Lens in Contact Method
Materials required:
- Illuminated wire gauze
- Lens stand
- Screen
- Metre scale
- A convex lens of short focal length
- Concave lens
Real Lab Procedure:
- Keep the given convex lens of known focal length, f1, in contact with the concave lens whose focal length to be determined. This forms the combination of lenses.
- It is placed between the illuminated wire gauze and the screen at a fixed distance away from the wire gauze. It is taken as u cm.
- The position of the screen is adjusted to get a clear image of the wire gauze on the screen.
- Measure the distance between the combination of lenses and the screen. It is taken as v cm.
- Calculate the focal length of the combination using the formula:

- Repeat the experiment by placing the combination of lenses at different distances away from the wire gauze.
- Then calculate the mean value of F.
- Using the values of f1 and F, the focal length of the given concave lens can be calculated using the formula:

Observations:
SI No. |
Distance between the combination of lenses and |
Focal Length

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Object u (cm) |
Image v (cm) |
1 |
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2 |
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3 |
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4 |
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5 |
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Calculations:
- Focal length of the combination lens, Mean F = …………cm
- Focal length of convex lens, f1 = ……………cm
- Therefore, the focal length of the given concave lens,

Results:
Focal length of the given concave lens by lens in contact method = …………cm
Lens Out of Contact Method
Materials required:
- Illuminated wire gauze
- Two stands
- Screen
- Metre scale
- A convex lens of short focal length
- Concave lens
Real Lab Procedure:
- The given convex lens is mounted on a stand.
- It is placed between the illuminated wire gauze and the screen at a fixed distance away from the wire gauze.
- The position of the screen is adjusted to get a clear image of the wire gauze on the screen at I1.

- Now, the given concave lens (L) is mounted between the screen and the convex lens without any rearrangement.
- The distance between the screen and the concave lens LI1 is measured as u cm.
- Now, the screen alone is moved back to obtain a clear image I2 on the screen.
- The distance between the concave lens and the screen, LI2 is measured as v cm.

- Using the values of u and v, the focal length of the concave lens is calculated using the formula, f= uv/u-v.
- To repeat the experiment, remove the concave lens and bring the screen to the initial position. Then place the concave lens at a different position in between the convex lens and screen and record the values in the tabular column.
Observations:
SI No. |
Distance between concave lens and |
Focal length of concave lens
f = uv/u-v cm
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First image(u cm) |
Second Image (v cm) |
1 |
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2 |
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3 |
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4 |
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5 |
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Calculation:
Focal length of concave lens, Mean f = …………….cm
Result:
The focal length of the concave lens by combination method = …………..cm
Simulator Procedure (as performed through the Online Labs)
Select the method from the drop down list.
Select the concave lens from the drop down list.
Lens in Contact
- Select the distance between the lens (combination of convex lens and concave lens) and the object (wire gauze) using the slider.
- To turn On the wire gauze, click on the ‘Light On’ button.
- Adjust the distance between the lens and the screen using the slider to get a clear image of the wire gauze on the screen.
- Note the value of u and v.
- You can calculate the focal length (F) of combination of lenses using the formula F = uv/(u+v).
- The focal length of convex lens (f1) is shown inside the simulator window.
- You can calculate the focal length of the concave lens using the formula f2 = (f1F/(f1-F).
- You can verify your result by clicking on the ‘Show result’ button.
Lens Out of Contact
- To turn On the wire gauze, click on the ‘Light On’ button.
- Adjust the distance between the convex lens and the screen using the slider to get a clear image of the wire gauze on the screen.
- When a clear image is formed, click on the ‘Insert concave lens’ button to insert the concave lens in between the screen and the convex lens.
- Set the position of the concave lens from the screen using the slider. It is taken as u.
- Adjust the distance between the concave lens and the screen using the slider to get a clear image of the wire gauze on the screen. It is taken as v.
- You can calculate the focal length of the concave lens using the formula f = uv/(u-v).
- You can verify your result by clicking on the ‘Show result’ button.
To redo the experiment, click on the ‘Reset’ button.