CBSE Class 10 Practicals Science Chapter 10 Refraction of light for Topic To Find the Focal Length of Convex Lens. Students get here clear concept at this page.
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To Find the Focal Length of Convex Lens
Hello students we are familiar with the concepts of light such as reflection, refraction etc. To study refraction of light we use Lenses, which are divided into two type i.e. convex lens and concave lens.
Convex lens has ability to produce convergence of refracted rays. Due to this it is used at higher extend in day to day life applications. In this experiment we are going to find the focal length of convex lens.
Objective of the experiment….
To find the focal length of convex lens.
Instruments required for the experiment…..
Convex lens, screen, lens stand, meter scale, object (paste white paper on hard bound paper and draw arrow on it), pencil etc.
Assumptions used for the experiment…..
When ray of light is made incident on the convex lens it follows specific paths,
- When ray of light is parallel to principal axis, it gets refracted through principal focus.
- When a ray of light passes through the focus, it gets refracted and parallel to principal axis.
- When ray of light passes through optical centre of lens, it traces the same path after refraction.
Formula used…..
If u = object distance, v = image distance and
f = focal length of lens, then the lens formula is given by
1/f = 1/v – 1/u
According to sign conventions we need to assume the distance as follows,
- Object distance, u= -ve,
- Image distance, v = -ve/+ve (depends upon the position of image) and
- Focal length, f= +ve for convex lens
Diagram showing the experimental condition….
Ray diagram showing the experimental conditions are as follows,
Procedure to find the focal length….
- Place object and screen on table such that their centres should be collinear as shown in fig.
- Now place lens holder with lens between object and screen. Now move the screen to and fro slowly such that the sharp image of arrow drawn on hard bound paper appears on the screen.
- When you find the sharp image of object don’t move the position of any one among three. Mark the positions of all on table with pencil.
- Measure the distance of object from the lens and record as ‘u’.
- Now Measure the distance of screen from the lens and record as ‘v’.
- Record the nature, size of image obtain on the screen.
- Repeat the same procedure for taking minimum three readings for different distances of object from lens with different screen distance.
- You can use the same procedure to find the focal length of concave lens also.
Observation table…
Sr No. | Distance of object from the lens (u) cm | Distance of image from the lens (v) cm | Focal length (f) cm | Mean focal length (F) cm |
1 | ||||
2 | ||||
3 |
Calculations……
Using mirror formula,
Results from the experiment…
- Focal length of convex mirror =…………..cm
Precautions that should be followed while performing experiment….
- Position object, lens and screen should be such that the centre of all should lie on same line.
- Mark the positions accurately to measure the distances, and measure all distances carefully.
- Repetition of same positions of object should be avoided.
- Use sign conventions proper accuracy in calculation
Some important questions that needs to revised for the experiment…
Q.1) Define lens. State its types
Answer: Transparent object bounded by two surfaces out of which at least one should be spherical is known as lens. There are two types of lens i.e. convex lens and concave lens.
Q.2) Define: 1) principal focus 2) Optical centre.
Answer:
Principal focus: When beam of incident ray is parallel to principal axis, after refraction all the refracted rays’ gets passes through same point which lies on principal axis called as Principal focus.
Optical centre: The central point lies on the lens surface is known as pole of mirror.
Q.3) What do you mean by convergence of refracted rays?
Answer:
When a parallel beam of light falls on convex lens, after refraction all the rays meet a same point which lies on the principal axis. This point of intersection of all the refracted rays is called as principal focus of lens and the phenomenon is known as convergence of the ray. See the figure.