Many so-called cycle lights for the British market produce a conical beam of light, like a torch. Typically the light beam will have a bright but narrow central cone, and a dimmer part that spreads out over a much wider angle. Most people then just mount this on their bike pointing straight ahead. This will shine directly into the eyes of oncoming vehicle operators (including other cyclists). They then cannot easily see where they are going. Additionally, since the beam is not illuminating the ground in front of you, you will think you need a more powerful light. By the time you have a light bright enough to see by, oncoming vehicle operators will be completely blinded until you have passed, and they will have poor night vision for some time after. Note that a large proportion of the light is wasted, illuminating things way above your head. This means reduced battery life (if you are using batteries).
