Understanding Periscopes and Optics

A periscope is an optical instrument that allows an observer to see objects located at a higher or lower level than the observer, or objects that are hidden behind an obstacle. It works by using a series of reflections to change the direction of light rays.

Imagine you are looking through a periscope. Light from the object you want to see enters the periscope at the top. This light needs to travel down to your eye at the bottom. To do this, the direction of the light is changed using optical components.

How a Simple Periscope Works

A basic periscope uses two optical components placed parallel to each other at the top and bottom ends of a tube. These components are angled at 45 degrees with respect to the horizontal or vertical axis of the tube. Light from the object hits the top component, is reflected downwards, travels through the tube, hits the bottom component, and is reflected horizontally towards the observer's eye.

Optical Components in a Periscope

The question asks what is used in a periscope. Let's look at the options:

• Plane mirror
• Prism
• Concave lens
• Convex lens

Analyzing the Options

• Plane mirror: A plane mirror is a flat reflective surface. When light strikes a plane mirror, it reflects at the same angle it arrived, but in a different direction. This is perfect for changing the direction of light rays by 90 degrees if placed at a 45-degree angle.
• Prism: Prisms can also be used in periscopes, often using total internal reflection to achieve the same 90-degree change in light direction as a mirror. Prisms can sometimes provide a clearer image because they don't suffer from the reflective coating issues of mirrors, but mirrors are simpler and cheaper for basic periscopes.
• Concave lens: A concave lens is thicker at the edges than at the center and diverges (spreads out) light rays. It is used to form virtual, reduced, and upright images for certain object positions. It is not used for simply changing the direction of light in the way needed for a periscope's main function.
• Convex lens: A convex lens is thicker at the center than at the edges and converges (focuses) light rays. It is used to form real or virtual images depending on the object's position. Like concave lenses, it is primarily for focusing/defocusing, not for the simple reflection needed to change direction in a periscope.

In a simple periscope, the function required is to change the direction of light through reflection so that the observer can see objects that are not in a direct line of sight. Both plane mirrors and prisms can perform this reflection. However, the question provides "Plane mirror" as an option, and it is a common component in periscopes, especially simpler ones.

Therefore, a plane mirror is used in a periscope to reflect light and change its direction, allowing observation over obstacles.

Conclusion

Based on how a periscope functions by reflecting light to change direction, and considering the common components used, the optical device typically employed is a plane mirror.

Revision Table: Periscope Components

Additional Information: Types of Periscopes

There are different types of periscopes. Simple periscopes, like the one described, use two plane mirrors. More complex periscopes, such as those used in submarines, often use prisms and sometimes include lenses to provide magnification or a wider field of view. The basic principle, however, remains the redirection of light using reflection.

The angle of the mirrors or prisms is crucial. Placing them at a 45-degree angle ensures that the incoming light ray is reflected at a 90-degree angle, effectively changing its path from vertical to horizontal or vice versa, depending on the placement.