Concave Lens - Overview, Structure, Properties & Uses

Concave Lens

Edited By Vishal kumar | Updated on Jul 02, 2025 04:42 PM IST

One of the most important discoveries made by humans is the lens. Although it is impossible to establish when or how the lens was found, it is obvious that ancient people realised at some time that they could alter light using a piece of glass. Humans, for concave lens examples, employed lenses to make distant items appear closer, small objects appear larger, and fuzzy objects appear clearer (i.e. magnifying glasses and corrective lenses). Convex and Concave lenses are the two types of Spherical lenses.
If both surfaces of a lens are convex, it is called biconvex (or double convex, or just convex). The lens is biconvex if both surfaces have the same radius of curvature. Biconcave refers to a lens with two concave sides (or just concave). The lens is plano-convex or plano-concave depending on the curvature of the other surface if one of the surfaces is flat. Convex-concave or meniscus lenses have one convex and one concave side. This is the most frequent type of lens used in corrective lenses.

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What is concave lens? Define concave lens.
The formula for a concave lens
Uses of concave lens

Concave Lens

What is concave lens? Define concave lens.

Concave lens definition (concave definition) and Concave lens meaning (concave meaning): A concave lens is a lens that diverges a straight light beam from the source to create a reduced, upright virtual picture. Concave lenses feature at least one curved surface inside. A concave lens is also known as a diverging lens because it is shaped round inwards in the centre and bulges outwards through the edges, causing light to diverge. They are used to cure myopia .

concave lens

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What types of images are formed by concave lenses?

Only imaginary pictures are created by concave lenses. There will be no true images because the rays never converge after being refracted. All concave lens images will be virtual, decreased, and upright, and will be discovered between the F and the lens. A concave lens is also known as a diverging lens.

The formula for a concave lens

The concave lens formula is used to determine the nature and position of the image created by the lens. The following is the lens formula:

1/f=1/v+1/u

Where:

f is the focal length,

v is the image's distance from the centre,

u is the object's distance from the centre.

Similarly, the image magnification is provided by the equation,

M=hi/ho=v/u

Where:

M is the magnification,

hi is the image height,

ho is the object height.

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Uses of concave lens

Spectacles are the first thing that comes to mind while thinking of lenses. Concave lenses are commonly used in spectacles to correct myopia (nearsightedness). Myopia affects people's ability to view distant objects. This occurs as the distance between the eye lens and the retina of a myopic person increases, causing light rays to collide considerably before they reach the retina's surface, resulting in no picture being formed on the retina, and the person seeing a blurred image of the far-away object. This can be remedied by inserting a concave lens in front of the nearsighted eye, which distributes the light and increases the focal length, allowing the picture to form on the retina. As a result, a person can see far away.

Peepholes are small lenses fitted on the walls or doors to allow you to see who is outside. Concave lenses are used in peepholes, allowing for a better view of the items outside the door. The majority of peepholes have three concave lenses and one convex lens. We can see the person outside the door through a peephole, but the person outside cannot see us. A person staring through the peephole from the outside can only get a magnified image of a very small portion of the light coming from the inside of the room due to the design of the concave lenses used in the peepholes.
Telescopes: Telescopes with only convex lenses produce distorted and hazy views from time to time. Concave lenses, as well as convex lenses, are employed in telescopes to remedy this anomaly. Concave lenses are put in front of the eyepieces of telescopes to assist focus the image more effectively, resulting in a crisp magnified image.
Laser Scanners: Many optical devices that utilise a laser beam, such as printers, scanners, and some medical equipment, use concave lenses. Laser beams have a very high intensity and are concentrated on a specific location, which might pose problems with optical systems. Concave lenses are employed because they disperse light and spread it over a larger region, improving the performance of laser beam systems.
Cameras: Only convex lenses are used in these cameras, which results in distorted images and a high risk of chromatic aberration. Chromatic aberration is an optical problem that arises when a convex lens fails to bring distinct colour wavelengths to the same focal plane; a focal plane is a plane perpendicular to the lens's major axis that passes through the focal point. Because a concave lens increases the focal length of the lens system and so minimises chromatic aberration, we can overcome the problem of chromatic aberration by utilising a concave lens in conjunction with a convex lens in cameras.
Flashlights: When parallel light rays released by a source collide with the surface of a concave lens, the light diverges on the other side, resulting in greater radii of the light source and a broader light spread. This is why concave lenses are commonly employed in flashlights. It's also utilised in car headlights since it can diverge light over greater distances, allowing the driver can see distant vehicles well at night.
Binoculars: The image supplied by the binoculars appears to be three-dimensional because, due to the space between our eyes, we see two images at slightly different angles; these two images merge, and we get a three-dimensional representation of the item. Both a concave and a convex lens are used in binoculars. The objective lens, which captures the initial image, is a concave lens, and the eyepiece is a convex lens. The objective lens collects light from the item and puts it into focus, while the eyepiece magnifies the image of the object.

