Concave and Convex Lens

Difference between Concave and Convex lens

A lens is an optical device that converges or diverges a ray of light by means of refraction. Unlike a prism, a lens can focus a ray of light to form an image, while prism simply refracts the light beam without focusing. All those devices which can focus a light beam are known as lenses, for e.g. microwave lens, acoustic lens, etc. A lens can be divided into two types, based on its shape. Hence, the lens which is thicker at the centre and gets thinner as we move towards the edges is known as a convex lens, while the lens which is thinner at the centre and gets thicker as we move towards the edges is called as Concave lens.

Convex Lens


The type of lenses which converge the light beam coming from the source to one point on the other side to form an image is called as a convex lens or converging lens. If the lens contains two spherical surfaces then it is said to be a biconvex lens, and if the lens contains one flat and one spherical surface then it is said to be Plano-Convex lens. The point where the converging light rays meet is known as the focus point or the principal focus of the lens and the distance between the center of the lens and the principal focus is said to be focal length (which is positive in this type of lens).

When the object is placed in the middle of the focus and optical center then the image formed is virtual in nature. Generally, a convex lens forms a real and inverted image. Basically, convex lens gives a clearer and larger image of an object by focusing the light beam at one point. As the center of the convex lens is thicker, it makes the object image at the focus point look larger and closer. Convex lens is used for various purposes like in a magnifying glass, camera, microscope, correction of Hypermetropia, etc.

Some of the uses of the Convex lens are stated below:-


  • Magnifying glasses:

  • It is the most common use of the convex lens, in which the ray of light after converging through the convex lens forms an image at the focus point which provides the maximum magnification to the object.

  • Eyeglasses:

  • A person can face the problems of farsightedness or nearsightedness due to the failure of the lens of his eye in focusing the light on the retina properly. The convex lens is used to solve the problem of farsightedness or Hypermetropia, by bending the light ray which shortens the focal length and makes the light ray focus on the retina in a proper way.

  • Cameras:

  • The convex lens is used in the camera for focusing the image and also for magnifying it. The magnification of the image in the camera is done by adding multiple lenses one after another like a convex lens followed by a concave lens and again followed by a convex lens. The magnification of the image in the camera is controlled by moving the front convex lens.

  • Microscopes:

  • Convex lens is used in microscopes to magnify the images of very small objects. Generally, microscopes consist of three lenses in which the end lens generates a magnified and inverted image.

    Concave lens


    The type of lens which diverges the straight light beam coming from the source to form a diminished, upright and virtual image in front of it is called a Concave lens. Depending on the light source, it can form both real and virtual images. The lens which has at least one surface curved inwards are called concave or diverging lens. This lens is shaped round inwards at the center and bulges outwards through the edges, making the ray of light diverge on it. As these lenses are used for the treatment of short-sightedness or myopia, it makes the objects look smaller which are far away, then they really are. The point where the ray of light seems to refract and diverge is called as the principal focus, and the distance between the principal focus and center of the lens is called the focal length (which is negative in this type of lens) of the concave lens. If the thickness of the lens is very less when compared to the radius of the arc then the lens is called a thin lens. Its formula can be given as below:

    1/f = (1/v) + (1/u)
    Where, f = focal length;
                  V = Distance of the image from the center
                  U = Distance of the object from the center
    Magnification of the image can be given by;
    M = hi / ho = v / u
    Where, M = Magnification
                   Hi = Height of image
                   Ho = Height of object

    Some uses of the concave lens are stated as follows:-


  • Telescopes and Binoculars:

  • To make a person focus more clearly and see the far objects clearly through telescope or binoculars, a concave lens is used. It is most preferred for these devices as the convex lens helps in seeing a blurry image.

  • Eyeglasses:

  • The concave lens is used to treat the problem of nearsightedness or myopia by diverging the light ray and making the clear image of the object on the retina.

  • Lasers:

  • Small concave lenses are used to widen the laser beam which occupies more area and makes the laser beam producing equipment work properly.

  • Camera: The magnification of the image in the camera is done by adding multiple lenses one after another like a convex lens followed by a concave lens and again followed by a convex lens. By combining a concave lens with a convex lens, the undesirable effects are eliminated.

  • Flashlights:

  • To magnify the light produced by the light beam, the concave lens is used which increases the radius of the beam by diverging the light ray falling on the hollowed side to the other side, making the light ray wider and magnified.

  • Peepholes:

  • Peepholes in doors provide the panoramic view of the objects on the other side of the door, works as a security device in which concave lens is used to minimize the proportion of objects providing a wider view of the objects outside the door.

    Difference between Convex and Concave lenses:-


    BasisConvex LensConcave Lens
    Other nameA convex lens is called as ‘converging lens’ or ‘positive lens’.A concave lens is called as ‘Diverging lens’ or ‘Negative lens.
    Light raysWhen light passes through the lens, it bends the light rays towards each other.The concave lens spreads out the light when it passes through the lens. An image cannot form on a screen in this case.
    Structure This lens is thick at the center as compared to the edge. It helps to magnify the things and make them look bigger.A concave lens is exactly opposite to a convex lens. It is thin at the center and thick at the edge.
    Parallel raysAs the parallel rays converge, the convex lens is termed as a converging lens.Similarly, a concave lens is called diverging lenses because they cause parallel rays to diverge.
    Curve The convex lens is curved towards the inside.The concave lens is curved towards the outside.
    UseIt is used to correct long-sightedness or hypermetropia.It is used to correct short-sightedness or myopia.
    For instanceThe human eye, camera, telescope, microscope etc. are some examples of the convex lens.Lights, flashlights, laser, binocular etc. are the examples of the concave lens.
    Focal lengthIt is called positive lens because of its positive focal length nature.The concave lens is called a negative lens because of its negative length nature.