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If you live in Abu Dhabi City and you are searching for a good Physics Tutor in Abu Dhabi City, then you are at the right place. Many students study in CBSE, IB, IGCSE, A-Level, AP Physics, NEET and IIT JEE, but they still struggle in Physics because concepts are not clear.
Sometimes school teaching is fast, sometimes numericals are difficult, and sometimes students do not understand how to apply formulas. In Physics, only memorising formulas is not enough. You must understand the concept, sign convention, diagram, derivation and application.
At Kumar Physics Classes, Kumar Sir teaches Physics in a simple and result-oriented way. If you are in Abu Dhabi City and need online Physics tuition, you can contact Kumar Sir.
Physics Tutor in Abu Dhabi City for CBSE, IB, IGCSE, A-Level, AP, NEET and IIT JEE
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Physics Concept: Combination of Convex and Concave Lens
Question:
A convex lens A of focal length 20 cm and a concave lens B of focal length 5 cm are kept along the same axis with distance d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then find d.
Given
Convex lens A:
f1 = +20 cm
Concave lens B:
f2 = -5 cm
Final emergent beam is parallel.
So equivalent focal length should be infinity.
Therefore:
1/F = 0
Formula for Two Thin Lenses Separated by Distance d
1/F = 1/f1 + 1/f2 – d/(f1 f2)
Since final beam is parallel:
F = infinity
So:
1/F = 0
Therefore:
0 = 1/f1 + 1/f2 – d/(f1 f2)
Now put values:
0 = 1/20 + 1/(-5) – d/[(20)(-5)]
0 = 1/20 – 1/5 + d/100
Now:
1/20 – 1/5 = 1/20 – 4/20
1/20 – 4/20 = -3/20
So:
0 = -3/20 + d/100
d/100 = 3/20
d = 100 × 3/20
d = 15 cm
Final answer:
d = 15 cm
Sign Convention Explanation
Convex lens has positive focal length:
f1 = +20 cm
Concave lens has negative focal length:
f2 = -5 cm
Because the second lens is concave, we must write its focal length with negative sign. This is the main trick of the question.
If the student forgets the negative sign, the answer becomes wrong.
Kumar Sir Style Explanation
देखो बेटा, question में दो lenses हैं. पहला convex है, इसलिए उसका focal length positive होगा. दूसरा concave है, इसलिए उसका focal length negative होगा.
अब parallel beam पहले lens पर आ रही है और second lens से भी parallel निकल रही है. इसका मतलब पूरा combination afocal है.
Afocal system में:
F = infinity
इसलिए:
1/F = 0
अब formula लगाओ:
1/F = 1/f1 + 1/f2 – d/(f1 f2)
बस sign convention सही लगाना है. यही पूरा game है.
Final answer:
d = 15 cm
Why Students Need Kumar Sir for Physics
Physics becomes easy when the teacher explains the logic behind every formula. Kumar Sir teaches every topic step by step:
Concept
Formula
Sign convention
Diagram
Derivation
Numerical application
Exam-level practice
Whether you are preparing for CBSE, IB, IGCSE, A-Level, AP Physics, NEET or IIT JEE, Kumar Physics Classes can help you build strong Physics fundamentals.
Contact Kumar Physics Classes
Kumar Physics Classes
Website: kumarphysicsclasses.com
Phone / WhatsApp: +91-9958461445
Email: kumarsirphysics@gmail.com
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25 Conceptual Questions on Lens Maker Formula
Kumar Sir Style
1. What is Lens Maker Formula?
Answer:
Lens Maker Formula tells the focal length of a lens using refractive index and radii of curvature.
Formula:
1/f = (μ – 1)(1/R1 – 1/R2)
2. Why is it called Lens Maker Formula?
Answer:
Because lens manufacturers use it to design lenses of required focal length by choosing material and curvature.
