Convex Lens - Definition, Types, Uses, FAQs

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

Define Convex lens.

Convex lens Definition and Convex meaning: The convex lens converges light rays that travel parallel to its primary axis (i.e. converges incident rays towards the principal axis) and is comparatively thick in the middle and thin at the lower and upper edges. Convex lens is also known as converging lens because it converges parallel beam of light on point called principal focus. A convex lens can generally converge a beam of parallel rays to point on the other side of the lens. This point is known as the lens' focus, and the focal length is the distance between it and the Optical Centre of the beam. An approximate equation connects radius of curvature R1 and R2 of spherical surfaces as well as focal length of lens ‘f'.

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Define Convex lens.
Types of convex lenses:
Uses of Convex lenses

Convex Lens - Definition, Types, Uses, FAQs

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For a Mathematical Equation, use the following formula:

1/f = (n–1)(1/R₁–1/R₂)

The refractive index of the material is denoted by n.

R₁ and R₂ are the curvature radii.

R₁ refers to the surface that is closest to the light source.

R₂ refers to the surface that is far away from the light source.

The focal length of a Double Convex lens is longer due to the presence of a second curved surface. Double Convex lenses are popular because many optical systems demand larger focal lengths.

Why is a convex lens called a Converging lens?

Because it converges a parallel beam of light on a point called the principal focus, a convex lens is called a converging lens.

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Magnification of Convex lens:

Convex lens magnification is defined as the ratio of picture height to object height. A magnification of 2 means that the picture is twice the size of the object; whereas a magnification of 1 means that the image is the same size as the object. When the magnification is positive, the picture is vertical in relation to the object (virtual image). The picture is reversed in comparison to the object when magnification is negative (real image).

Types of convex lenses:

Convex lenses come in a variety of shapes and sizes.

1. Plano-convex lens: This lens has a plano-convex shape.

It is curled outwards on one side and flat on the other. Positive focal length elements with one spherical and one flat surface are used. These lenses are made for non-critical applications that require unlimited parallel light.. It's used in pharmaceuticals, defence, and robots, among other things.

2. Double Convex lens:

From both sides, it curves outwards. The Biconvex lens, or just convex, is another name for it. Plano-convex lenses of identical diameter and surface radius have a lower focal length. Longer focal lengths are required by so many optical devices. As a result, double convex lenses are chosen. It is used in projectors, monoculars, telescopes, and cameras, among other things. It served as a virtual image for the human eye and a real image for photography, as well as an optical sensor and a burner for glass.

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3. Concave-Convex lenses:

From one side, it curves inwards, while from the other, it curves outwards. It can be used to compensate for other lenses' spherical aberrations. Its purpose is to control the laser beam. A concave-convex lens or meniscus is a combination of lenses with one convex lens along with one concave lens side.

Convex lens's image formation:

The real image is produced by the convex lens at the focus when an object is positioned at infinity. The image is a fraction of the size of the actual object.
The real image is produced by the convex lens, between the center of curvature and the focus when an object is positioned behind the centre of curvature. When compared to the size of the thing, the image is the same size.
The real picture of an object is produced by the convex lens at the other center of curvature when it is at the center of curvature. When compared to the size of the object, the image is the same size.
When an object is placed between the center of curvature and the focus of the convex lens then, the real image is created behind it. The image is much larger than the size object.
A real image is produced at infinity when an object is put at the focus of the convex lens. The image is much larger than the actual object.
A virtual image is produced when an object is placed between the focus and the pole of the convex lens. The image is significantly larger than the actual object.

NCERT Physics Notes :

Convex lens: Real Image and Virtual Image

Real Image:

When an object is positioned at a distance of more than one focal length from the lens, a convex lens can be utilised to provide a genuine/real image. It can be projected in front of the lens and captured on a screen. It's what you'd find in a movie theatre, along with a projector and other equipment.

Imaginary Image:

If the item is in front of the focal point, a convex lens will produce a virtual picture. It is utilised to provide crisp images in eyewear.

Uses of Convex lenses

For hyperopia correction, a convex lens is used. Farsightedness, also known as hypermetropy, is the inability to see distant objects clearly while also having difficulty focusing on close objects.
In microscopes and magnifying glasses, a convex lens converges all incoming light rays to a single point.
In camera systems, the convex lens is used. Because it focuses light and generates a clear and crisp image, it is utilised in cameras.
The projector also uses a convex lens, also called a converging lens. A projector is used to create an enlarged, real, and inverted image of an item that is slightly beyond the focus length of a thin convex lens and can be projected onto a screen.
A magnifying glass is one of the most straightforward and simplest uses of a convex lens. When light rays enter the magnifying glass's convex lens, they focus on a certain focal point in front of the lens's center.