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Frequently Asked Questions (FAQs)

1. What is the term for a concave lens?

Diverging lens: A diverging lens is also known as a concave lens because it is formed around inwards at the centre and bulges outwards through the edges, causing light to diverge.

2. What types of images are formed by concave lenses?

3. What is a concave lens?

A concave lens is a lens that diverges a straight light beam from the source to create a reduced, upright virtual picture. It can generate both real concave lens and virtual visuals. Concave lenses feature at least one curved surface inside. A concave lens is also known as a diverging lens because it is shaped round inwards in the centre and bulges outwards through the edges, causing light to diverge. They are used to cure myopia because they make distant objects appear smaller than they are.

4. What is a concave lens?

A concave lens is a type of optical lens that is thinner at its center than at its edges. It causes light rays to diverge (spread out) after passing through it. Concave lenses are also known as diverging lenses because of this property.

5. What are the uses of a concave lens?

To diverge incident rays, a concave lens is used. This contributes to the formation of a virtual picture on the opposite side of the refracting surface. As a result, these lenses are commonly found in binoculars, telescopes, cameras, spotlights, and eyeglasses. Unlike real concave lens photographs, the images are erect and upright.

6. What is the lens equation for a concave lens?

The lens equation for a concave lens is the same as for all thin lenses: 1/f = 1/v - 1/u, where f is the focal length, v is the image distance, and u is the object distance. However, for concave lenses, f is always negative.

7. What is the focal length of a concave lens?

The focal length of a concave lens is the distance from the center of the lens to its virtual focus. It is always negative for concave lenses, indicating that the focus is on the same side as the incoming light.

8. How does the shape of a concave lens affect its focal length?

The more curved the surfaces of a concave lens, the shorter its focal length. A flatter concave lens will have a longer (more negative) focal length, while a more deeply curved lens will have a shorter (more negative) focal length.

9. What is the significance of the optical center in a concave lens?

The optical center is a point on the principal axis of a concave lens through which light rays pass undeviated. It's important because it helps in ray diagrams and calculations involving the lens equation.

10. What is the sign convention for concave lenses in ray diagrams?

In ray diagrams for concave lenses, the following sign conventions are typically used:

11. How does a concave lens correct myopia (nearsightedness)?

Concave lenses correct myopia by diverging light rays before they enter the eye. This helps focus the image onto the retina rather than in front of it, allowing a person with myopia to see distant objects clearly.

12. Can a concave lens ever form a real image?

No, a concave lens cannot form a real image. It always forms a virtual, upright, and diminished image, regardless of the object's position. This is because the lens always causes light rays to diverge.

13. How does a concave lens affect the apparent depth of an object?

A concave lens makes objects appear closer to the surface than they actually are. This is because the diverging light rays create a virtual image that appears closer to the lens than the actual object.

14. Why is the image formed by a concave lens always virtual?

The image formed by a concave lens is always virtual because the diverging light rays never actually meet on the other side of the lens. Instead, they appear to originate from a point on the same side as the object, creating a virtual image that cannot be projected onto a screen.

15. How does the size of an image change as an object moves closer to a concave lens?

As an object moves closer to a concave lens, the size of the virtual image increases. However, the image remains upright and smaller than the object, regardless of the object's position.

16. How does a concave lens affect parallel light rays?

When parallel light rays pass through a concave lens, they diverge or spread out. The lens causes the rays to appear as if they are originating from a single point on the same side of the lens as the incoming light. This point is called the virtual focus of the lens.

17. What is the difference between a concave lens and a convex lens?

A concave lens is thinner at the center and causes light rays to diverge, forming virtual, upright, and diminished images. A convex lens is thicker at the center and causes light rays to converge, potentially forming real, inverted images or virtual, upright images depending on the object's position.

18. What happens to light rays that pass through the edges of a concave lens compared to those passing through the center?

Light rays passing through the edges of a concave lens are refracted more strongly than those passing through the center. This causes the rays to diverge more, contributing to the overall diverging effect of the lens.

