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To find focal length of a Concave Mirror
To determine the focal length of a concave mirror, by obtaining image of a distant object.
Theory
- A concave mirror, like a plane mirror, obeys the laws of reflection of light.
- Rays of light from object - The rays of light coming from a distant object e.g. sun or a distant building can be considered to be parallel to each other. When these parallel rays of light fall on a concave mirror along its axis, reflect and meet at a point in front of the mirror, which is called as Principal focus of the mirror.
- A real, inverted and very small image size is formed at the focus of the mirror.
- Focal Length - The distance between the pole P of the concave mirror and the focus F is the focal length of the concave mirror. Thus, the focal length of a concave mirror can be estimated by obtaining a 'Real image' of a distant object at its focus, as shown in the figure.
Before getting started with the experiment, it is important to understand a few terms that are used in a concave mirror and convex lens: Below is an experiment to determine the focal length of a concave mirror and convex lens. To determine the focal length of: Following are the ways to obtain the focal length of the concave mirror: Following are the ways to obtain the focal length of convex lens:
Aim
Theory
How to find focal length of concave mirror?
How to find the focal length of convex lens?
To determine focal length of a given concave mirror
Materials Required
Procedure
Experimental Setup
Observation Table
Sl.no | Position of concave mirror (M) | Position of screen (S) | Focal length = (M-S)
cm |
1 | 60 cm | 50 cm | 10 cm |
2 | 60 cm | 50 cm | 10 cm |
3 | 60 cm | 50 cm | 10 cm |
Calculation
Following is the mean value of the focal length of concave mirror:
\(\begin{array}{l}\frac{f_{1}+f_{2}+f_{3}}{3}\;cm=10\;cm\end{array} \) |
Result
10 cm is the focal length of the concave mirror.
Precautions
- To get a well illuminated and distinct image of the distinct object, the distant object should be well illuminated.
- A concave mirror should be always placed near an open window.
- The polished surface of the concave mirror and the distinct object should be facing each other.
- There should not be any hurdle between the rays of light from the object and the concave mirror.
- The screen and the concave mirror stand should be parallel to the measuring scale.
- The mirror holder along with the mirror should be perpendicular to the measuring scale.
To determine focal length of a convex lens
Materials Required
- A wooden bench
- A convex lens
- A lens holder
- A screen fixed to a stand
- A measuring scale
Experimental Setup
Procedure
- Without disturbing the lens and screen, arrange both of them on the wooden bench.
- Place the lens on the holder facing a distant object.
- Place the holder with the screen on the bench.
- The position of the screen should be such that the sharp image of the distant object is obtained on it.
- The difference between the position of the lens and the screen is equal to the focal length of the given convex lens.
- Now shift the focus to towards various other distant object and calculate the focal length of the convex lens.
Observation Table
Sl.no | Position of convex lens (L) | Position of screen (S) | Focal length = (L-S) cm |
1 | 60 cm | 50 cm | f1 = 10cm |
2 | 60 cm | 50 cm | f2 = 10cm |
3 | 60 cm | 50 cm | f3 = 10cm |
Calculation
Following is the mean value of the focal length of convex lens:
\(\begin{array}{l}\frac{f_{1}+f_{2}+f_{3}}{3}\;cm=10\;cm\end{array} \) |
Result
10 cm is the focal length of the convex lens.
Precautions
- The placing of the convex lens should be vertical.
- There should not be any hurdle between the rays of light from the object and the convex lens.
- To get a well illuminated and distinct image of the distinct object, the distant object should be well illuminated.
- The convex lens stand and the screen should be parallel to the measuring scale.
Viva Questions
Q1. Differentiate between convex lens and concave lens.
Ans: Following is the table explaining the difference between the convex lens and the concave lens:
Convex lens | Concave lens |
The focal length is positive | The focal length is negative |
Image obtained can be either real or virtual | Image obtained is always virtual |
The light rays are converged towards the principal axis | The light rays diverge away from the principal axis |
Q2. Name two types of spherical mirrors
Ans: Following are the two types of spherical mirror:
- Concave mirror
- Convex mirror
Q3. What is the relationship between focal length T and the radius of curvature R of a concave mirror?
Ans: Following is the relationship between focal length and the radius of curvature:
\(\begin{array}{l}f=\frac{R}{2}\end{array} \) |
Q4. What is the mirror formula?
Ans: Following is the mirror formula:
\(\begin{array}{l}\frac{1}{f}=\frac{1}{v}+\frac{1}{u}\end{array} \) |
Q5. What is law of reflection for a mirror?
Ans: Following is the law of reflection for a mirror:
- The incident ray, the reflected ray and the normal all lie in the same plane.
- The angle of incidence is equal to the angle of reflection.
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