Determining the Focal Length of a Convex Lens: A Physics Practical

Determination of focal length of a convex lens

10th Science : Physics Practicals

To determine the focal length of a convex lens by using 1. Distant object method 2. uv method

Aim:

To determine the focal length of a convex lens by using

1. Distant object method

2. uv method

Apparatus required:

A convex lens, stand, wire gauze object, screen and measuring scale.

Formula:

$$f = \frac{uv}{u+v}$$

Here,

u is the distance between the object (light source) and the convex lens

v is the distance of the image (screen) from the convex lens

f is the focal length of the convex lens

1. Distant Object Method:

Fix the given convex lens vertically on the stand and place it on the table near an open window of the laboratory. Locate a distant object (tree or building) through the open window. Place the screen behind the convex lens. Adjust the position of the convex lens and the screen so as to get a sharp, inverted and diminished image. Measure the distance between the screen and the convex lens with the help of the measuring scale. This distance is equal to the approximate focal length of the convex lens (f)

2. uv - Method:

Fix the given convex lens vertically on the stand and place it on the table. Place the wire gauze object on the left side of the convex lens (say at a distance greater than 2f). Measure the distance between the object and the lens (u). Place the screen on the right side of the convex lens and adjust its position to get a sharp, inverted and diminished image. Measure the distance between the screen and the lens (v). Repeat the same procedure, by changing the distance of the object (u) and tabulate your observations.

Observation:

Focal length of the convex lens (By distance object method) is (f) = …………cm

2f = …….cm

Result:

The focal length of the given convex lens

1. By distance object method f = ………cm

2. By ‘uv’ method f = … .…..cm

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.

Applications of Convex Lenses: Uses in Cameras, Microscopes, and Vision Correction

Applications of Convex Lenses

10th Science | Chapter 2: Optics

Key Applications of Convex Lenses

1. Convex lenses are used as camera lenses
2. They are used as magnifying lenses
3. They are used in making microscope, telescope and slide projectors
4. They are used to correct the defect of vision called hypermetropia

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Understanding Image Formation by a Convex Lens: A Detailed Guide

Refraction Through a Convex Lens

10th Science : Chapter 2 : Optics : Refraction Through a Convex Lens

REFRACTION THROUGH A CONVEX LENS

Let us discuss the formation of images by a convex lens when the object is placed at various positions.

Object at infinity

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

Figure 2.6: Object at infinity

Object placed beyond C (>2F)

When an object is placed behind the center of curvature(beyond C), a real and inverted image is formed between the center of curvature and the principal focus. The size of the image is the same as that of the object.

Figure 2.7: Object placed beyond C (>2F)

Object placed at C

When an object is placed at the center of curvature, a real and inverted image is formed at the other center of curvature. The size of the image is the same as that of the object.

Figure 2.8: Object placed at C

Object placed between F and C

When an object is placed in between the center of curvature and principal focus, a real and inverted image is formed behind the center of curvature. The size of the image is bigger than that of the object.

Figure 2.9: Object placed between F and C

Object placed at the principal focus F

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.

Figure 2.10: Object placed at the principal focus F

Object placed between the principal focus F and optical centre O

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

Figure 2.11: Object placed between the principal focus F and optical centre O

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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