# Help With a Difficult Lenses Question: Find the Answer!

• Rheegeaux22
In summary, the conversation discusses a question about the focal length of a converging lens and the correct answer of 10m. The person asking for help initially assumed the focal length was 3m, but after further clarification, it was revealed to be 2m.
Rheegeaux22

## Homework Statement

The question is : https://postimg.org/image/lntyejfvp/

1/s +1s' =1/f

## The Attempt at a Solution

Hello, I have an upcoming test on Monday on lenses and while reviewing I've come across a question that confused me.

My understanding is that the image of the first lens is used as the object of the second lens. I keep coming up with the answer below.

My solution:
https://postimg.org/image/vqerb9suf/

Thank you for helping me. I really need to get a high score in this test to get exempted for the finals. I really don't want to take the finals :(

Last edited by a moderator:
Welcome to PF Rheegeaux!

What is the focal length of the converging lens?

AM

Rheegeaux22
Andrew Mason said:
Welcome to PF Rheegeaux!

What is the focal length of the converging lens?

AM

Yes, you need the focal length of the converging lens. From the wording of the problem I suspect you were told the focal length of the converging lens in the problem immediately preceeding this one.

In your solution you assumed the focal length of the converging lens is 3m, perhaps guessing they meant to say both lenses are 3. However when you look back st the previous problem I bet you will find the focal length is 2 which will fix the discrepancy.

Rheegeaux22
Cutter Ketch said:
Yes, you need the focal length of the converging lens. From the wording of the problem I suspect you were told the focal length of the converging lens in the problem immediately preceeding this one.

In your solution you assumed the focal length of the converging lens is 3m, perhaps guessing they meant to say both lenses are 3. However when you look back st the previous problem I bet you will find the focal length is 2 which will fix the discrepancy.
Thanks :), you're right the focal length is 2.

## 1. How do I approach a difficult lenses question?

When approaching a difficult lenses question, it is important to first understand the fundamentals of lenses and their properties. This includes knowing the difference between convex and concave lenses, understanding the concept of focal length, and being familiar with the lens formula. Additionally, it can be helpful to break the question down into smaller, more manageable steps, and to draw diagrams to visualize the problem.

## 2. What is the lens formula and how do I use it?

The lens formula is a mathematical equation used to calculate the position and size of an image formed by a lens. It is written as 1/f = 1/u + 1/v, where f is the focal length, u is the object distance, and v is the image distance. To use the lens formula, plug in the known values and solve for the unknown. It is important to use consistent units (e.g. meters or centimeters) for all values.

## 3. What are some common mistakes to avoid when solving a difficult lenses question?

One common mistake when solving a difficult lenses question is not taking into account the sign conventions. In the lens formula, object distance (u) and image distance (v) are considered positive when measured in the same direction as the incident light. However, when using the magnification formula, the image distance (v) is considered negative when the image is formed on the opposite side of the lens from the incident light. It is important to pay attention to these conventions to ensure accurate calculations.

## 4. How do I determine the type and size of the image formed by a lens?

The type and size of the image formed by a lens can be determined by using the magnification formula, M = -v/u. The magnification (M) is negative for images that are virtual (formed on the same side as the object) and positive for images that are real (formed on the opposite side as the object). The absolute value of the magnification also indicates the size of the image relative to the object - a magnification greater than 1 indicates an enlarged image, while a magnification less than 1 indicates a reduced image.

## 5. What are some real-world applications of lenses and their properties?

Lenses and their properties have many practical applications in various fields, including photography, ophthalmology, and microscopy. Lenses are used in cameras to focus light and create clear images, and in glasses and contact lenses to correct vision. In ophthalmology, lenses are used to diagnose and treat vision problems. In microscopy, lenses are used to magnify and visualize tiny objects, such as cells and bacteria.

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