How can you find focal length of a converging lens?

Answer 1

To find the approximate value of the focal length of a convex lens, let us do an experiment.

Place a convex some distance away from a light source. At the opposite side of the lens, place a white paper.

Now adjust the position of the lens till a sharp point of light on the white paper is obtained.

Measure the distance between the lens and the white paper. This gives the value of the focal leng

Answer 2

he focal length of convex lens (Graphical method)

PURPOSE: To determine the focal length of converging lens by using a graphical method .

APPARATUS:

1.converging lens .

2.Holder.

3.A plan mirror.

4.Meter scale.

5.Object.

6.Screen.

METHOD;

1. Place an object pin at distance from the lens equal to 2F.

2. Measure the distance between the object and lens which is called "u".

3. Locate the position of its real image on the other side of the lens by using screen ,measure the distance between the image and lens which is called "V".

4. Move the object to other position both nearer to and farther way from the lens, location the new position of the image each time.

READING;

1/V (1/cm)

1/U(1/cm)

Distance of image from lens "v" cm

Distance of object from lens "u" cm

1. Plot a graph of (1/U) against (1/V).

2. Draw the straight line through the pointed and produce it to intersect both axes.

THEORY AND CALCULATION;

1/F=1/U+1/V

1. Straight line included each axis is obtained.

2.The intercept on the (1/v)axis is the numerical value for which (1/u)=0

1/F1=1/V

F1=V

3. Similarly for the intercept on the (1/U) axis

Then F2=U

4. Take the mean value of the two intercept.

F=F1+F2/2

MEDICAL APPLICATION:

Glasses (corrective lenses)help defective vision were among the first prosthetic devices invented in order to discuss the strength of the corrective lens for defective eye we need to review the basic negation of simple lens .

1/F=1/U+1/V

F is measured in meters ,1/F is the strength of the lens in diopter (D)

Now let us discuss defective eyesight due to refractive problems -ametropia.

A metropia affects over half of population of united states.it is often possible to correct it completely with glasses .

There are four general type of ametropia;

1. Myopia (near-sightedness).

2. Hyperopia or Hypermetropia (far-sightedness).

3. Astigmatism (asymmetricalfocusing).

4. Presbyopia (old sight).

The myopia individual has too long an eye ball or too much curvature of the cornea ,distant objects come to a focus in front of the retina ,and the rays diverge to cause a blurred image at the retina .This condition is easily corrected with negative lens.

Answer 3

Focal lengh by displacement method.

[sorry , i can't upload the diagram here as i can't write or upload the image. i hope you can undertstand by explanation.]

First , take the lens and object (like a hole or something, clearly visible). And move the

lens from the object. and a certain length , the image of the object on the screen will be

clearer and equal in the size of the object. This is the nearer focal length. and move the

screen to such a distance such that D=4f. And then do as the same. but at a certain length,

you will see a clear big image and note that length, And an small clear image.take the

Differece of them. that will be S.

Do the same procedure for different values of D (increase D by 10-20cm). and note down the

readings.

Use the formula to calculate the focal length. f=(D2- s2)/4D

and take the mean. That will be your lens focal length.

Answer 4

First and foremost by distance object method we can get approximate focal length

Then base on that value we can assign u values and get required v values and find focal length by u-v method.

Plane mirror method is also there

Two position method would help to find the same

Answer 5

You can place the length in the sunlight, see where the light converges, and measure the distance.