Lens in Contact Method

Materials required:

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:

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

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

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

Focal length of the given concave lens by lens in contact method = …………cm

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.

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

Focal length of concave lens, Mean f = …………….cm

The focal length of the concave lens by combination method = …………..cm

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: