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Materials Required
  • Concave mirror
  • Stand
  • Screen
  • Illuminated 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.

Observations

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Calculations:

Calculate the value of focal length (f) each time and find its mean.

Plot a graph with u along X axis and v along Y axis by taking same scale for drawing the X and Y axes. Draw the bisector OA and join OC and OB. Thus, OC=OB= 2f. Calculate the focal length from this.

Plot a graph with 1/u along X axis and 1/v along Y axis by taking same scale for drawing the X and Y axes. The graph is a straight line intercepting the axes at A and B. Then OA=OB= 1/f. Calculate the focal length from this.

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From u-v graph,

OB =……..….cm

OC = ………..cm

1212

= ……….. cm

From 1/u – 1/v graph,

OA =…………cm

OB =………… cm

1231

= ………… cm

Results

The focal length of the given concave mirror, f

By u-v method = ……..×10-2 m

From u-v graph = .……..×10-2 m

From 1/u- 1/v graph = ………×10-2 m

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