JEE Mains 2025 8th April Shift 2 Physics Solution – Convex Concave Lens System
The JEE Mains 2025 Physics paper featured a conceptually rich question from the topic of combination of lenses. In this post, we will solve a question from the 8th April Shift 2 that involves a convex lens and a concave lens placed in contact. Our goal is to understand the image formation using lens formulas, step-by-step.
Question
JEE Mains 2025 8th April Shift 2:
A convex lens of focal length 30 cm is placed in contact with a concave lens of focal length 20 cm. An object is placed at 20 cm to the left of this lens system. The distance of the image from the lens in cm is:
- (1) 30
- (2) 60/7
- (3) 15
- (4) 45
More JEE Mains 2025 8th April Shift 2 Physics Solution and questions
Detailed Step-by-Step Solution
Step 1: Use the Lens Combination Formula
When two lenses are in contact, the equivalent focal length F is given by:
1/F = 1/f₁ + 1/f₂
Here, f₁ = +30 cm (convex lens), f₂ = –20 cm (concave lens)
So,
1/F = 1/30 – 1/20 = (2 – 3)/60 = –1/60
F = –60 cm
Step 2: Apply the Lens Formula
Lens formula is:
1/v – 1/u = 1/F
Object distance u = –20 cm (left of the lens, so negative)
Now,
1/v – (–1/20) = –1/60
⇒ 1/v + 1/20 = –1/60
⇒ 1/v = –1/60 – 1/20 = –(1 + 3)/60 = –4/60 = –1/15
v = –15 cm
Step 3: Interpret the Result
The negative sign indicates that the image is formed 15 cm to the left of the lens system.
Correct Answer: (3) 15 cm
Watch the Full Video Solution
For better understanding, watch the complete solution on YouTube:
Conclusion
This question tested your understanding of the lens combination formula and sign conventions. Solving such problems step-by-step improves both your conceptual clarity and exam speed.
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