CBSE Class 10 Science: Lens Power & Myopia Question Solved

Understanding the Integrated Lens and Human Eye Question

Many CBSE Class 10 students struggle when examiners combine concepts from Light and Human Eye chapters. The 2025 Science Board Exam (Paper Code 3153) featured precisely such an integrated question that tested conceptual clarity. Question 29 presented a lens power value of -0.25 D and required analysis across three interconnected parts. After reviewing this problem, I believe its brilliance lies in testing whether students can apply theoretical knowledge to practical scenarios. Let's break down each section systematically.

Lens Type and Focal Length Calculation

Given lens power (P) = -0.25 dioptre, we apply the fundamental formula:
P = 1/f
Where f is focal length in meters. Rearranging:
f = 1/P = 1/(-0.25) = -4 meters

Key implications:

• Negative power indicates a concave lens (as per NCERT Class 10 Science Chapter 10)
• The negative focal length confirms light divergence
• Magnitude shows moderate corrective strength

Identifying Correctable Eye Defects

Concave lenses specifically correct myopia (near-sightedness), as established in Chapter 11 of the CBSE syllabus. Here's why this lens works:

1. Myopia causes light to focus before the retina
2. Concave lenses diverge incoming light rays
3. This shifts focus backward onto the retina

Important distinction: Hypermetropia requires convex lenses. Examiners often trick students by reversing defect types, but the negative power here confirms myopia correction.

Image Formation Analysis

When placing an object between f and 2f of a concave lens:

• Nature: Always virtual and erect (regardless of object position)
• Size: Diminished (smaller than object)
• Position: Between optical center and focus

CBSE-style ray diagram demonstrating image characteristics

Critical exam insight: Unlike convex lenses, concave lenses never produce real or inverted images. This consistent behavior makes them predictable in problem-solving.

Essential Problem-Solving Toolkit

Action Checklist for Similar Questions:

1. Verify lens power sign first (negative = concave, positive = convex)
2. Link defect to lens type: Concave → Myopia, Convex → Hypermetropia
3. Recall image properties: Concave = always virtual/erect/diminished
4. Double-check units: Power in dioptres, focal length in meters

Recommended Resources:

• NCERT Exemplar Class 10 Science - Provides advanced lens problems with solutions
• PhET Lens Simulation (University of Colorado) - Interactive tool to visualize ray diagrams
• CBSE Marking Scheme 2025 - Official answer keys showing step-wise marking

Key Takeaways and Engagement

This integrated question demonstrates how CBSE tests conceptual application beyond rote learning. By understanding why concave lenses correct myopia and how they form images, you'll tackle any variant confidently.

Question for you: Which ray diagram scenario do you find most challenging? Share your exam preparation hurdles in the comments!