Understanding Luminosity and Wavelength

The question asks about how the luminosity of light changes as its wavelength increases from violet to red. Luminosity, in this context, refers to the perceived brightness of light by the human eye. The human eye is not equally sensitive to all wavelengths (colors) of visible light. This varying sensitivity is described by the photopic luminosity function, which peaks at a certain wavelength.

Luminosity and the Visible Spectrum

The visible spectrum ranges from approximately 380 nanometers (nm) for violet light to about 740 nm for red light. The wavelengths increase as we move from violet towards blue, green, yellow, orange, and finally red.

• Violet: Short wavelength (around 380-450 nm)
• Blue: (around 450-495 nm)
• Green: (around 495-570 nm)
• Yellow: (around 570-590 nm)
• Orange: (around 590-620 nm)
• Red: Long wavelength (around 620-740 nm)

The photopic luminosity function, denoted as V($\lambda$), describes the average human eye's sensitivity to light of different wavelengths ($\lambda$) under bright conditions. This function represents how bright a light of a certain wavelength appears compared to a light of the same power at the peak sensitivity wavelength.

How Luminosity Changes with Wavelength

The peak sensitivity of the human eye under bright light (photopic vision) occurs at a wavelength of approximately 555 nm, which corresponds to a yellow-green color. As the wavelength moves away from this peak in either direction (towards shorter wavelengths like violet or longer wavelengths like red), the eye's sensitivity decreases.

• Starting from violet (short wavelength), as the wavelength increases towards the yellow-green peak (around 555 nm), the human eye's sensitivity, and thus perceived luminosity for a given power, increases.
• After reaching the peak sensitivity around 555 nm, as the wavelength continues to increase towards red (long wavelength), the human eye's sensitivity, and thus perceived luminosity, decreases.

Therefore, as the wavelength is increased from violet (low sensitivity) through the spectrum to red (low sensitivity), the luminosity first increases (from violet towards yellow-green) and then decreases (from yellow-green towards red).

This behavior aligns with the shape of the standard photopic luminosity function curve.

Conclusion

Based on the human eye's sensitivity curve, as the wavelength increases from the violet end of the spectrum towards the red end, the perceived luminosity first increases, reaching a maximum around yellow-green, and then decreases.