The longest wavelength within the visible spectrum is light.
a. blue
b. orange
c. red
d. violet

The longest wavelength within the visible spectrum is the red light. The answer is letter C. It is called visible light because it is the only light that can be seen by the human eye. Red light is the longest wavelength around 620 to 750 nanometer. It is followed by orange which has a wavelength of 590 t 620 nanometer. And then blue which has a wavelength of 450 to 495 nanometer. And the shortest wavelength is violet which has a wavelength of 380 to 459 nanometer.