Learn the definition and examples of a concave lens, and its many uses throughout human history. Discover how much concave lenses are used in...
What is a concave lens? As discussed previously, a concave or a diverging lens is characterized by a thinner center and thicker edges. Because of its shape, parallel light rays passing through the lens refract and spread out. Shown in the diagram are the parts of a concave lens and what occurs to light rays as they pass through it.
A concave or diverging lens spreads out the parallel light rays passing through it.
A concave lens and its corresponding ray diagram include the focal point, focal length, and principals axis. The focal point, F, of a concave lens is the point where all refracted light rays seem to emerge when traced from the parallel light rays. Since a typical biconcave or double concave lens has two concave sides, it also has two foci. The focal length, f, is the distance from the focal point and the center of the lens. A concave lens has a negative focal length, thus it is also known as a negative lens. Lastly, the principal axis refers to the horizontal line passing perpendicular through the lens.
Parallel light rays passing through the concave lens are refracted and do not pass through the focal point behind the lens. When the refracted rays are traced, they all seem to diverge at the focal point in front of the lens, producing a virtual image. It is formed when refracted rays do not pass through or cross each other to form an image.
What are the characteristics of an image formed by a concave or a diverging lens? These image characteristics can be determined by drawing a ray diagram. Consider the following steps in ray diagramming:
Principal rays included when creating a ray diagram for a concave lens
All concave or diverging lenses form upright, virtual, and diminished images. The image formed is also always located between the object and lens, as demonstrated in the series of ray diagrams below.
Images formed by a concave lens are always virtual and upright no matter what the position of the object is.
Concave or diverging lenses have different uses. Some of the uses of concave lenses include eyeglasses, telescopes, flashlights, and peepholes.
Concave lenses are used to correct nearsightedness or myopia. It is an eyesight problem where the eye can only focus on nearby objects but not on far objects. It occurs when the eyeball is too long or when the curvature of the cornea is too large. Because of this, light rays converge in front of the retina. To correct this problem, a concave or a diverging lens in eyeglasses is used to allow the light to slightly diverge before it enters the cornea, enabling the light rays to converge at the retina.
A concave lens is used to correct nearsightedness or myopia.
Terrestrial telescopes, such as a Galilean telescope, are used to observe objects on Earth. It requires an upright image, that is why a concave lens is used as an eyepiece while a convex lens is used as an objective lens. When light rays from an object pass through the telescope, the refracted rays pass through the diverging lens before they reach the focal point of the converging lens. The refracted rays form an upright and a virtual image. However, these types of telescopes have a smaller field of view compared to other telescopes. Opera glasses still use the same setup until today.
A general setup for a Galilean telescope, which uses a concave lens as an eyepiece.
In general, binoculars, telescopes, and cameras use concave lenses to reduce the chromatic aberrations (color) or blurriness produced by a convex lens. It helps produce a clearer and more focused image. In addition, convex lenses also produce an upright image, an important image characteristic in binoculars and some telescopes.
Concave or diverging lenses are also used in flashlights. Since the purpose of a flashlight is to produce light with a wider beam, it is done using a diverging lens. The parallel rays from the flashlight hit the diverging lens, refracting and scattering the rays which result in a wider beam.
Door peepholes or spyholes are used to check the identity or presence of the visitor outside the door before opening it. It contains three concave lenses and one convex lens that allow a wide field of view and form an upright image. A person outside the door cannot use the peephole to check inside the room since they can only observe a magnified image of a small section of the room.
Typical door peepholes use three concave lenses to produce an upright image while covering a wider field of view.
A lens is a circular optical device composed of two sides or faces cut out from a sphere. There are two types of lens—convex and concave lenses. Convex or converging lenses are used to refract and converge parallel light rays to a specific point. They form varying image characteristics in terms of magnification, type, and position. Concave or diverging lenses, on the other hand, refract and scatter the light rays as they pass through them. They always form upright, virtual, and smaller images. In addition, the image formed is always located between the focal point and the lens.
Concave lenses are used in various applications, including but not limited to corrective eyeglasses, terrestrial telescopes, flashlights, and door peepholes/spyholes. A concave lens is used to correct nearsightedness or myopia since it allows the light rays to slightly diverge and converge exactly on the retina. It is also used to form upright images in terrestrial telescopes. In binoculars, telescopes, and cameras, concave lenses are used to fix the orientation of the image and the chromatic aberrations of the images formed by convex lenses. In general, it helps these optical devices to focus the images better. It is also used in flashlights to produce a wider beam of light. In door peepholes or spyholes, a combination of concave and convex lenses is used to form an upright image in a wider field of view.