To find the focal length and radius of curvature of concave/convex mirror
Hello students we are familiar with the concepts of light such as reflection, refraction etc. To study reflection from different objects we use mirrors i.e. plane mirror and spherical mirrors. Spherical mirrors are of two types, concave mirror and convex mirrors.
We have learnt in theory about the convergence and divergence of reflected ray from the mirrors which gives rise terms like focus and focal length. In this experiment we are going to discuss the activity/experiment to find the focal length of given mirror.
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Objective of the experiment….
To find the focal length of the concave/concave mirror using object and image distances.
Instruments required for the experiment…..
Concave mirror, convex mirror, screen, lens stand, meter scale, object (bulb or candle etc.), and plane table.
Assumptions used for the experiment…..
When ray of light is made incident on the concave/convex mirror it follows specific paths,
- When ray of light is parallel to principal axis, it gets reflected back through focus.
- When a ray of light passes through the focus, it gets reflected parallel to principal axis.
Formula used…..
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 concave mirror/+ve for convex mirror.
Diagram showing the experimental condition….
Ray diagram showing the experimental conditions are as follows,
Procedure to find the focal length….
- Place stands on the table such that it appears collinear.
- Place pin/pencil on 1st stand, mirror on the 2nd stand at approximate same height.
- Hold the stand of pin close to mirror and move it slowly away from the mirror and observe the image formed in mirror.
- Keep your chin on the point marked on table.
- Now try to observe the sharp image of tip of pin/pencil in mirror by looking in to mirror behind object. (Here we are going to observe the image by looking in to mirror such that object (pin) is in middle of mirror and our eyes.)
- Mark the position of mirror, pin and our eyes on the table.
- Measure the distances of image from the mirror (v) and object/pin (u) from the mirror.
- Repeat the same procedure for minimum three different positions for object and image and complete the given table.
- Using same procedure we can find the focal length of concave mirror.
Observation table…
Sr No. | Distance of object from the mirror (u) cm | Distance of image from the mirror (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 concave mirror =…………..cm
- Radius of curvature of concave mirror =……………cm
Precautions that should be followed while performing experiment….
- Position of our neck/eyes, tip of pin and mirror should be collinear.
- Mark the positions accurately to measure the distances.
- Repetition of same positions of object should be avoided.
Some important questions that needs to revised for the experiment…
Q.1) What are the rules of reflection for the concave mirror
Answer: The rules of reflection are
- If incident ray is parallel to principal axis, reflected ray passes through focus
- If incident ray passes through the focus, reflected ray becomes parallel to principal axis.
- If the incident ray passes through centre of curvature, traces same path after reflection.
Q.2) Define: 1) principal focus 2)radius of curvature 3)pole of spherical mirror.
Answer:
Principal axis: When beam of incident ray is parallel to principal axis, after reflection all the reflected rays’ gets passes through same point which lies on principal axis called as Principal focus.
Radius of curvature: The radius of the imaginary sphere of which the mirror is part is called as radius of curvature.
Pole: The central point lies on the mirror surface is known as pole of mirror.