Key Differences Between Convex and Concave Lenses Explained

Differences between a Convex Lens and a Concave Lens

A convex lens is thicker in the middle than at edges. A concave lens is thinner in the middle than at edges.

Convex Lens

1. A convex lens is thicker in the middle than at edges.
2. It is a converging lens.
3. It produces mostly real images.
4. It is used to treat hypermeteropia.

Concave Lens

1. A concave lens is thinner in the middle than at edges.
2. It is a diverging lens.
3. It produces virtual images.
4. It is used to treat myopia.

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10th Science : Chapter 2 : Optics : Differences between a Convex Lens and a Concave Lens

Lens Formula Explained: A Key Concept in 10th Science Optics

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Lens Formula

10th Science : Chapter 2 : Optics : Lens Formula

Defining the Lens Formula

LENS FORMULA

Like spherical mirrors, we have lens formula for spherical lenses. The lens formula gives the relationship among distance of the object (u), distance of the image (v) and the focal length (f) of the lens. It is expressed as

$$ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} $$

Application and Key Considerations

It is applicable to both convex and concave lenses. We need to give an at most care while solving numerical problems related to lenses in taking proper signs of different quantities.

Understanding Refraction Through a Concave Lens: Image Formation Explained

Refraction Through a Concave Lens

REFRACTION THROUGH A CONCAVE LENS

Let us discuss the formation of images by a concave lens when the object is placed at two possible positions.

Object at Infinity

When an object is placed at infinity, a virtual image is formed at the focus. The size of the image is much smaller than that of the object (Figure 2.12).

Figure 2.12 Concave lens-Object at infinity

Object anywhere on the principal axis at a finite distance

When an object is placed at a finite distance from the lens, a virtual image is formed between optical center and focus of the concave lens. The size of the image is smaller than that of the object (Figure 2.13).

Figure 2.13 Concave lens-Object at a finite distance

But, as the distance between the object and the lens is decreased, the distance between the image and the lens also keeps decreasing. Further, the size of the image formed increases as the distance between the object and the lens is decreased. This is shown in (figure 2.14).

Figure 2.14 Concave lens- Variation in position and size of image with object distance

Image Formation by Convex and Concave Lenses: Rules of Refraction

Images Formed Due to Refraction Through a Convex and Concave Lens

When an object is placed in front of a lens, the light rays from the object fall on the lens. The position, size and nature of the image formed can be understood only if we know certain basic rules.

Rule 1:

When a ray of light strikes the convex or concave lens obliquely at its optical centre, it continues to follow its path without any deviation (Figure 2.3).

Rule 2:

When rays parallel to the principal axis strikes a convex or concave lens, the refracted rays are converged to (convex lens) or appear to diverge from (concave lens) the principal focus (Figure 2.4).

Rule 3:

When a ray passing through (convex lens) or directed towards (concave lens) the principal focus strikes a convex or concave lens, the refracted ray will be parallel to the principal axis (Figure 2.5).

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10th Science : Chapter 2 : Optics : Images Formed Due to Refraction Through a Convex and Concave Lens

Understanding Lenses: Types, Functions, and Diagrams | Optics for Class 10

Lenses

What is a Lens?

LENSES

A lens is an optically transparent medium bounded by two spherical refracting surfaces or one plane and one spherical surface.

Lens is basically classified into two types.

Primary Types of Lenses

They are:

• (i) Convex Lens
• (ii) Concave Lens

(i) Convex or bi-convex lens:

It is a lens bounded by two spherical surfaces such that it is thicker at the centre than at the edges. A beam of light passing through it, is converged to a point. So, a convex lens is also called as converging lens.

(ii) Concave or bi-concave Lens:

It is a lens bounded by two spherical surfaces such that it is thinner at the centre than at the edges. A parallel beam of light passing through it, is diverged or spread out. So, a concave lens is also called as diverging lens.

Other types of Lenses

Plano-convex lens:

If one of the faces of a bi-convex lens is plane, it is known as a plano-convex lens.

Plano-concave lens:

If one of the faces of a bi-concave lens is plane, it is known as a plano-concave lens.

All these lenses are shown in Figure 2.2 given below:

Figure 2.2: Types of lenses

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10th Science : Chapter 2 : Optics : Lenses