Introduction
Light is a fascinating chapter included in the science syllabus. Students learn a lot of new concepts in this chapter. As they proceed to the new classes, they delve deeper into the basic and advanced concepts of light. First, they learn the features of light rays and then proceed to learn what reflection of light rays is. On proceeding further, they come to know how refraction of light rays occurs in a transparent medium. We will study the features of a concave and convex lens in this section.
One of the most significant parts of this chapter is the lens. It is a transparent medium made of glass but has one or two curved surfaces. Lenses are of two types, concave and convex lenses. The nature of the curved surface determines what kind of lens it is. Studying the behaviour of the lenses will help distinguish between a concave and convex lens. Let us learn what a concave and convex lens is first.
What is a Convex Lens?
A curved transparent medium made of glass that looks like a part of a compact sphere is called a convex lens. The surface of this lens has an external curve that looks like the surface of a glass ball. When light rays fall on the convex surface of this lens, they tend to converge from their paths.
When both the surfaces are convex, they are called a biconvex lens. When one surface is a plane and the other is convex, it is called a plano-convex lens. The behaviour of the lenses depends on the degree of curvature of a convex lens. When parallel rays fall on the convex surface of this lens, they tend to converge and meet at a single point. This point is called the focus of the lens. The focal length is the measurement of the focal point and the centre of the lens. The focal point is also called the principal focus of a lens.
In this section, you will learn how the position of an object on the principal axis and its distance from the focus determine the size and type of the images formed. The thumb rule of the convex lens regarding image formation dictates the dimension and nature of images. This lens is used in different ways.
Uses of a Convex Lens
Convex lenses are used in microscopes, magnifying glasses and eyeglasses. They are also used in the cameras to create real images of objects present at a distance. The nature of the images depends on the way these lenses are used. The prime reason for learning the uses of convex lenses is to find how light refraction is used to see things properly.
The above example of a convex lens signifies how these lenses are used to create real images to serve a purpose. In eyeglasses, the convex lenses remain closer to the eyes creating a virtual image. You will also learn why the images formed are either real or virtual and how they are used for various purposes.
Check the convex lens used and find how a lens is used to serve a purpose. Every use will define how the lenses are used in different ways. It will help you remember the features of this lens.
What is a Concave Lens?
A concave lens is a type of transparent medium made of glass that has one or two concave surfaces. The best way to recognize a concave lens is by checking the curved surface. It resembles the inner surface of a hollow sphere, almost like the mouth of a cave. These lenses are also called divergent lenses as the parallel beams incident on their surface tends to diverge from their paths.
Concave lenses can never produce a real image as their property is to diverge light rays away from their path. It means that the light rays will not converge and meet at a point physically. When the rays are produced backwards in a virtual way, they meet at a point. This is the prime difference between concave and convex lens. If you follow the equation of the focal length and object distance, you will find that the image distance always comes negative. It means that the image will form on the opposite side of the refracted rays.
Uses of a Concave Lens
A concave lens is used to diverge incident rays. This helps to create a virtual image on the opposite side of the refracting surface. Hence, these lenses are used in binoculars, telescopes, cameras, flashlights and eyeglasses. The images are erect and upright, unlike the real images. This is how you can distinguish between concave and convex lens by learning the features of light rays refracting inside the lenses.
When you follow the structure and traits of a concave and convex lens, you will clearly understand the difference in the formulas. The same formula of focal length, image distance and object distance can be adjusted using the traits of each lens.
Why should You differentiate between Concave and Convex Lens?
The difference between a concave and convex lens will help you understand the traits of each lens. It will also help you figure out the adjustments in the formula used to calculate image distance, object distance and focal length of a lens.
Difference between Convex Lens and Concave Lens
The appearance of convex lenses is thicker in the middle and thinner at the edges whereas the appearance of concave lenses is thinner in the middle and thicker at the edges.
A convex lens is also known as a converging lens whereas a concave lens is also known as a diverging lens.
Convex can be used in a lot of things like overhead projector, camera, focus sunlight, simple telescope, projector microscope, magnifying glasses etc whereas concave lens can also be used in a lot of things like spy holes in the doors, glasses, some telescopes etc.
