Which light is designed to show an arc of 135 degrees, 67.5 degrees from each side of the vessel?

The light designed to show an arc of 135 degrees, specifically 67.5 degrees from each side of a vessel, is the stern light. This configuration is important for providing visibility to other vessels approaching from behind. The stern light is placed at the stern (the back) of the vessel and helps other vessels assess the position and direction of the vessel at night or in reduced visibility conditions.

In contrast, the masthead light is a white light displayed on the fore and aft centerline of the vessel and has a wider arc of 225 degrees, illuminating the area in front of the vessel. An all-around light projects light over a full 360 degrees, which is useful for vessels at anchor, as it indicates their presence in all directions. A towing light is also used, but its arc is different and is specifically intended for vessels engaged in towing operations.

Each type of light serves a specific purpose in ensuring safety at sea, by helping to prevent collisions and ensuring that vessels are identifiable to one another based on their orientation and activity.