The correct answer is Option 3: Infra-red waves.

Night vision apparatus, also known as night vision goggles or devices, are designed to enhance visibility in low-light conditions or complete darkness. These devices allow individuals to see in the dark by detecting and amplifying available light sources, including light from the infrared (IR) spectrum.

Infrared waves, which have longer wavelengths than visible light, fall within the electromagnetic spectrum. The infrared spectrum is typically divided into three regions: near-infrared (NIR), mid-infrared (MIR), and far-infrared (FIR). Night vision apparatus primarily utilize the near-infrared portion of the spectrum.

The operation of night vision devices involves a process known as "active" or "passive" night vision, depending on the technology used. In the case of passive night vision, the devices detect and amplify the existing infrared light emitted or reflected by objects in the environment. This natural infrared radiation is emitted by warm objects or reflected off surfaces. Humans and animals, for example, emit infrared radiation due to their body heat.

Infrared detectors, such as photomultiplier tubes or charge-coupled devices (CCDs), are used to sense the incoming infrared light. These detectors convert the IR radiation into electrical signals, which are then processed and amplified to create a visible image for the viewer.

The amplified image is usually displayed on a phosphor screen or converted into a digital signal and displayed on a screen, allowing the user to see the scene even in complete darkness. The image appears as various shades of green because the human eye is more sensitive to green light, and it helps provide better contrast and detail.

It`s important to note that radio waves and microwaves, mentioned in Options 1 and 2, respectively, are not used in night vision apparatus. Radio waves are typically used for communication purposes, such as radio broadcasting, cellular networks, and Wi-Fi. Microwaves, on the other hand, are commonly used for various applications like cooking, radar systems, and wireless communications. Neither of these wave types is directly related to night vision technology.

In conclusion, night vision apparatus primarily utilize infrared waves, specifically near-infrared, to enhance visibility in low-light or dark conditions. Infrared radiation emitted or reflected by objects is detected, amplified, and converted into a visible image for the viewer, enabling them to see in the dark.