The lenses with one or both of their sides having inward curved surfaces are known as Concave lenses. A concave lens is thinner across the center comparatively to its upper and lower edges. When a parallel beam of light rays falls on the surface of the concave lens, it deflects the light rays outward, and they appear to diverge from a point called the focal point. The distance between the center of the lens and its focal point is called the focal length. Due to its diverging property, a concave lens is also known as a diverging lens.
Types of Concave Lens
Concave lenses are broadly categorized into the following types.
Plano concave lens has one surface curved inward, and the other surface is flat. They have a negative focal length. These lenses are widely used in increasing the focal length of the optical instruments.
Biconcave lens is also known as a double concave lens because both of its surfaces are curved inward, and the radius of curvature of both the surfaces is equal. These lenses are used in expanding light beams in projection systems.
The Convexoconcave lenses have both convex and concave surfaces. However, The degree of curvature of the concave face is more than that of the convex face.
Image Formation by Concave Lens
The concave lens diverges the light that falls on its surface due to its inward curved shape and forms real and erect images on the same side as that of the object. Let’s discuss image formation by a concave lens.
Object at Infinity
The concave lens forms a virtual image at its focal point when the object is placed at infinity. The size of the image formed at the focal point is very small (nearly point-sized) as compared to the size of the object.
Object at a Finite Distance
The concave lens forms a virtual and erect image in between its optical center and the focal point when the object is placed at a finite distance from the concave lens. The image formed is of a smaller size than that of the object.
Uses of Concave Lens
Concave lenses are widely used in spectacles to correct near-sightedness or Myopia. People suffering from myopia can’t see far away objects clearly. This happens because of the increase in the distance between the eye lens and the retina of the myopic person, which results in the convergence of light rays much before the surface of the retina, and no image is formed on the retina, hence the person sees a blurred image of the far-away object. This can be corrected by placing a concave lens in front of the nearsighted eye; the concave lens spreads the light and increases the focal length, which results in the formation of the image on the retina. Thus, a person can see the far-away objects clearly by using the concave lens in their spectacles.
The small lenses that are fixed on the walls or doors to view the person present outside are called Peepholes. A concave lens is used in making peepholes as they provide a wider view of the objects outside the door. Most of the peepholes have a setup of three concave lenses and one convex lens. We can see the person outside the door through a peephole, but the person who is outside cannot see us. If a person peeks through the peephole from the outside, then his/her eyes could only see the magnified view of a very small part of the light coming from the inside of the room because of the arrangement of the concave lenses used in the peepholes.
Those telescopes that consist of the convex lenses only, sometimes create distorted and blurred images. To correct this abnormality, concave lenses are used in the telescopes along with convex lenses. A concave lens is placed in front of the eyepieces of the telescopes that help to focus the image better, hence a clear magnified image is obtained using concave lenses.
4. Laser Devices
Concave lenses are also used in many optical devices such as printers, scanners, and some medical equipment that use a laser beam. Laser beams are of very high intensity and focused on a specific area that may cause disruption in the proper working of the optical devices. That is why concave lenses are used as they disperse the light and spread it to a wider area, which enhances the working of the laser beam devices.
The cameras consisting of only convex lenses produce distorted images, and the chances of chromatic aberration are high in these cameras. Chromatic aberration is an optical problem that occurs when the convex lens fails in bringing different color wavelengths to the same focal plane; a focal plane is a plane through the focal point perpendicular to the principal axis of the lens. We can overcome the problem of chromatic aberration by using a concave lens along with the convex lens in the cameras because a concave lens increases the focal length of the lens system, hence it reduces the chromatic aberration.
6. Flash Lights
When the parallel light rays emitted by a source fall on the surface of the concave lens, then it diverges the light on the other side, which results in the increased radii of the light source and gives wider light spread, that is why the concave lens is widely used in flashlights. It is also used in car headlights as it is capable of diverging the light to longer distances, which helps the driver view distant vehicles clearly during the night.
The image provided by the binoculars looks like three-dimensional images because when we look through the binoculars, we see two images at slightly different angles due to the space between our eyes; these two images merge together, and we see a 3D image of the object. Generally, binoculars consist of convex lenses along with the prism, but some binoculars use both the concave lens and convex lens, where a concave lens is used as the objective lens, and a convex lens is used as an eyepiece. The objective lens captures the light from the object and brings it into focus, while the eyepiece provides a clear and magnified image of the object.