# Concave and Convex Lenses: Image Formation

Spherical lenses are lenses formed by connecting two spherical transparent surfaces. In general, there are two types of spherical lenses. So, lenses formed by binding two spherical surfaces bulging outward are known as convex lenses, while the lenses formed by binding two spherical surfaces such that they are curved inward are known as concave lenses. This is one of the basic differences between concave and convex lenses.

Convex lenses are also known as converging lenses since the rays converge after falling on the convex lens. The concave lens are known as diverging lenses, as the rays diverge after falling on the concave lens. In this article, we will learn about image formation by concave and convex lenses.

Image formation by Concave Lens

## Guidelines for rays falling on the concave and convex lenses

1. When a ray strikes concave or convex lenses obliquely at its optical centre, it continues to follow its path.
2. When a ray, parallel to the principal axis strikes concave or convex lenses, the reflected ray passes through a focus on the principal axis.
3. When a ray, passing through focus strikes concave or convex lenses, the reflected ray will pass parallel to the principal axis.

## Image Formation by Concave and Convex Lenses:

### Convex Lenses

• When an object is placed at infinity, the real image is formed at the focus. The size of the image is highly diminished and point size.

• When an object is placed beyond the centre of curvature, the real image is formed between the centre of curvature and focus. The image size will not be the same as the object. It will be diminished in size..

• When an object is at the centre of curvature, the real image is formed at the other centre of curvature. The size of the image is the same as compared to that of the object.

• When an object is placed in between the centre of curvature and focus, the real image is formed behind the centre of curvature. The size of the image is larger than that of the object.

• When an object is placed at the focus, a real image is formed at infinity. The size of the image is much larger than that of the object.

• When an object is placed in between focus and optical centre, a virtual image is formed. The size of the image is larger than that of the object.

## Watch the video and understand the concepts of Lens Formula, Magnification and Power of Lens

### Concave Lenses

• When an object is placed at infinity, a virtual image is formed at the focus. The size of the image is highly diminished and point size.

• When an object is placed at a finite distance from the lens, a virtual image is formed between the optical centre and the focus of the convex lens. The size of the image is smaller than that of the object.

## Summary of Image Formation by Concave and Convex Lens

 Image formation by Concave Lens Object Location Image Location Image Nature Image Size Infinity At F1 Virtual and Erect Highly diminished, point-sized Beyond Infinity and Optical Centre Between Focus (F1) and Optical center (O) Virtual and Erect Diminished
 Image formation by Convex Lens Object location Image location Image nature Image size Infinity At F2 Real and Inverted Diminished, point sized Beyond 2F1 Between 2F2 and F2 Real and Inverted Diminished Between 2F1 and F1 Beyond 2F2 Real and Inverted Enlarged At F1 At infinity Real and Inverted Infinitely large or highly enlarged At 2F1 At 2F2 Real and Inverted Same size Between F1 and 0 On the same side as the object Virtual and Erect Enlarged

## Frequently Asked Questions – FAQs

Q1

### Which type of lens is known as a converging lens?

Convex lenses are also known as converging lenses.

Q2

### Why concave lenses are known as diverging lenses?

Concave lenses are known as diverging lenses since they diverge rays after falling on the concave lens.

Q3

### Which lens has negative focal length?

Concave has negative focal length.
Q4

### Which lens is thicker in the middle and thinner at the edges?

Convex lens is thicker in the middle and thinner at the edges.
Q5

### Where are concave lenses used?

Concave lenses are used in telescopes and spy holes in the doors.

#### The below video helps to revise the topic reflection in the chapter Light Reflection and Refraction Class 10

Stay tuned with BYJU’S to learn more about image formation by concave and convex lenses and explore more on physics formulas and calculators.

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