Before getting started with the experiment, it is important to understand a few terms that are used in a concave mirror and convex lens:
- Centre of curvature: For a curved mirror, the centre of curvature, C can be defined as the centre of a hollow glass sphere of which a curved mirror is a part.
- Radius of curvature: For a curved mirror, the radius of curvature, R can be defined as the radius of the hollow glass sphere of a spherical mirror is part.
- Principal axis: Principal axis is defined as the imaginary line passing through the pole and centre of curvature of the spherical mirror.
- Principal focus: Principal focus is defined as a point at which the reflected rays meet or appear to meet for the spherical mirror. For a concave mirror, the principal focus is in the front and for a convex mirror, the principal focus is at the behind.
Below is an experiment to determine the focal length of a concave mirror and convex lens.
Aim
To determine the focal length of:
- Concave mirror
- Convex lens by obtaining the image of a distant object.
Theory
How to find focal length of concave mirror?
Following are the ways to obtain the focal length of the concave mirror:
- A concave mirror is defined as the spherical mirror whose reflecting surface is curved inwards and follows laws of reflection of light.
- The light rays coming from a distant object can be considered to be parallel to each other.
- If the image formed is real, inverted and very small in size, then the parallel rays of light meet the point in the front of the mirror.
- The image formed by the convex lens is real and can be obtained on the screen.
- f is used to denote the difference between the principal axis P and the focus F of the concave mirror.
How to find the focal length of convex lens?
Following are the ways to obtain the focal length of convex lens:
- The convex lens is thicker in the middle and thinner at the edges and is also known as the converging lens.
- The refracted rays from the parallel beam of light converge on the other side of the convex lens.
- If the image is obtained at the focus of the lens, the image would be real, inverted and very small.
- f is the focal length which is the difference between the optical centre of the lens and the principal focus.
- The image can be obtained on the screen as the image formed by the lens is real.
To determine focal length of a given concave mirror
Materials Required
- A concave mirror
- A measuring scale
- A screen holder
- A mirror holder
- A mirror stand
Procedure
- The distance between the selected distinct object should be more than 50 ft.
- The concave mirror placed on the mirror stand and the distant object should be facing each other.
- The screen should be placed in front of the reflecting surface of the mirror. To obtain a clear, sharp image the screen should be adjusted.
- Using a metre scale the distance between the concave mirror and screen can be determined. The distance is the same as the focal length of the given concave mirror.
- Repeat the above procedure thrice to calculate the average focal length.
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:
\(\frac{f_{1}+f_{2}+f_{3}}{3}\;cm=10\;cm\) |
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 | f_{1} = 10cm |
2 | 60 cm | 50 cm | f_{2} = 10cm |
3 | 60 cm | 50 cm | f_{3} = 10cm |
Calculation
Following is the mean value of the focal length of convex lens:
\(\frac{f_{1}+f_{2}+f_{3}}{3}\;cm=10\;cm\) |
Result
10 cm is the focal length of the concave mirror.
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 concane 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:
\(f=\frac{R}{2}\) |
Q4. What is the mirror formula?
Ans: Following is the mirror formula:
\(\frac{1}{f}=\frac{1}{v}+\frac{1}{u}\) |
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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