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Frequently Asked Questions (FAQs)

1. What are the convex lens rules?

Image formation using a convex lens:

Rule 1: After refraction via the lens, a beam of light that was previously parallel to the major axis passes through the focus.

Rule 2: After refraction, a ray of light travelling through the optical centre of a convex lens does not bend but instead passes straight.

2. What does it mean to use a convex lens?

Convex lenses are used in eyeglasses to treat farsightedness, a condition in which the distance between the lens of the eye and the retina is excessively short, causing the focal point to be behind the retina. Convex lenses increase refraction and, as a result, shorten the focal length of eyeglasses.

3. Do convex lenses have a positive focal length?

Item distances (u) are always negative when the object is located to the left of the mirror/lens, according to Cartesian sign convention. A convex lens and convex mirror have a positive focal length (f). Concave lenses and concave mirrors have a negative focal length.

4. What are the different types of convex lenses?

Convex lenses Come in a Variety of Shapes and Sizes

The plano convex lens, double convex lens, along with finally the concave-convex lens are the three forms of these lenses.

5. What is a convex lens?

A convex lens is a type of optical lens that is thicker at the center and thinner at the edges. It causes light rays to converge (come together) at a focal point after passing through the lens. This shape allows convex lenses to form real images and magnify objects.

6. What is the focal point of a convex lens?

The focal point of a convex lens is the point where parallel light rays converge after passing through the lens. It is located on the principal axis of the lens, and its distance from the lens center is called the focal length.

7. How does a convex lens differ from a concave lens?

A convex lens is thicker in the middle and thinner at the edges, causing light rays to converge. In contrast, a concave lens is thinner in the middle and thicker at the edges, causing light rays to diverge (spread out). Convex lenses can form real images, while concave lenses only form virtual images.

8. What is the significance of the principal axis in a convex lens system?

The principal axis is an imaginary line that passes through the center of curvature of both lens surfaces and the optical center of the lens. It is significant because:

9. How does the refractive index of a convex lens material affect its focusing power?

The refractive index of a lens material directly affects its focusing power. A higher refractive index results in greater bending of light rays, leading to a shorter focal length and higher focusing power. Conversely, a lower refractive index results in less bending, a longer focal length, and lower focusing power.

10. What are the main types of convex lenses?

The main types of convex lenses are:

11. How does the shape of a convex lens affect its focal length?

The shape of a convex lens directly affects its focal length. A more curved lens (with a smaller radius of curvature) will have a shorter focal length, while a less curved lens will have a longer focal length. This is because a more curved lens bends light rays more sharply, causing them to converge closer to the lens.

12. How does the thickness of a convex lens affect its optical properties?

The thickness of a convex lens affects its optical properties in several ways:

13. What is chromatic aberration in convex lenses, and how can it be minimized?

Chromatic aberration is the failure of a lens to focus all colors to the same point, resulting in colored fringes around images. It occurs because different wavelengths of light refract at slightly different angles. To minimize chromatic aberration, achromatic doublets (combinations of convex and concave lenses with different refractive indices) or apochromatic lenses (using special glass types) can be used.

14. What is the difference between linear and angular magnification in convex lenses?

Linear magnification refers to the ratio of image size to object size and is used for real images formed by convex lenses. Angular magnification, used for virtual images and in instruments like magnifying glasses, refers to the ratio of the angle subtended by the image at the eye to the angle subtended by the object. Angular magnification depends on where the eye is placed relative to the lens.

15. How does a convex lens form a real image?

A convex lens forms a real image when an object is placed beyond the focal point. Light rays from the object pass through the lens and converge on the opposite side, creating an inverted image that can be projected onto a screen. The size and position of the image depend on the object's distance from the lens.

16. Under what conditions does a convex lens produce a virtual image?

A convex lens produces a virtual image when the object is placed between the lens and its focal point. In this case, the light rays diverge after passing through the lens, creating an upright, magnified virtual image on the same side of the lens as the object.

17. What is the difference between a real and virtual image formed by a convex lens?

A real image is formed when light rays actually converge at a point after passing through the lens. It can be projected onto a screen and is inverted. A virtual image is formed when light rays appear to diverge from a point but don't actually meet there. Virtual images cannot be projected onto a screen and are upright.