19. Can a concave lens magnify an object?

No, a concave lens cannot magnify an object. It always produces a diminished (smaller) virtual image. The image appears smaller than the object, regardless of the object's position relative to the lens.

20. How does the power of a concave lens relate to its focal length?

The power of a lens is the reciprocal of its focal length in meters. For a concave lens, the power is always negative and is calculated as P = -1/f, where f is the focal length in meters. The unit of power is diopters (D).

21. What is the difference between a plano-concave and a biconcave lens?

A plano-concave lens has one flat surface and one concave surface, while a biconcave lens has two concave surfaces. Biconcave lenses generally have a stronger diverging effect than plano-concave lenses of the same material and thickness.

22. How does the refractive index of the lens material affect the behavior of a concave lens?

A higher refractive index material will cause greater refraction of light rays, resulting in a stronger diverging effect for the same lens shape. This means a concave lens made of a higher refractive index material will have a shorter focal length (more negative) than one with the same shape but lower refractive index.

23. What is chromatic aberration in concave lenses?

Chromatic aberration in concave lenses occurs because different colors of light are refracted by slightly different amounts. This causes the virtual focal points for different colors to be at slightly different distances from the lens, leading to color fringing in images.

24. How does a concave lens affect wavefronts?

A concave lens transforms plane wavefronts into diverging spherical wavefronts. The center of curvature of these spherical wavefronts appears to be at the virtual focus of the lens.

25. Can two concave lenses be combined to form a magnifying glass?

No, two concave lenses cannot be combined to form a magnifying glass. Concave lenses always produce diminished virtual images. To create a magnifying glass, at least one convex lens is necessary.

26. What is the relationship between the radius of curvature and the focal length of a concave lens?

For a thin concave lens, the relationship between the radius of curvature (R) and focal length (f) is given by the lens maker's formula: 1/f = (n-1)(1/R1 - 1/R2), where n is the refractive index of the lens material, and R1 and R2 are the radii of curvature of the two surfaces. For a concave lens, at least one R value will be negative.

27. How does the thickness of a concave lens affect its optical properties?

While thin lens approximations are often used in calculations, the thickness of a real concave lens does affect its optical properties. A thicker lens will generally have a slightly longer focal length than a thinner lens of the same curvature due to the increased path length of light through the lens material.

28. What is the principal axis of a concave lens?

The principal axis of a concave lens is an imaginary line that passes through the centers of curvature of both surfaces of the lens. It is perpendicular to the lens surface at its optical center.

29. How does a concave lens affect the wavefront of light?

A concave lens transforms a plane wavefront into a diverging spherical wavefront. The wavefront appears to originate from the virtual focus of the lens.

30. What is the difference between the principal focus and the focal plane of a concave lens?

The principal focus of a concave lens is a point on the principal axis from which light rays appear to diverge after passing through the lens. The focal plane is an imaginary plane perpendicular to the principal axis that passes through the principal focus.

31. How does the image formation in a concave lens differ from that in a plane mirror?

Both concave lenses and plane mirrors form virtual, upright images. However, concave lenses always form diminished images, while plane mirrors form images of the same size as the object. Additionally, the image distance in a plane mirror is equal to the object distance, which is not the case for concave lenses.

32. Can a concave lens be used to correct any type of vision defect?

Concave lenses are primarily used to correct myopia (nearsightedness). They are not suitable for correcting hyperopia (farsightedness) or astigmatism on their own. However, they may be used in combination with other lens types in more complex corrective lenses.

33. What happens to the focal length of a concave lens when it is immersed in water?

When a concave lens is immersed in water, its focal length increases (becomes less negative). This is because the difference in refractive indices between the lens material and its surrounding medium (now water instead of air) is reduced, leading to less refraction and a weaker diverging effect.

34. How does the concept of optical power apply to concave lenses?

The optical power of a lens is the reciprocal of its focal length in meters. For concave lenses, the optical power is always negative and is measured in diopters (D). A concave lens with a focal length of -0.5 m has an optical power of -2 D.

35. What is the significance of the center of curvature in a concave lens?

The center of curvature is the center of the sphere that would contain the curved surface of the lens. In a concave lens, there are two centers of curvature (one for each surface). These points are important in constructing accurate ray diagrams and understanding the geometry of the lens.

36. How does a concave lens affect the apparent position of an object viewed through it?

When viewed through a concave lens, an object appears to be closer to the lens than it actually is. This is because the diverging light rays create a virtual image that is always between the object and the lens.