3. On what factors does focal length of a lens depend?
Answer:
Focal length depends on:
Refractive index of lens material
Refractive index of surrounding medium
Radius of curvature of both surfaces
4. What is Lens Maker Formula in a medium?
Answer:
If lens is placed in a medium:
1/f = (μlens/μmedium – 1)(1/R1 – 1/R2)
5. What happens when a lens is placed in water?
Answer:
Focal length increases because relative refractive index decreases.
So power decreases.
6. Why does focal length increase in water?
Answer:
Because bending of light becomes less in water. Lens becomes less powerful.
7. Can a convex lens behave like a concave lens in a liquid?
Answer:
Yes. If surrounding liquid has refractive index greater than lens material, then convex lens behaves like concave lens.
8. When does a lens have infinite focal length?
Answer:
When refractive index of lens and surrounding medium are equal.
Then:
μlens/μmedium = 1
So:
1/f = 0
Therefore:
f = infinity
Lens behaves like plane glass.
9. What happens to power of lens in water?
Answer:
Power decreases because focal length increases.
P = 1/f
10. Why does a lens become invisible in a liquid of same refractive index?
Answer:
Because there is no refraction at the boundary. Light passes without bending.
11. What is the role of R1 and R2?
Answer:
R1 and R2 represent curvature of two lens surfaces. More curvature means smaller radius and greater power.
12. If radius of curvature decreases, what happens to focal length?
Answer:
If radius decreases, curvature increases, so focal length decreases and power increases.
13. Why is violet light bent more than red light?
Answer:
Violet light has higher refractive index than red light, so it bends more.
14. Which has smaller focal length: red or violet?
Answer:
Violet has smaller focal length because it bends more.
15. Which has greater power: red or violet?
Answer:
Violet has greater power because its focal length is smaller.
16. Why does focal length depend on colour?
Answer:
Because refractive index depends on wavelength. This is called dispersion.
17. What happens if red light is replaced by violet light in a convex lens?
Answer:
Focal length decreases because violet light bends more.
18. What happens if violet light is replaced by red light?
Answer:
Focal length increases because red light bends less.
19. Why does white light split through a lens?
Answer:
Different colours have different refractive indices, so they focus at different points.
20. What is chromatic aberration?
Answer:
Chromatic aberration is the defect in which different colours focus at different points.
21. Which colour focuses closest to the lens?
Answer:
Violet focuses closest to the lens because it has maximum bending.
22. Which colour focuses farthest from the lens?
Answer:
Red focuses farthest because it bends least.
23. Does Lens Maker Formula explain dispersion?
Answer:
Yes. Since μ changes with colour, focal length also changes with colour.
24. Why is sign convention important in Lens Maker Formula?
Answer:
Because wrong signs of R1 and R2 give wrong focal length and wrong nature of lens.
25. What is the most important concept in Lens Maker Formula?
Answer:
The main concept is:
Focal length depends on relative refractive index and curvature.
If medium changes, focal length changes.
If colour changes, focal length changes.
If curvature changes, focal length changes.
Sign Convention in Lens Maker Formula
In Lens Maker Formula, sign convention is very important because one wrong sign can change the complete answer. The formula is:
1/f = (μ – 1)(1/R1 – 1/R2)
Here R1 is the radius of curvature of the first surface and R2 is the radius of curvature of the second surface. We use the Cartesian sign convention. Light is assumed to travel from left to right. Any distance measured in the direction of light is positive, and any distance measured opposite to the direction of light is negative.
For a convex lens, the first surface has its centre of curvature on the right side, so R1 is positive. The second surface has its centre of curvature on the left side, so R2 is negative. Therefore, for a convex lens:
R1 = positive
R2 = negative
For a concave lens, signs are opposite:
R1 = negative
R2 = positive
If the lens is placed in water or any other medium, we use relative refractive index:
μ = μlens / μmedium
Kumar Sir style: बेटा, बस light की direction देखो. जिस side centre of curvature है, उसी हिसाब से sign लगाओ. Sign convention clear है तो Lens Maker Formula बहुत easy है.