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Concave 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:

                                                                                  

  • 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

                 
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,  

                                                                  

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: