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9.5.4 - Combination of thin lenses in contact

Interactive Audio Lesson

Listen to a student-teacher conversation explaining the topic in a relatable way.

Introduction to Lens Combinations

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Teacher
Teacher

Today we're going to learn about how two thin lenses can work together when placed in contact. Can anyone explain what happens when light passes through a single lens?

Student 1
Student 1

When light passes through a lens, it bends, and depending on the shape and material, it can focus the light to form an image.

Teacher
Teacher

Exactly! Now, when we combine two lenses, how might that change the behavior of light?

Student 2
Student 2

I think it could change where the image is formed, right?

Teacher
Teacher

Yes, very good! The first lens creates an image which then becomes the object for the second lens. Let's explore how these images are formed mathematically.

Image Formation with Two Lenses

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Teacher
Teacher

For our two lenses, let's define their focal lengths as \( f_1 \) and \( f_2 \). We can use the lens formulas to find the resultant image. Can anyone recite the lens formula?

Student 3
Student 3

Sure! The lens formula is \( \frac{1}{v} - \frac{1}{u} = \frac{1}{f} \).

Teacher
Teacher

Great! Now for the first lens, how do we define \( u_1 \) and \( v_1 \)?

Student 4
Student 4

If the object is on the left side, then \( u_1 \) is negative, and \( v_1 \) would be positive since the image formed will be on the right!

Teacher
Teacher

Correct! Now, once we find \( v_1 \), we use it as the object distance for the second lens as \( u_2 = -v_1 \) depending on its location. Let’s calculate an example.

Effective Focal Length Calculation

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Teacher
Teacher

Let’s go a bit deeper! When it comes to two lenses in contact, what would be our equation for the effective focal length?

Student 1
Student 1

It would be \( \frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2} \)!

Teacher
Teacher

Exactly! This means that if one lens has a focal length of 10 cm and the second has 15 cm, can someone find the effective focal length?

Student 2
Student 2

Using the formula, I get \( \frac{1}{f} = \frac{1}{10} + \frac{1}{15} = \frac{3 + 2}{30} = \frac{5}{30} \), so \( f = 6 \) cm!

Teacher
Teacher

Well done! Remember that combining lenses can allow us to design glasses, cameras, and more! Let's wrap up with the magnification.

Magnification in Lens Combinations

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Teacher
Teacher

Now let’s discuss magnification. If each lens has its own magnification given by \( m \), how to find the total magnification of our two lenses?

Student 3
Student 3

I think the total magnification would be \( m = m_1 \times m_2 \)!

Teacher
Teacher

Absolutely right! Can someone come up with a scenario where we would need to calculate total magnification?

Student 4
Student 4

Using a microscope? Each lens would help in magnifying the tiny details of a specimen!

Teacher
Teacher

Exactly! Let’s make an example. If the objective lens has a magnification of 10 and the eyepiece 5, what’s the total?

Student 1
Student 1

That would be 10 times 5, equal to 50!

Teacher
Teacher

Great job! Understanding these concepts is foundational for our studies in optics. Let's summarize.

Wrapping Up Lens Combinations

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Teacher
Teacher

To conclude, we discussed how two lenses can be combined to yield an effective focal length and magnification. What is the key equation for calculating the effective focal length?

Student 2
Student 2

It's \( \frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2} \)!

Teacher
Teacher

And for total magnification?

Student 3
Student 3

It's \( m = m_1 \times m_2 \)!

Teacher
Teacher

Excellent! Remember these relationships and they will serve you well in optics as we delve deeper into complex systems. Keep practicing!

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

This section discusses how multiple thin lenses placed in contact with each other interact and create an effective focal length and magnification for optical systems.

Standard

The combination of two thin lenses in contact allows the formation of a new effective focal length, where the image formed by one lens serves as the object for the next. This concept is crucial in the design and functioning of various optical instruments, providing enhanced control over image formation and magnification.

Detailed

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Audio Book

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Image Formation by the First Lens

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Consider two lenses A and B of focal length f₁ and f₂ placed in contact with each other. Let the object be placed at a point O beyond the focus of the first lens A (Fig. 9.19). The first lens produces an image at I₁.

Detailed Explanation

In this section, we're discussing how two lenses can work together when they are placed in contact. The first lens (A) takes light from an object positioned at point O. When light passes through this lens, it refracts the light rays and creates an image at point I₁. This image is referred to as a 'real image' because the light rays actually converge at this point, allowing a screen to capture the image.

Examples & Analogies

Think of the first lens like a projector lens showing a movie on a screen. Just as the projector lens forms a clear image on the screen by collecting and focusing light from the film reel, the first lens forms an image from the object placed before it.

Second Lens as a Virtual Object

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Since image I₁ is real, it serves as a virtual object for the second lens B, producing the final image at I₂.

Detailed Explanation

After the first lens forms the image at I₁, this image now acts as the object for the second lens (B). A 'virtual object' means that rather than light emanating directly from the physical object, we are now using the light coming from I₁ to create a new image. The second lens adjusts the direction of rays from this image to create a final image at point I₂.

Examples & Analogies

Consider it like using a magnifying glass after looking into a projector. The image you see on the screen (like I₁) can be viewed again using another lens (like lens B) that magnifies or alters that image further for a better view.

Lenses as Thin and Coincident Optical Centers

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Since the lenses are thin, we assume the optical centres of the lenses to be coincident. Let this central point be denoted by P.

