Concave Lens
A concave lens is an important topic in optics and is widely studied in Class 10 and 12 Physics. It is a type of spherical lens that is thinner at the centre and thicker at the edges, and it is also known as a diverging lens. A concave lens spreads parallel light rays outward and forms a virtual, erect, and diminished image on the same side of the object. Understanding the image formation by a concave lens, lens formula, magnification formula, ray diagrams, and its uses is essential for board exams and competitive exams. Concave lenses are commonly used in spectacles for myopia, door viewers, telescopes, and optical instruments. In this article, we will learn about the definition, formula, image formation, and uses of a concave lens in simple language.
This Story also Contains
- What is Concave Lens?
- Image Formed by a Concave Lens
- Formula for a Concave Lens
- Uses of Concave Lens
What is Concave Lens?
A concave lens is a type of spherical lens that is thinner at the centre and thicker at the edges. It is also called a diverging lens because it spreads out (diverges) parallel rays of light passing through it.
When a beam of parallel light rays falls on a concave lens, the rays spread out after refraction and appear to come from a point called the principal focus, which lies on the same side of the lens as the object.
Also read -
Image Formed by a Concave Lens
A concave lens is a diverging lens, so it always spreads the light rays passing through it. Because of this property, the image formed by a concave lens has fixed characteristics.
Characteristics of Image Formed:
- The image is always virtual (cannot be obtained on a screen).
- The image is always erect (upright).
- The image is always diminished (smaller than the object).
- The image is formed between the optical centre and the principal focus.
- The image is formed on the same side of the lens as the object.
Formula for a Concave Lens
A concave lens is a diverging lens that always forms a virtual, erect, and diminished image for a real object. The position and nature of the image formed by a concave lens can be determined using the lens formula.
Lens Formula
$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$
Where:
- f=Focal length of the lens
- $\ {v}=$ Image distance from the optical centre
- $\mathbf{u}=$ Object distance from the optical centre
Note: According to the sign convention, for a concave lens, the focal length (f) is always negative.
Magnification Formula
The magnification produced by a concave lens is given by:
$
M=\frac{h_i}{h_o}=\frac{v}{u}
$
Where:
- $\mathbf{M}=$ Magnification
- $\mathbf{h}_{\mathbf{i}}=$ Height of the image
- $\mathbf{h}_{\mathbf{o}}=$ Height of the object
- $\mathbf{v}=$ Image distance
- $\mathbf{u}=$ Object distance
Uses of Concave Lens
- In Spectacles (Myopia Correction): Used to correct short-sightedness (myopia) by diverging light rays before they enter the eye.
- Door Viewers (Spy Holes): Fitted in doors to provide a wide field of view of the outside area.
- In Telescopes: Used in combination with convex lenses in Galilean telescopes.
- In Cameras and Optical Instruments: Used to correct image distortion and improve image quality.
- Laser Devices: Used to expand laser beams by spreading light rays.
- In Binoculars: Used in certain optical systems to adjust and improve viewing.
Also check-
Frequently Asked Questions (FAQs)
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.
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.
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.
| Convex Lens | Concave Lens |
| Thicker at the centre and thinner at the edges | Thinner at the centre and thicker at the edges |
| Converges (brings together) parallel light rays | Diverges (spreads out) parallel light rays |
| Also called a converging lens | Also called a diverging lens |
| Can form real and inverted image (and sometimes virtual image) | Always forms virtual, erect, and diminished image |
| Image may be formed on either side of the lens | Image is always formed on the same side as the object |
| Focal length is positive | Focal length is negative |
| Used in magnifying glass, camera, microscope | Used in spectacles for myopia, door viewers |
