Our eyes can only detect a limited range of the electromagnetic spectrum. This range is visible light, with wavelengths from about 400 nm (violet) to about 700 nm (red). It is a very small part of the whole spectrum.
Waves just outside this range are invisible to us. Infrared has a longer wavelength than red light and ultraviolet has a shorter wavelength than violet light. Radio waves, microwaves, X-rays and gamma rays are also invisible to our eyes.
We need other detectors to find these waves. A thermal imaging camera detects infrared, and a phone camera can show the infrared from a TV remote control as a purple flash on its screen. Radio waves are detected by aerials, and X-rays by photographic film or electronic detectors.