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.
Simulator Procedure (as performed through the Online Labs)
- Select the focal length using the slider.
- Click on the ‘Light On/Off’ button to turn On/Off the wire gauze.
- Select the distance between the mirror and the object (u) using the slider.
- Change the distance between the mirror and the screen (v) suing the slider to get a clear image of the wire gauze on the screen.
- You can calculate the focal length of the mirror using the equation,
- You can verify your result by clicking on the ‘Show result’ button.
- To redo the experiment, click the ‘Reset’ button.
Observations
| No. |
Distance between the mirror and
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1/u
(cm-1)
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1/v
(cm-1)
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(cm)
|
Object, u
(cm)
|
Image, v
(cm)
|
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| 3 |
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| 5 |
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Mean, f = ………. cm
= ………. ×10-2 m
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.
u -v Graph 1/u - 1/v Graph
From u-v graph,
OB =….....….cm
OC = ...……..cm
= ........... cm
From 1/u – 1/v graph,
OA =…………cm
OB =…...…... cm
= ............ 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
