Reflectors are required on bicycles in many countries, including the United States and the United Kingdom, in order for a rider to use their bike legally. Reflectors are a type of passive bike lighting, meaning they do not need to be actively turned on in order to work. Instead, they reflect the light of other vehicles or sources on the road in order to increase the visibility of the bike and its rider. While some states in the US do not require reflectors, they can still be a useful safety measure, especially when riding at night or in low light conditions. It is important to check the specific laws and regulations in your area concerning bike lights and reflectors in order to avoid any issues. In the UK, the "Road Vehicles Lighting Regulations" require bikes to be equipped with appropriate lights and reflectors during the night on public roads.