DEPARTMENT OF CIVIL ENGINEERING 
Program: B. Sc in Civil Engineering

Course Title:

Mechanics, Heat, Thermodynamics, Waves, Oscillations and Physical Optics- Lab

Course code:        PHY 103

Assignment

Experiment No-01                                   Experiment Done Date: 28.01.2015

Name of the Experiment:

To determine the focal length and hence the power of a convex lens by displacement method with the help of an optical bench.

Submitted by:

Name: Mohammed Abdur Razzak

Roll No:  3326,  Registration No: WUB/10/13/60/3326, 

Batch No: 60/C, Semester No: Fourth, Year: Second.

Submitted To:

Name:  Md. Shohel Rana

 Designation:  Lecturer in physics, Basic science Division (WUB)                 Date of Submission:

Experiment No.:04

Experiment Name: Determine the focal length and hence the power of a convex lens by displacement method with the help of an optical bench.

Theory: If the object and the image screen be so placed on an optical bench that the distance between them is greater than four times the focal length (f) of a given convex lens, then there will be two different positions of the lens which for which an equal sharp image will be obtained on the image screen.

                                                                                                                     

The fig represent respectively the positions of the object and the image screen and the two different positions of the lens for which an equally sharp image is obtained.

Let the distance Ol =D and L₁ – L₂ = x.

From the lens equation , we have

Or,   (since u + v =D)

Applying sign convention, u is negative.

Or,

Or, u² – ud =df = 0

Solving the above equation which is quartic, we have two values of u corresponding to the two positions of the lens. These are

u₁ =  -  position L₁ of the lens

And u₂ =  +  position L₂ of the lens

Then x = L₁  L₂ = u₁  u₂ =

Or   x² = D² – 4Df

Or   f =  ……………….. (1)

Where D is the distance between the object and the image and must be greater than 4f and x is the distance between two different positions of the lens.

The power P of the lens s as usual given by the relation,

P =  diopters

Results:

Table I

No. of obs.	Position of				Displacement of Lens X = L1 – L2 (cm)	Apparent distance between object and image D’ = O	Corrected distance between object and image D = D’ +
	Object (O)	Image (I)	Lens at
			L1	L2
1	0	45	16	28.2	12.20	45
2	0	50	14.4	34.4	20.00	50
3	0	55	14	40.30	26.30	55
4	0	60	12.4	46.00	33.60	60
5	0	65	12.1	51.00	38.90	65

Table II:

No. of obs.	Lens displacement (x) from Tab. II	Corrected Distance (d) from Tab. II	Focal length	Mean focal length (f) cm	Power P =
1	12.20	45	10.42	10.45	9.56
2	20.00	50	10.50
3	26.30	55	10.60
4	33.60	60	10.30
5	38.90	65	10.43