37. What is the relationship between the object distance, image distance, and focal length in a concave lens?

The relationship is described by the lens equation: 1/f = 1/v - 1/u, where f is the focal length, v is the image distance, and u is the object distance. For a concave lens, f is negative, and v is always negative (indicating a virtual image).

38. How does the magnification of an image change as an object moves away from a concave lens?

As an object moves farther from a concave lens, the magnification decreases. The image becomes smaller relative to the object, but it always remains upright and virtual.

39. Can a concave lens form an image at infinity?

No, a concave lens cannot form an image at infinity. The image formed by a concave lens is always virtual and located between the object and the lens, regardless of the object's position.

40. What is the difference between a concave lens and a concave mirror?

While both are concave, they behave very differently. A concave lens always forms virtual, upright, and diminished images by refracting light. A concave mirror can form both real and virtual images, which can be enlarged or diminished, by reflecting light.

41. How does the f-number concept apply to concave lenses?

The f-number, commonly used in photography, is not typically applied to concave lenses. It's more relevant to convex lenses in camera systems. However, the concept of relative aperture (the ratio of focal length to effective diameter) can still be calculated for concave lenses, though it's less practically useful.

42. What happens to light rays that pass through a concave lens parallel to, but not on, the principal axis?

Light rays parallel to, but not on, the principal axis will still diverge after passing through the concave lens. They will appear to come from a point on the focal plane, but not from the principal focus itself.

43. How does the curvature of a concave lens affect its diverging power?

The greater the curvature of a concave lens (i.e., the smaller the radius of curvature), the stronger its diverging power. This results in a shorter (more negative) focal length and a higher (more negative) optical power.

44. What is the significance of the virtual focus in a concave lens?

The virtual focus is the point from which light rays appear to diverge after passing through the lens. It's crucial for understanding image formation and for constructing ray diagrams. The distance from the lens to this point defines the focal length of the concave lens.

45. How does a concave lens affect the coherence of light?

A concave lens does not significantly affect the coherence of light. While it changes the direction and divergence of light rays, it doesn't alter the phase relationships between different parts of the wavefront in a way that would destroy coherence.

46. Can a concave lens be used to create a beam expander?

Yes, a concave lens can be used as part of a beam expander system. When placed before a convex lens, it can help to create a wider, collimated beam of light. This combination is often used in laser systems and some types of telescopes.

47. How does the thickness profile of a concave lens contribute to its optical properties?

The thickness profile of a concave lens - thinner at the center and thicker at the edges - is crucial to its diverging effect. This shape causes light rays passing through different parts of the lens to bend by different amounts, resulting in the overall diverging effect.

48. What is the role of a concave lens in a Galilean telescope?

In a Galilean telescope, a concave lens serves as the eyepiece. It intercepts the converging light rays from the objective lens before they reach a focus, creating an upright, virtual image. This design allows for a more compact telescope compared to designs using convex eyepieces.

49. How does the material dispersion of a concave lens affect its performance?

Material dispersion in a concave lens causes different wavelengths of light to refract by slightly different amounts. This leads to chromatic aberration, where the virtual focal points for different colors are at slightly different distances from the lens, potentially causing color fringing in images.

50. Can a concave lens be used to correct spherical aberration?

While concave lenses themselves can exhibit spherical aberration, they can be used in combination with convex lenses to correct spherical aberration in optical systems. This is often done in the design of high-quality camera lenses and telescopes.

51. How does the concept of vergence apply to light rays passing through a concave lens?

Vergence describes the degree of divergence or convergence of light rays. A concave lens always increases the divergence of light rays, meaning it decreases their vergence. For parallel incoming rays (zero vergence), the outgoing rays will have a negative vergence.

52. What is the effect of a concave lens on the wavefront of light from a point source?

For a point source at a finite distance, a concave lens transforms the incoming spherical wavefront into a more rapidly diverging spherical wavefront. The new wavefront appears to originate from a virtual point source closer to the lens than the actual source.

53. How does the concept of cardinal points apply to concave lenses?

Cardinal points, including focal points, principal points, and nodal points, are used to describe the optical properties of lenses. For a thin concave lens, the principal points and nodal points coincide at the optical center, while the focal points are virtual and located on either side of the lens at equal distances.

54. Can a concave lens be used in a phase-contrast microscope?

While concave lenses are not typically the primary optical elements in phase-contrast microscopes, they can be used in the illumination system to help shape the light beam. The main phase-contrast effect, however, is usually achieved using specially designed phase plates and objective lenses.