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Lenses are transparent objects which can be either made of glass or plastic that can bend or refract light rays. They are used in various optical devices such as cameras, telescopes, eyeglasses, and microscopes. In this blog, we will discuss the two types of lenses – concave and convex lenses. We will define them, explain their differences, formulas, and focal points, and provide examples and drawings. To understand the terms, concave and convex lens, first, let us understand what are lenses?
Table of Contents
What are Lenses?
Lenses are optical devices that refract light and focus it to form an image. They work by bending the light that passes through them, either converging it to a point or diverging it away from a point. The amount of bending depends on the shape and curvature of the lens. Have you ever wondered how a magnifying glass makes things look bigger, or why eyeglasses can help you see clearly? The answer is that they are different types of concave and convex lenses.
Types of Lenses
There are two types of Lenses – Concave and Convex Lenses.
- Concave Lenses: Concave lenses are also known as diverging lenses. They are thin at the center and thick at the edges. Concave lenses diverge light rays and form virtual images ( images that cannot be projected on a screen) that are always smaller than the object.
- Convex Lenses: Convex lenses are also termed converging lenses. They are thick at the center and thin at the edges. Convex lenses cause light rays to come together or converge after passing through them. This means that they produce real images (images that can be projected on a screen) that are larger than the object.
Also Read – Concept Of Parallel Axis Theorem: History, Definition, Formula
What are Concave Lenses?
A Concave lens, also called a diverging lens, is a transparent optical device that has a curved shape curving inward, with thicker edges and thinner centres. The parallel rays of light spread apart or diverge when passing through this type of lens. They are commonly used in cameras, projectors, and telescopes.
Types of Concave Lenses
There are two types of concave lenses:
- Biconcave Lenses: These lenses are curved inwards on both sides, making them thinner towards the center and dispersing rays, which creates virtual images. It is most commonly used in eyewear glasses for the purpose of nearsightedness.
- Plano Concave Lenses: These lenses are similar to a biconcave lens, and have one side (Plano) but curve inwards on the other side, diverging light. It finds its usage in specially designed optical systems like while setting up beam expansion.
What are Convex Lenses?
A Convex lens, also known as a converging lens, is an optical device that is transparent and has a curved shape that bulges from the center and tapers towards the edges. This lens can converge parallel rays of light that pass through it because it is thicker at the center than it is at the edges.
Types of Convex Lenses
There are three types of convex lenses:
- Biconvex Lens: The biconvex lens has a shape that bulges towards the center on both sides, and it is commonly used in magnifying glasses and simple camera lenses.
- Plano Convex Lens: The plano-convex lens has one side that is flat (plano), and the other side is curved outward. It is designed to focus rays to a specific point and is often found in optical instruments and projectors.
- Concavo Convex: The concavo-convex lens has a convex face and a concave face, and the convex face has a smaller curvature.
Also Read – Perpendicular Axis Theorem: Definition, Formula & More
Difference Between Concave and Convex Lens
Concave lenses, thinner in the middle and shaped like bowls, are light dispersers. They nudge incoming light rays apart, creating a virtual image that appears smaller and farther away compared to the actual object. In essence, convex lenses bring things closer and magnify them, while concave lenses make things seem smaller and further away.
In Contrast, Convex and concave lenses, despite their seemingly simple shapes, have opposite effects on light. Convex lenses, thicker in the middle and resembling magnifying glasses, act like tiny crowd pushers. They bend light rays inwards, focusing them to a single point and making objects appear larger or magnified.
Imagine a spoon. Concave lenses are like the inside of a spoon, curving inwards. They push light rays apart, making things look smaller. Convex lenses are like the outside of the spoon, bulging outwards. They squeeze light rays together, making things appear bigger.
Table of Differences Between Concave and Convex Lens
The differences between Concave and Convex Lens are –
| Feature | Concave Lens | Convex Lens |
| Shape | Curves inwards like a bowl | Curves outwards like a magnifying glass |
| Light Effect | Diverges light rays | Converges light rays |
| Focal Point | Virtual and on the opposite side of the lens | Real and on the same side of the lens |
| Image Formation (General) | Virtual and upright, smaller than the object | Can be virtual or real, magnified or inverted (depending on the object’s position) |
| Image Size (General) | Smaller than the object | Larger (magnified) than the object |
| Thickness at Center | Thinner | Thicker |
| Common Uses | Corrective lenses for farsightedness, diverging light source | Magnifying glasses, microscopes, eyeglasses for nearsightedness, converging light source |
| Symbol | ( NIL) | + |
| Magnification | Negative (always reduces image size) | Positive (can magnify image) |
| Application in Eyeglasses | Corrects farsightedness | Corrects nearsightedness |
| Effect on Field of View | Narrows field of view | Widens field of view |
| Aberration (color fringing) | Less prone | More prone |
| Example in Everyday Life | Fishbowl | Eyeglasses, magnifying glass |
| Formula (Simple) | Not applicable (no real focal point) | 1/f = 1/do + 1/di (f is focal length, do is object distance, di is image distance) |
| Image Location | Always on the same side as the object. | Can be on the same side (real image) or the opposite side (virtual image)convex lenses can also form virtual images on the same side if the object is placed between the lens and its focal point. |
| Object Position for Real Image Formation | Not applicable | Between the lens and its focal point |
| Object Position for Virtual Image Formation | Anywhere | Beyond the focal point on either side |
| Overall Effect | Makes objects appear smaller and farther away | Makes objects appear larger and closer |
Focal Length of Concave and Convex Lens
Focal length is all about where light rays converge (convex) or diverge (concave) after passing through a lens. Concave lenses are diverging lenses and have a negative focal length. Here, the focal length of concave lenses signifies a virtual point from where the diverging rays seem to originate after passing through the lens.
Conversely, Convex lenses are converging lenses, so their focal length is a positive value. The focal length of convex lenses represents the distance from the center of the lens to the point where parallel incoming rays meet.
Note: – There’s no actual convergence in a concave lens.
Concave and Convex Lens Application
| Lenses | Example | How It Works |
Concave Lenses | Eyeglasses for farsighted people | Diverges incoming light rays to focus them on the retina for clear vision. |
| Car rearview mirror (passenger side) | Spreads out reflected light to provide a wider field of view of approaching vehicles. | |
| Peephole in a door | Allows a wider view through a small opening by diverging light rays. | |
| Concave (in some designs) | Astronomical telescope eyepiece | Can be used to further magnify the image formed by the objective lens. |
Convex Lenses | Magnifying glass | Converges light rays to create a magnified virtual image of the object. |
| Eyeglasses for nearsighted people | Converges incoming light rays to focus them on the retina for clear vision. | |
| Microscope objective lens | Focuses light rays to create a magnified real image of the specimen. | |
| Overhead projector | Converges light rays from a light source to project an enlarged image onto a screen. | |
| Camera lens | Converges light rays to focus the image onto the camera sensor. |
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Hope this blog helps you. Keep reading more of our blogs to learn about the basic concepts of Physics!