A lens is a transparent piece of material that refracts or bends light. Concave lenses are thinner in the middle and thicker at their edges, creating an inward curve on at least one surface. This shape causes light to spread out rather than come together. When looking through a concave lens, objects appear smaller and seem further away than they actually are.
How Concave Lenses Interact with Light
Concave lenses are referred to as diverging lenses. When parallel rays of light strike a concave lens, they are bent outward. This bending, or refraction, occurs as light passes from the air into the lens material and then back into the air, with the lens’s shape causing the rays to spread.
As the light rays diverge after passing through the concave lens, they do not converge at a single point. If these diverging rays were traced backward, they would appear to originate from a specific point on the same side of the lens as the incoming light. This apparent point of origin is known as the virtual focal point. It is termed “virtual” because light rays do not physically converge there.
Characteristics of Images Formed by Concave Lenses
Regardless of how far an object is placed from a concave lens, the image formed is virtual, upright, and diminished. A virtual image is one where the light rays do not physically converge to form the image; instead, they only appear to originate from that location. This means a virtual image cannot be projected onto a screen.
The image produced by a concave lens is always upright, meaning it is oriented in the same direction as the original object. The image is always diminished, appearing smaller than the actual object. This reduction in size is a direct consequence of the light rays diverging as they pass through the lens, spreading out the visual information.
Everyday Uses of Concave Lenses
Concave lenses have practical applications. A common use is in eyeglasses designed to correct nearsightedness, also known as myopia. In nearsighted individuals, light focuses in front of the retina, and concave lenses spread out incoming light rays before they reach the eye, moving the focal point back onto the retina for clearer vision.
A use of concave lenses is in door peepholes. These security devices allow a wide view of the area outside a door. The concave lens minimizes object proportions, providing a broader field of view through a small aperture.
Concave lenses are also found in optical instruments like binoculars and telescopes. While convex lenses handle magnification, concave lenses can be incorporated into these systems. In older designs like Galilean telescopes, a concave lens acts as the eyepiece, helping to ensure the final image is upright. They can also serve to correct optical aberrations or adjust the overall focal length within complex lens systems.