A convex lens is being used for the correction of the long sight problem whereas a concave lens is being used for the correction of the short sight problem.
Convex lenses always have a positive focal length whereas concave lenses have a negative focal length.
Convex lens converges the incident rays towards the principal axis whereas concave lens diverges the incident rays away from the principal axis.
The image which is being formed through the concave lens is upright, smaller in size than the object and is virtual. The position of the image which is being formed is between the lens and the objects and is regardless of the position of the object. Whereas the image which is being formed through a convex lens is real, inverted and smaller in size than the object when the object is being placed at focus. When the object is being placed at 2F, the image which is going to be formed is inverted, of the same size as that of the object and is real. When the object is placed between 2F and F, the image which is going to be formed is inverted, larger than the object and is real. If and when the object is being placed at F (focus), there will be no image going to form. When the object is going to be placed on the same side of the lens, the image will be formed virtual, upright and larger than the object.
FAQs on Concave and Convex Lens
1. What is the main difference between a concave and a convex lens in terms of light refraction?
The main difference is that a convex lens converges (brings together) incoming parallel light rays to a single point known as the focus, while a concave lens diverges (spreads out) parallel light rays away from the axis. This fundamental property affects how each type of lens forms images.
2. How can I identify a convex lens and a concave lens by appearance?
A convex lens is thicker at the centre and thinner at the edges, resembling the shape of a lentil. A concave lens is thinner at the centre and thicker at the edges, similar to the mouth of a cave.
3. Why does a concave lens produce only virtual and erect images?
A concave lens always forms virtual and erect images because it diverges incoming light rays so they appear to originate from a point on the same side of the lens as the object. Thus, the rays do not physically meet after passing through the lens, resulting in a virtual image that cannot be projected onto a screen.
4. In what everyday devices are convex and concave lenses commonly used and why?
- Convex lenses are used in magnifying glasses, microscopes, telescopes, cameras, and for correcting hypermetropia (long-sightedness) because they converge light to form real images.
- Concave lenses are found in binoculars, peepholes, some telescopes, flashlights, and in eyeglasses for correcting myopia (short-sightedness) due to their diverging effect on light rays.
5. What are the different types of convex and concave lenses?
- Biconvex Lens: Both surfaces are convex (bulged outwards).
- Plano-convex Lens: One surface is flat, the other is convex.
- Biconcave Lens: Both surfaces are concave (hollow inwards).
- Plano-concave Lens: One surface is flat, the other is concave.
6. How do you use the lens formula to calculate image formation in lenses?
The lens formula is 1/f = 1/v - 1/u, where f is the focal length, v is the image distance, and u is the object distance. A positive focal length indicates a convex lens, and a negative focal length indicates a concave lens. Using correct sign conventions, this formula helps find the position and nature of the image.
7. Why is the focal length considered positive for convex lenses and negative for concave lenses?
Focal length is positive for convex lenses because the focus is on the opposite side of the light source (real focus), and negative for concave lenses because the focus is on the same side as the incoming light (virtual focus). This sign convention follows the Cartesian system used in optics calculations.
8. What happens to the size and nature of the image as an object is moved closer to a convex lens?
As an object moves towards the principal focus of a convex lens, the image changes:
- Beyond 2F: Smaller, real, inverted.
- At 2F: Same size, real, inverted.
- Between F and 2F: Larger, real, inverted.
- At F: Image at infinity.
- Between F and lens: Larger, virtual, and erect.
9. How do convex and concave lenses correct vision problems?
- Convex lenses are used to correct hypermetropia (long-sightedness) by converging light rays to focus on the retina.
- Concave lenses are used for myopia (short-sightedness) by diverging rays so they focus further back on the retina.
10. What are common misconceptions about image formation in concave and convex lenses?
- One misconception is that concave lenses can form real images – in reality, they only form virtual images.
- Another misconception is that convex lenses always form larger images. Their image size depends on the object's distance from the lens.
- It is also wrongly believed that the lens shape alone determines image type, but both object distance and lens type matter together.
