A periscope lets a submerged submarine see objects above the ocean surface without exposing the whole vessel. This matters because it allows sailors to observe ships, coastlines, aircraft, and weather while staying hidden and protected underwater. The basic idea is simple: light from the scene above the water is redirected down a vertical tube to an observer or camera inside the submarine.
Traditional periscopes use two angled mirrors or prisms to bend light through about 90 degrees at the top and again at the bottom. The two reflections keep the view upright while carrying the image down to eye level. Modern submarines often use photonics masts, which replace long optical tubes with cameras, sensors, fiber optics, and electronic displays inside the control room.
Understanding Ships and Submarines: The Periscope
A periscope has to preserve more than a path for light. It must give a useful view of direction, distance, and movement. The narrow tube limits the field of view, so an observer sees only a small part of the horizon at one time.
A wider viewing angle helps find objects faster, but it can make them look smaller or distorted. The height of the viewing head matters too.
From a higher point, the horizon is farther away because Earth is curved. Even a small change in height can change what is visible over waves or near a coastline.
Accurate alignment is essential. If either reflector is slightly turned, the viewed scene points in the wrong direction. This can make a ship seem to be in a different bearing from its true position.
Bearings are directions measured around the horizon. Sailors compare a target's bearing over time to judge whether it is moving closer, farther away, or across their path.
The viewing head may rotate while the submarine stays pointed in another direction. This separates where the vessel travels from where the crew looks, which is important for navigation and safe movement.
Prisms can be useful because light may reflect inside glass very efficiently. At some angles, light reaching the inside surface of a prism does not pass out into the air. It reflects back into the glass.
This effect is called total internal reflection. It can produce a bright image without the thin reflective coating used on many mirrors. Mirrors need clean surfaces.
Salt spray, dirt, scratches, and water drops can scatter light and reduce contrast. At dawn, dusk, or in fog, poor contrast can hide a distant object even when the object is technically within view.
The water surface creates another challenge. Waves constantly change the boundary between air and water. They can block the view for a moment or make objects seem to jump and bend.
Refraction near the surface can shift the apparent position of distant objects, especially when layers of air have different temperatures. Modern camera systems can record video, zoom in, and combine visible light with infrared images. Infrared sensors detect heat differences, so they can help find warm engines or people against cooler water.
Students should pay attention to the difference between an object's real position and its apparent position. Optics provides the image, but careful observation turns that image into reliable information.
Key Facts
- Law of reflection: angle of incidence = angle of reflection.
- A simple periscope uses two plane mirrors or prisms set at about 45 degrees to the tube.
- Each 45 degree reflector bends the light path by about 90 degrees.
- Two reflections in a basic periscope let the observer see an upright image.
- Refraction can occur when light passes between air, glass, and water, following n1 sin(theta1) = n2 sin(theta2).
- Modern photonics masts use cameras and electronic signals instead of a direct optical tube.
Vocabulary
- Periscope
- A periscope is an optical instrument that lets a person see over or around an obstacle by redirecting light.
- Reflection
- Reflection is the bouncing of light from a surface, such as a mirror or prism face.
- Prism
- A prism is a transparent solid that can bend, reflect, or separate light depending on its shape and material.
- Photonics mast
- A photonics mast is a modern submarine viewing system that uses cameras and sensors to send images electronically to displays.
- Line of sight
- Line of sight is the straight path along which light travels from an object to an observer or detector.
Common Mistakes to Avoid
- Thinking the periscope makes the submarine invisible is wrong because it only lets the crew observe while most of the submarine remains underwater.
- Drawing the mirrors flat across the tube is wrong because the mirrors or prisms must be angled, usually about 45 degrees, to redirect light downward and then sideways.
- Assuming light curves down the periscope by itself is wrong because light travels in straight lines unless it is reflected, refracted, or scattered.
- Forgetting the second reflection is wrong because one reflection changes the direction of the light, but a second reflection is needed in a simple periscope to deliver a useful upright view to the observer.
Practice Questions
- 1 A simple periscope has two plane mirrors each set at 45 degrees to the vertical tube. If light from a ship enters horizontally at the top, by how many degrees is the light path turned by the first mirror?
- 2 A submarine periscope extends 3.0 m above the submarine tower and the observer's eye is 7.0 m below the top mirror. What is the total vertical distance the light travels inside the periscope tube?
- 3 Explain why a modern submarine might use a photonics mast instead of a traditional optical periscope, and describe one advantage and one possible limitation.