Detailed Explanation

Thin lenses are treated as if they have negligible thickness, which simplifies calculations and assumptions in optical systems. By assuming that both lenses share the same central point—denoted as P—mathematical relationships regarding distances and image formation can be easily derived.

Examples & Analogies

Imagine stacking two pieces of thin glass together; if they fit snugly, you could see through both at once as if they were one single lens. This represents how the optical centers are treated in calculations—simplifying their interaction.

Deriving the Condition for Image Formation

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For the image formed by the first lens A, we get 1/f₁ = 1/v₁ + 1/u₁. For the image formed by the second lens B, we get 1/f₂ = 1/v₂ + 1/v₁.

Detailed Explanation

Each lens has its own focal length (f₁ for lens A and f₂ for lens B). The lens formulas indicate relationships between object distance (u), image distance (v) and focal length (f). Rearranging and adding these equations illustrates how the image formed by one lens serves as the object for the next lens, allowing us to understand the combined optics of multiple lenses.

Examples & Analogies

Think of two mirrors in a funhouse setup where reflections occur. The light reflecting off the first mirror becomes the object for the second mirror. Similarly, the images from both lenses interact in a structured way that can be calculated mathematically.

Effective Focal Length of Lens Combinations

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If the two lens-system is regarded as equivalent to a single lens of focal length f, we have 1/f = 1/f₁ + 1/f₂.

Detailed Explanation

This formula expresses the effective focal length of a system comprising multiple lenses. By taking the reciprocal of the individual focal lengths, we derive a single equivalent focal length for the combination, simplifying the analysis of the overall system. This means we can treat two lenses as if they were one effective lens when determining how light is focused and where images form.

Examples & Analogies

Imagine combining two magnifying glasses for reading small print. Instead of calculating how each glass affects the vision separately, we can compute how they work together as a singular tool—it’s more practical when making adjustments or understanding their combined effect.

Total Magnification of the Lens Combination

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The total magnification m of the combination is a product of magnification (m₁, m₂) of individual lenses.

Detailed Explanation

The magnification resulting from multiple lenses in contact is the result of multiplying the magnification produced by each lens. This is important in applications such as microscopes and cameras, where the goal is to achieve high magnification through combinations of optics. Thus, total magnification can be determined accurately by understanding how each lens contributes to the final image.

Examples & Analogies

Consider using multiple sets of binoculars; the way they enhance your view is multiplied with each set you add. Each magnifying lens in a telescope or microscope acts like a pair of binoculars, enhancing what you see by combining their effects.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Image Formation: The image produced by the first lens (Lens A) serves as a virtual object for the second lens (Lens B). The positioning of these lenses and their individual focal lengths determine the final position of the image produced by the system.

  • Lens Equations: For the first lens, the lens formula is given by:

  • \[

  • \frac{1}{v_1} - \frac{1}{u_1} = \frac{1}{f_1}

  • \] where \(v_1\) is the image distance, \(u_1\) is the object distance, and \(f_1\) is the focal length of Lens A.

  • For the second lens, the formula is similarly defined:

  • \[

  • \frac{1}{v_2} - \frac{1}{u_2} = \frac{1}{f_2}

  • \] where \(v_2\) is the image distance for Lens B and \(u_2\) is the object distance derived from the image formed by Lens A.

  • Effective Focal Length: If the two-lens system is seen as a single lens of focal length \(f\), the effective focal length for the system can be determined from the individual focal lengths:

  • \[

  • \frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}

  • \]

  • This relationship holds true for any number of thin lenses in contact, providing versatility in achieving specific focal configurations.

  • Power of Lenses: The power of the combined lens system, noted as \(P\), equals the algebraic sum of the individual lenses’ powers, as expressed by:

  • \[

  • P = P_1 + P_2

  • \]

  • Magnification: The overall magnification of the lens combination is the product of the magnifications of the individual lenses:

  • \[

  • m = m_1 \times m_2

  • \]

  • where \(m_1\) and \(m_2\) are the magnifications produced by each lens.

  • These principles enable enhanced optical systems, such as compound microscopes and cameras, to achieve desired optical properties and resolutions.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • Combining a converging and diverging lens to create a specific optical effect, like focusing light for a camera.

  • Using a pair of thin lenses to design a microscope that can magnify small specimens.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • Lenses stacked tight, bring focus a sight, when combined they create, an image just right.

📖 Fascinating Stories

  • Two lens friends met one day, decided to work, come what may. One focused light with flair, the other joined in, they're quite the pair!

🧠 Other Memory Gems

  • L.E.M (Lenses, Effective focal length, Magnification) - Remembering the primary elements of lens combinations.

🎯 Super Acronyms

F.O.C.U.S (Focal length, Object distance, Combined image, Useful system) - Helps remember the characteristics of a lens system.

Flash Cards

Review key concepts with flashcards.

Glossary of Terms

Review the Definitions for terms.

  • Term: Focal Length

    Definition:

    The distance from the lens at which parallel rays of light converge or diverge.

  • Term: Effective Focal Length

    Definition:

    The resultant focal length of a combination of lenses placed in contact, calculated using the lens formula.

  • Term: Magnification

    Definition:

    The ratio of the size of the image formed to the size of the object.

  • Term: Lens Formula

    Definition:

    An equation relating the object distance, image distance, and focal length of a lens.

  • Term: Power of a Lens

    Definition:

    A measure of the degree to which a lens converges or diverges light, defined as the inverse of the focal length in meters.