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

Understanding the Power of a Lens: Definition, Formula, and Units (Dioptre)

Power of a Lens

Definition of Power of a Lens

When a ray of light falls on a lens, the ability to converge or diverge these light rays depends on the focal length of the lens. This ability of a lens to converge (convex lens) or diverge (concave lens) is called as its power. Hence, the power of a lens can be defined as the degree of convergence or divergence of light rays. Power of a lens is numerically defined as the reciprocal of its focal length.

The formula for the power of a lens is:

\( P = \frac{1}{f} \)       ……. (2.6)

Unit and Convention

The SI unit of power of a lens is dioptre. It is represented by the symbol D. If focal length is expressed in ‘m’, then the power of lens is expressed in ‘D’. Thus 1D is the power of a lens, whose focal length is 1 metre.
\( 1D = 1m^{-1} \)

Sign Convention

• By convention, the power of a convex lens is taken as positive.
• By convention, the power of a concave lens is taken, as negative.

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Lens Maker’s Formula Explained | 10th Science Optics Study Material

Study Material | 10th Science | Chapter 2: Optics

Lens Maker’s Formula

LENS MAKER’S FORMULA

All lenses are made up of transparent materials. Any optically transparent material will have a refractive index. The lens formula relates the focal length of a lens with the distance of object and image. For a maker of any lens, knowledge of radii of curvature of the lens is required. This clearly indicates the need for an equation relating the radii of curvature of the lens, the refractive index of the given material of the lens and the required focal length of the lens. The lens maker’s formula is one such equation. It is given as

where µ is the refractive index of the material of the lens; R₁ and R ₂ are the radii of curvature of the two faces of the lens; f is the focal length of the lens.

Units in Science and Technology

Units

1.    Charge – The coulomb
2.    Current – The ampere
3.    Potential Difference  - The Volt
4.    Resistance -  The ohm
5.    Resistivity – The ohm metre
6.    Energy – The joule
7.    Power – The watt
8.    Focal length – The metre
9.    Power of a lens – The dioptre.