Concave and Convex Lens

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Light/visible light is defined as the electromagnetic radiation which can be detected by the human eye. There are several features of light including reflection and refraction. Let’s study the features of concave and convex lenses in detail.

Lens is defined as a transparent medium which is made of glass, having either one or two curved surfaces. There are two types of lenses, concave and convex lenses. The nature of the curved surface is key in determining the kind/type of lens. Learning the behavior of lenses will help us in distinguishing between a concave and convex lens. 

Convex Lens

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A convex lens is a curved transparent medium which is made of glass. It looks like a part of a compact sphere. The external curve on the surface of this lens looks similar to the surface of a glass ball. As light rays fall on the convex surface of the lens, they will converge from their paths. 

If both surfaces are convex, then it is known as a biconvex lens. If one surface is plane and other is convex, it is known as plano-convex lens. The degree of curvature of a convex lens decides the behavior of the lenses. As parallel rays fall on the convex surface of such types of lenses, they will converge and meet at a single point. The point is referred to as the focus of the lens. The measurement of the focal point and the center of the lens is called the focal length. The focal point is also known as the principal focus of a lens.

The size and type of the images formed depends on the position of an object on the principal axis and its distance from the focus. The dimension and nature of images are determined by the thumb rule of the convex lens. 

Uses of Convex Lens


Microscopes, magnifying glasses and eyeglasses make use of convex lenses. They are also used in cameras for creating real images of objects present at a distance. The way these lenses are used decides the nature of the images. Applications of convex lenses help us to figure out how light refraction is used for seeing things properly.

Hence, convex lenses are used for creating real images for serving a particular purpose.

Convex lenses remain closer to the eyes in eyeglasses helping to create a virtual image. 

Concave lens

A transparent medium of glass which has one or two concave surfaces is called a concave lens. We can identify a concave lens by checking the curved surface. It looks like the mouth of a cave. A concave lens resembles the inner surface of a hollow sphere. They are also known as divergent lenses. The parallel beams incident on their surface diverge from their paths. 

Concave lenses will never produce real images. They have the property of diverging light rays away from their path. This implies that light rays will not converge and meet at a point physically. If the rays are produced backwards in a virtual way, they will meet at a point. This is the primary difference between concave and convex lenses. By observing the equation of the focal length and object distance, you will notice that image distance is negative. The image will be produced on the opposite side of the refracted rays. 

Uses of Concave Lens


A concave lens will be used for diverging incident rays. This helps in creating a virtual image on the opposite side of the refracting surface. As a result, concave lenses are used in binoculars, telescopes, cameras, flashlights and eyeglasses. Unlike real images, the images will be erect and upright. Hence, concave and convex lenses can be distinguished by observing the features of light rays refracting inside the lenses.

Concave and convex lenses have different traits. Formulas help in calculating image distance, object distance and focal length of a lens.

Difference between Convex Lens and Concave Lens

  • Convex lens is thicker in the middle and thinner at the edge. Whereas, the concave lens is thinner in the middle and thicker at the edges. 
  • Convex lens is also called a converging lens. Concave lens is also known as a diverging lens. 
  • Convex lenses are used in overhead projectors, cameras, simple telescopes and magnifying glasses. Concave lens is used in spy holes in doors, glasses etc ..
  • Convex lens is used for correcting long sight problems whereas concave lens is used for correcting short sight problems. 
  • Convex lenses will always have a positive focal length. On the other hand, concave lenses have a negative focal length. 
  • When a convex lens converges incident rays towards the principal axis, the concave lens diverges the incident rays away from the principal axis. 
  • The image formed through the convex lens will be upright, virtual, and smaller in size than the object. The position of the image formed will be between the lens and the objects, regardless of the position of the objects. On the other hand, the image formed through a convex lens is real, inverted, and smaller in size than the object when it is placed at focus. If the object is placed at 2F, the image will be inverted. It will be real and of the same size as that of the object. In case the object is placed between 2F and F, the image formed will be inverted, real and larger than the object. In case the object is placed at F (Focus), no image will be formed. If the object is being placed on the same side of the lens, a virtual image will be formed, which is upright and larger than the object.

Check your knowledge

 Answer) The difference between a convex and concave lens is determined by the type of curvature of the refracting surface. There is a difference in the nature of refraction occurring in these lenses as well.

      Answer) Diverging lens.

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