What we perceive as light is a part of the vast spectrum of “electromagnetic radiation.” Each segment of this spectrum transitions smoothly into the next, yet each has distinct characteristics. For example, radio waves can travel long distances, while X-rays can penetrate thick steel or damage human tissue. However, most of this radiation is not visible to the human eye, which can only detect wavelengths from about 400 nm to 700 nm, known as the visible spectrum.
When a light source emits a relatively even mix of all visible wavelengths, the light appears white and colorless. If only certain wavelengths are present, the light appears colored. For instance, wavelengths between 400 nm and 450 nm are seen as dark purple. If the wavelengths shift to 450-500 nm, the light appears blue. Between 500 nm and 580 nm, the light looks more blue-green, and from around 580 nm to 600 nm, we perceive yellow. As the wavelengths increase, the yellow turns to orange, and at 650 nm, it becomes red, deepening until the visible limit at 700 nm. Thus, the colors of the spectrum – purple, blue, green, yellow, and red – are all present in various types of white light.
The human eye contains three types of light receptors, each corresponding to broad, overlapping bands of blue, green, and red wavelengths. When these receptors are equally stimulated by light, we perceive it as white or neutral gray. If the stimulation is uneven due to an imbalance in wavelengths, such as more red (long) waves compared to blue (short) waves, the light can appear tinged with orange, as seen during sunrise or sunset.
Remembering the order of the colors in the visible spectrum is useful for understanding the color response of black-and-white films or selecting color filters and darkroom-safe lamps. Later, you will learn how the concept of three types of visual receptors collectively responding to the entire color spectrum is essential for the functioning of color photo films and digital sensors.
