The correct answer is option 1: Ultraviolet waves. Ultraviolet waves are a type of electromagnetic wave with a wavelength shorter than visible light but longer than X-rays. These waves are used for detecting forgery in currency notes because certain security features are added to currency notes that are only visible under ultraviolet light.

Ultraviolet (UV) detectors, also known as UV lamps or UV lights, emit UV radiation that causes these security features to become visible. These security features may include fluorescent inks or coatings that glow or change color under UV light, making it easier to identify counterfeit currency notes.

In contrast, the other options mentioned are not typically used for detecting forgery in currency notes. Infrared waves (option 2) have longer wavelengths than visible light and are used in various applications such as heat detection and communication. Radio waves (option 3) and microwaves (option 4) also have longer wavelengths and are commonly used in communication and cooking, respectively.

Therefore, the use of ultraviolet waves for detecting forgery in currency notes is a specific and important application of this type of electromagnetic wave.