The longest wavelength that we can detect depends on the sensory capabilities of our instruments and our biological senses. For human vision, the longest wavelength we can perceive is typically considered to be around 700 nanometers (nm), which corresponds to the deep red part of the visible spectrum. Wavelengths longer than 700 nm are typically referred to as "infrared" and are beyond the range of colors that human eyes can detect.
However, there are certain specialized instruments and sensors that can detect longer wavelengths in the infrared region, beyond what our eyes can see. Infrared radiation is used in various applications, such as thermal imaging, night vision, remote sensing, and astronomy. The range of wavelengths that these instruments can detect can extend from hundreds of nanometers to millimeters or even longer.
For example, thermal imaging cameras can detect infrared radiation emitted by warm objects, allowing us to "see" heat patterns. These cameras typically operate in the range of a few micrometers (µm) to several thousand micrometers (millimeters).
Beyond the infrared region, even longer wavelengths are found in microwave and radio wave frequencies. These waves are used in communication, radar, and remote sensing applications. The longest wavelengths in this category can be many meters to kilometers long, which allows for long-distance communication and the study of cosmic microwave background radiation in astronomy.
In summary, the longest wavelength we can detect with our biological senses is in the infrared region, around 700 nm. However, with the use of specialized instruments and sensors, we can detect much longer wavelengths in the infrared, microwave, and radio wave regions for various practical and scientific purposes.