The lens is a transparent material which is bound by two surfaces. It has a principal axis, principal focus, centre of curvature of lens, aperture and optical centre. There are two types of lenses and they are a convex lens and concave lens. The images obtained from these lenses can be either a real image or a virtual image. Below is an experiment to find the image distance for varying object distances of a convex lens with ray diagrams.
Aim
To find the image distance for varying object distances in case of a convex lens and drawing corresponding ray diagrams to show the nature of image formed.
Theory
What is convex lens?
Convex lens is defined as a lens which is thick at the centre and thin at the edges. A convex lens is also known as a converging lens as it converges the light beam incident on it. There are three types of convex lens:
 Double convex lens
 Planoconvex lens
 Concavoconvex lens
What is lens formula?
Lens formula is defined as the relationship between object distance (u), imagedistance (v) and the focal length (f). Following is the mathematical representation of lens formula:
\(\frac{1}{f}=\frac{1}{v}\frac{1}{u}\) 
Where,
 f is the focal length of the lens
 v is the distance of the image from the optical centre of the lens
 u is the distance of an object from the optical centre of the lens
Materials Required
Following are the materials required for the experiment:
 A convex lens with a focal length of 1220 cm.
 Measuring scale
 Optical bench
 A needle
 A candle
Experimental setup
Procedure
 Place the convex lens on a lens holder, placing the screen on the other side of the lens.
 Try focusing the image to get a sharp, clear and inverted image on the screen. The focal length that is obtained is a rough focal length which can be measured using a meter scale.
 The position at which the lens is fixed, mark it as O.
 Once the focal length is calculated, mark point F on both the sides of the lens.
 Mark a point 2F, as the distance between the lens is double the focal length of the lens.
 Place the candle at a distance beyond 2F and adjust the height of the centre of the lens with the height of the flame.
 To obtain a sharp image of the candle flame in the convex lens, adjust the position of the screen and record the observations.
 Record the observations by placing the needle or lighted candle at 2F.
 Record the observations by shifting the object between F and 2F.
 Record the observations by placing the object at F.
 Record the observations by placing the object between O and F.
 Draw the ray diagrams for all the positions of the object.
Ray Diagrams
Observation Table
Sl. no 
Postion of the optical centre O of the lens l (cm) 
Position of candle a (cm) 
Position of screen s (cm) 
Distance between lens and candle (object distance) u=al (cm) 
Distance between lens and screen (image distance) v=sl (cm) 
Focal length (l) 
1 
50 
30 
70 
20 
20 
10 cm 
2 
50 
35 
80 
15 
30 
10 cm 
3 
50 
20 
65 
30 
15 
10 cm 
4 
50 
40 
No image obtained on screen 
10 
Infinity 

5 
50 
45 
No image obtained on screen 
5 
Virtual image obtained and cannot be taken on screen 
Calculations
Following is the calculations of f for different cases explaining the focal length of the given lens is 10cm:
Case 1: \(\frac{1}{f}=\frac{1}{v}\frac{1}{u}\Rightarrow \frac{1}{f}=\frac{1}{20}(\frac{1}{20})=\frac{2}{20}=\frac{1}{10}\Rightarrow f=10\;cm\) 
Case 2: \(\frac{1}{f}=\frac{1}{v}\frac{1}{u}\Rightarrow \frac{1}{f}=\frac{1}{30}(\frac{1}{15})=\frac{3}{30}=\frac{1}{10}\Rightarrow f=10\;cm\) 
Case 3: \(\frac{1}{f}=\frac{1}{v}\frac{1}{u}\Rightarrow \frac{1}{f}=\frac{1}{15}(\frac{1}{30})=\frac{3}{30}=\frac{1}{10}\Rightarrow f=10\;cm\) 
Result
S.no 
Position of the object 
Position of the image 
Relative size of the image 
Nature of the image 
1 
At 2F_{1} 
At 2F_{2} 
Same size 
Real and inverted 
2 
Between F_{1 }and 2F_{1} 
Beyond 2F_{2} 
Enlarged 
Real and inverted 
3 
Beyond 2F_{1} 
Between F_{2} and 2F_{2} 
Diminished 
Real and inverted 
4 
At focus F_{1} 
At infinity 
Infinitely large or highly enlarged 
Real and inverted 
5 
Between focus F_{1 }and optical center O 
On the same side of the lens as the object 
Enlarged 
Virtual and erect 
6 
At infinity 
At focus F_{2} 
Highly diminished, pointsized 
Real and inverted 
The focal length of the given lens is 10cm.
Precautions
 The convex lens must have the focal length between 15 to 20 cm.
 The convex lens must have a small aperture.
 To avoid the flickering of the candle flame, the experiment can be conducted in calm air.
 Perform the experiment in a dark room so as to obtain a distinct and sharp image of the candle flame.
 Image screen shouldnâ€™t be shaky.
Viva Questions
Q1. What is a lens?
Ans: Lens is defined as a transparent material with curved sides on both the sides or one curved and one plane surface.
Q2. Define power of a lens.
Ans: Power of a lens is defined as the reciprocal of the focal length. The mathematical formula is given as:
\(P=\frac{1}{f}\) 
Where,
 P is the power of a lens
 f is the focal length
Q3. What is one dioptre?
Ans: One dioptre is defined as a unit of measurement of the optical power of a curved mirror or a lens. It is given as follows:
1 dioptre = 1m^{1} 
Q4. What is meant by principal of axis?
Ans: Principle of axis is defined as the line passing through the centre of the lens or a spherical mirror.
Q5. What is the focal length of a lens?
Ans: Focal length of a lens is defined as the distance between the principal focus and the optical centre.
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