18. How does the position of an object relative to a convex lens affect the image formed?

The position of an object relative to a convex lens determines the characteristics of the image formed:

19. What is the relationship between focal length and lens power?

The power of a lens is inversely proportional to its focal length. A lens with a shorter focal length has a higher power, while a lens with a longer focal length has a lower power. The formula for lens power (P) in diopters is P = 1/f, where f is the focal length in meters.

20. What is the lens maker's equation, and how is it used?

The lens maker's equation relates the focal length of a thin lens to its refractive index and radii of curvature:

21. What is the relationship between object distance, image distance, and focal length in a convex lens system?

The relationship between object distance (u), image distance (v), and focal length (f) in a convex lens system is given by the lens equation:

22. How does a convex lens magnify objects?

A convex lens magnifies objects by bending light rays to make them appear to come from a larger object. When an object is placed between the focal point and the lens, the refracted rays diverge as if coming from a larger, upright virtual image behind the object. The magnification depends on the lens's focal length and the object's position relative to the focal point.

23. How do convex lenses correct farsightedness (hyperopia)?

Convex lenses correct farsightedness by converging light rays before they enter the eye. In a farsighted eye, the focal point falls behind the retina. The convex lens brings the focal point forward onto the retina, allowing clear vision of nearby objects. The power of the lens is chosen to compensate for the eye's refractive error.

24. What are the primary uses of convex lenses in everyday life?

Convex lenses have numerous applications in everyday life:

25. How do convex lenses function in a compound microscope?

In a compound microscope, convex lenses serve two main functions:

26. What is the thin lens approximation, and when is it valid?

The thin lens approximation assumes that the thickness of a lens is negligible compared to its focal length and the radii of curvature of its surfaces. It simplifies calculations by treating the lens as a single refracting surface. This approximation is valid when:

27. How does the f-number of a convex lens relate to its light-gathering ability?

The f-number (f/#) of a lens is the ratio of its focal length to its diameter. A smaller f-number indicates a larger aperture relative to focal length, allowing more light to pass through. This results in:

28. What is spherical aberration in convex lenses, and how can it be reduced?

Spherical aberration occurs when light rays passing through different parts of a spherical lens converge at slightly different points, resulting in a blurred image. It can be reduced by:

29. How do convex lenses contribute to the formation of rainbows?

While convex lenses themselves don't form rainbows, they help explain the phenomenon. Raindrops act like tiny convex lenses, refracting and reflecting sunlight. The curved surface of a raindrop separates white light into its component colors through dispersion, similar to how a convex lens causes chromatic aberration. The different angles of refraction for various wavelengths result in the separation of colors we see in a rainbow.

30. What is the center of curvature of a convex lens, and how does it relate to the focal point?

The center of curvature of a convex lens is the point at the center of the sphere that would form the lens surface if extended. For a thin lens:

31. How does the index of refraction of the surrounding medium affect the behavior of a convex lens?

The index of refraction of the surrounding medium affects a convex lens by changing the relative refractive index (the ratio of the lens material's index to the surrounding medium's index). This impacts:

32. What is the difference between a converging and a diverging lens, and how can you identify them visually?

A converging lens (like a convex lens) brings parallel light rays together at a focal point, while a diverging lens (like a concave lens) spreads light rays apart. To visually identify them:

33. How does the curvature of a convex lens affect its ability to form images?

The curvature of a convex lens directly affects its ability to form images:

34. What is the significance of the optical center in a convex lens?

The optical center of a convex lens is a point within the lens through which light rays pass without deviation. Its significance includes:

35. How do convex lenses function in a refracting telescope?

In a refracting telescope, convex lenses serve two main functions:

36. How does the size of a convex lens affect its optical properties?

The size of a convex lens affects its optical properties in several ways:

37. What is the difference between a simple and a compound lens?

A simple lens consists of a single piece of glass or other transparent material with two refracting surfaces. A compound lens is a combination of two or more simple lenses, either cemented together or separated by a small distance. Key differences include:

38. How does the concept of wavefronts apply to convex lenses?

Wavefronts are surfaces of constant phase in a wave. In the context of convex lenses:

39. What is the principle of reversibility in convex lenses, and how does it apply to image formation?

The principle of reversibility states that light rays can be traced in either direction through an optical system and will follow the same path. For convex lenses, this means:

40. How do convex lenses contribute to the working of a camera obscura?

A camera obscura is a dark room or box with a small hole that projects an inverted image of the outside scene. Conv