The visible region of the electromagnetic spectrum corresponds to the range of frequencies that our eyes are sensitive to. Our eyes contain specialized cells called photoreceptors that are responsible for detecting and responding to light. These photoreceptors are of two types: rods and cones.
Rods are more sensitive to lower light levels and are responsible for our vision in dim lighting conditions. However, rods are not sensitive to different colors of light and do not provide color vision. Cones, on the other hand, are responsible for our color vision and are sensitive to different wavelengths of light.
Within the visible spectrum, the different wavelengths of light correspond to different colors. The shortest wavelengths are associated with violet light, while the longest wavelengths are associated with red light. The visible spectrum encompasses a range of wavelengths that our cones are sensitive to, allowing us to perceive a range of colors.
The reason why only specific frequencies can be observed in the visible region is due to the nature of the photoreceptor cells in our eyes. Cones have different types that are sensitive to different ranges of wavelengths. These cones are commonly referred to as red, green, and blue cones. Each cone type is most sensitive to a specific range of wavelengths, corresponding to the colors red, green, and blue, respectively.
The combination of signals from these different cone types, as well as the relative intensity of the light at different wavelengths, allows us to perceive a wide range of colors. Other frequencies outside the visible spectrum, such as infrared or ultraviolet, are not detected by our cones and are therefore not perceived as colors by our visual system.
In summary, the specific frequencies observed in the visible region of the electromagnetic spectrum correspond to the range of wavelengths that our cone photoreceptors are sensitive to, allowing us to perceive different colors.