the boundaries slightly differently:

390-770 nm: This range often includes some near-ultraviolet (UV) light at the lower end and some near-infrared (IR) light at the upper end. 380-740 nm: This range is more conservative, excluding the extreme ends of the spectrum. 400-700 nm: This is a commonly cited range, particularly in educational contexts, and it focuses on the most perceptible part of the visible spectrum.

In summary, while the exact boundaries can vary, the generally accepted range for visible light is around 380 to 750 nm. This range is broad enough to encompass the most relevant wavelengths for human perception and application in various scientific and technological fields.

Challenges in Defining the Cutoff Wavelengths

The cutoff wavelengths for visible light are not absolutely precise. Just like you, I sometimes need to verify the information from reliable sources. The limits of visibility depend on the person's eyesight and the brightness of the light source. Therefore, the edges of this spectrum are relative and can vary from individual to individual.

The human eye's sensitivity to light is not a sharp cutoff. The eye's sensitivity to light is represented by a bell-shaped curve, which peaks at 555 nm and decreases toward the edges. For example, the eye can detect extremely bright 770 nm sources but not dimmer ones. Some individuals can perceive lower wavelengths, and those with certain genetic conditions (such as protanopia) might have a different sensitivity curve.

Light Perception and Sensitivity

The eye's sensitivity to light is influenced by both the brightness of the source and the individual's visual capability. The wavelength range for light perception can differ based on whether the light is bright or dim. The sensitivity of the eye is a logarithmic response, meaning that even a large decrease in sensitivity (by a factor of 1000 or more) can still be perceived. Therefore, sources that are very bright can extend the visible spectrum beyond their nominal limits.

Visual Perception at the Extremes

For brighter light, the practical range of the eye is about 440 to 670 nm. In laboratory settings, when a light source is passed through a monochromator, the eye's sensitivity shows a gradual dimming toward both ends of this spectrum. Specifically, at 420 nm and 690 nm, the light must be extremely bright to be visible, whereas a pulsed ruby laser with megawatt output pulse can be seen with a wavelength of 694 nm, despite being below the nominal upper range.

Visible light at the edges of the spectrum can be difficult to perceive. For instance, ultraviolet light at 400 nm is often seen through fluorescence, making it appear as though one is seeing 405 nm laser light when in fact, they are seeing the fluorescence of the object the laser is shining on. This is often mistaken for "purple" lasers.

Eye Safety and Concerns

It is important to note that when light is bright enough to be seen at the edges of the visual spectrum, it can also pose a risk of eye damage. For example, collimated infrared light from lasers around 1000 nm can cause a 2-photon reaction in the retina, leading to a green color perception, usually accompanied by serious eye damage. Therefore, it is crucial to handle and use bright light sources with caution to ensure eye safety.

By understanding the nuances of the visible light spectrum, we can better appreciate the complex nature of light perception and its practical applications in science and technology.