Question

Determine the focal length of concave mirror by two pin - method

Answer 1

Materials Required

Concave mirrorStandScreenIlluminated wire gauze.Metre scale

Real lab Procedure

By distant object method :

Fix the given concave mirror on the stand and place it on a table, facing towards a distant object.Arrange the screen on the table so that the image of the distant object is obtained on it.Measure the distance between mirror and screen using a metre scale. It can be taken as the focal length (f) of the mirror.

By u-v method : 

Using the focal length obtained by distant object method set the values of u (distance between mirror and object) ranging from 1.5f to 2.5f. Divide the range into a number of equal steps.Place the mirror in front of an illuminated wire gauze. It acts as the object.Now, fix the mirror at the distance u (which is obtained as 1.5f) from the wire gauze.Place the screen on the table facing the mirror in such a way that the reflected image lies on the screen.We can adjust the position of the screen to get the clear image of the wire gauze.Keeping the distance between object and mirror fixed, adjust the position of screen in order to get the clear image of the object.Measure the distance between mirror and wire gauze, as well as mirror and screen. Take these values as u and v respectively.Record the values of u and v in a tabular column.Calculate the focal length of the given concave mirror by using the relation, f = uv/(u+v).Repeat the experiment for different values of u (up to 2.5f) and in each time, measure v and record it in the tabular column.Calculate the focal length (f) of the concave mirror each time.Calculate the mean of all focal lengths to get the correct focal length of the given concave mirror.The focal length of the mirror can also be measured graphically by plotting graphs between u & v, and 1/u & 1/v.We can repeat the experiment with concave mirrors of different focal lengths.

Answer 2

AIM: To find the focal length of concave mirror using two pin method.

APPARATUS: Concave mirror optical bench, object andimage pins, meter rule.

PROCEDURE:

1)  Approximate focal length of the given concave mirror is found by obtaining on the wall the image of a distant tree.

2) The concave mirror is then placed on a stand and one pinis fixed on the other stand, then placed on the optical bench.

3)  The object needle is kept in front of the mirror. A second needle is taken and placed in between the mirror and the object needle. The needle I is moved, until there is no parallax error between the image of O and I. Distance MO ( u ) and MI ( v ) are measured. This gives the observed object and image distance.  

4) Position of the object is changed bringing it closer to the mirror gradually taking care to see that a real image is obtained. This happens when object is at a distance greater than the focal length from the mirror.  

This is repeated to find the value of MO and MI in each case (six observations).

5) Graph v vs u is plotted. It will be a curve. A line OP is drawn making an angle of 45˚ . Both axis should meet the curve at point P.

This graph is used along with the formula for finding focal length to determine the result.