Humans experience the longest visible electromagnetic waves as

Red light

Explanation:

The electromagnetic spectrum classifies all the electromagnetic waves according to their wavelength/frequency. In order from shortest to longest wavelength, we have

Gamma rays

X rays

Ultraviolet

Visible light

Infrared

Microwaves

Radio waves

All of these waves are invisible to human eye, except for the "visible light" part of the spectrum. Waves in the visible light part are perceived by human as different colors, depending on their wavelength. More specifically, they appear as follows:

Violet (380-450 nm)

Blue (450-495 nm)

Green (495-570 nm)

Yellow (570-590 nm)

Orange (590-620 nm)

Red (620-750 nm)

So we see that the longest electromagnetic wave in the visible light part of the spectrum corresponds to red light.