Conave Lens - Focal Length

# 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:

$F=\frac{uv}{u+v}\;cm$

• 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:

$f_{2}=\frac{F f_{1}}{f_{1}-F} \;cm$

## Observations:

 SI No. Distance between the combination of lenses and Focal Length                  $F=\frac{uv}{u+v}\;cm$ Object u (cm) Image v (cm) 1 2 3 4 5

## 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,

$f_{2}=\frac{F f_{1}}{f_{1}-F} \;cm$

## 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 First image(u cm) Second Image (v cm) 1 2 3 4 5

## 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.

Cite this Simulator: