The document Theory & Procedure, Concave Lens (Focal Length) Class 12 Notes | EduRev is a part of the JEE Course Physics Class 12.

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**Our Objective:**

To find the focal length of a concave lens using a convex lens by:

- Lens in contact method
- Lens out of contact method

**The Theory:**

**Concave lens**

A concave lens is thinner at the center that at the edges. So the light beams passing through the lens are spread out or diverged. Therefore, the concave lens is called a diverging lens. The image formed by a concave lens is virtual and diminished.

Since a concave lens will not produce a real image, a convex lens is used to measure its focal length. There are two methods to find the focal length of a concave lens.

**a) Lens in contact method**

When a concave lens of focal length f_{2} is placed coaxially in contact with a convex lens of focal length f1 and if F is the focal length of the combination, then,

Therefore, the focal length of the concave lens,

**b) Lens out of contact method**

The real image (I_{1}) formed by the convex lens will act as the virtual object for the concave lens. When concave lens is interposed between the convex lens and the real image I_{1}, the new real image is formed at I_{2}. If u is the distance of the concave lens from the virtual object I_{1} and v is the distance of the concave lens from the real image I_{2}, then the focal length of the given concave lens is,

**Learning outcomes:**

- Students list the properties of concave lens.
- Students deduce and apply the lens formula.
- Students identify the two methods used to determine the focal length of concave lens.

**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, f
_{1}, 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 = â€¦â€¦â€¦â€¦cm
- Focal length of convex lens, f
_{1}= â€¦â€¦â€¦â€¦â€¦cm - Therefore, the focal length of the given concave lens,

**Results:**

Focal length of the given concave lens by lens in contact method = â€¦â€¦â€¦â€¦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 I
_{1}.

- 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 LI
_{1}is measured as u cm. - Now, the screen alone is moved back to obtain a clear image I
_{2 }on the screen. - The distance between the concave lens and the screen, LI
_{2}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 = â€¦â€¦â€¦â€¦â€¦.cm

**Result:**

The focal length of the concave lens by combination method = â€¦â€¦â€¦â€¦..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 â€˜Light Onâ€™ 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 (f
_{1}) is shown inside the simulator window. - You can calculate the focal length of the concave lens using the formula f
_{2}= (f_{1}F/(f_{1}-F). - You can verify your result by clicking on the â€˜Show resultâ€™ button.

**Lens Out of Contact**

- To turn On the wire gauze, click on the â€˜Light Onâ€™ 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 â€˜Insert concave lensâ€™ 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 â€˜Show resultâ€™ button.

To redo the experiment, click on the â€˜Resetâ€™ button.

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