NCERT Books for Class 12
FAQs on NCERT Books for Class 12 Physics Chapter 9 - Ray Optics and Optical Instruments Free PDF Download
1. What are the most frequently asked 5-mark questions from Chapter 9, Ray Optics and Optical Instruments, for the CBSE 2025-26 board exam?
For the Class 12 board exams, the important long-answer (5-mark) questions from Ray Optics typically involve derivations combined with a numerical problem. Key expected questions include:
Derivation of the lens maker's formula, followed by a problem on finding the focal length of a lens with a given refractive index and radii of curvature.
A detailed explanation of a compound microscope or an astronomical telescope, including a labelled ray diagram, derivation of its magnifying power, and its limitations.
Derivation of the relation between refractive index and minimum deviation for a prism.
Questions on the combination of thin lenses, requiring finding the effective focal length, power, and nature of the final image.
2. Which derivations are essential to practice from the Ray Optics chapter for scoring well in the Physics exam?
To secure full marks in derivation-based questions from this chapter, students must thoroughly practice the following:
The relationship between object distance (u), image distance (v), and focal length (f) for a spherical mirror, known as the mirror formula (1/v + 1/u = 1/f).
The lens maker's formula: 1/f = (n₂/n₁ - 1) * (1/R₁ - 1/R₂).
The formula for refraction at a single spherical surface.
The derivation for the magnifying power of both a compound microscope and an astronomical telescope.
The formula for the refractive index of a prism in terms of the angle of prism (A) and angle of minimum deviation (δm).
3. What types of numerical problems are commonly asked from the lens maker's formula and combination of lenses?
Important numericals from these topics often test your application of formulas and sign conventions. You can expect questions that require you to:
Calculate the focal length of a biconvex or biconcave lens when the radii of curvature and refractive index are given.
Determine the change in focal length when a lens is immersed in a liquid with a different refractive index.
Find the equivalent focal length and power of a system of two or more thin lenses in contact.
Calculate the position, nature (real/virtual, inverted/erect), and size of the image formed by a combination of lenses.
Always pay close attention to the Cartesian sign convention to avoid errors.
4. From the Optical Instruments section, which topics carry high weightage in exams?
In the Optical Instruments part of the chapter, the compound microscope and the astronomical telescope (both refracting and reflecting types) are the most important topics. Exam questions frequently focus on:
Drawing neat, labelled ray diagrams showing the final image formation (at infinity or near point).
Deriving the expression for magnifying power in both adjustment cases.
Comparing the advantages of a reflecting telescope over a refracting one.
Conceptual questions on resolving power and its dependence on the objective's aperture.
5. Why is it crucial to master questions on Total Internal Reflection (TIR) for the board exam?
Total Internal Reflection (TIR) is a high-yield topic because it tests the fundamental understanding of refraction and its practical applications. Examiners favour this topic as it allows them to frame questions that connect theory to real-world phenomena. Important areas include:
The two necessary conditions for TIR: light must travel from a denser to a rarer medium, and the angle of incidence must be greater than the critical angle.
Applications like the working of optical fibres, the brilliance of diamonds, and natural phenomena like mirages and looming.
Numerical problems to calculate the critical angle for different media pairs.
6. How can a student effectively use ray diagrams to score full marks in questions on lenses and mirrors?
Ray diagrams are not just illustrations; they are a critical part of the solution and carry significant marks. To score full marks, ensure the following:
Use a ruler and protractor for neat and accurate lines and angles.
Clearly mark the principal axis, pole (P), focus (F), and centre of curvature (C).
Show the direction of light rays with arrows. A diagram without arrows is often considered incorrect.
Draw at least two of the three principal rays to accurately locate the image.
The final diagram should correctly represent the nature, position, and relative size of the image as per your calculations.
7. Beyond memorising formulas, what is the key to solving HOTS questions from Ray Optics?
To tackle Higher Order Thinking Skills (HOTS) questions, you must move beyond formula-based learning. The key is to understand the 'why' behind the concepts. Focus on:
Conceptual Clarity: Understand why sign conventions are used, why a reflecting telescope is preferred, or how the medium affects a lens's focal length.
Application Skills: Be prepared for questions that modify standard setups, like cutting a lens into parts or combining a lens with a mirror.
Analytical Thinking: Practice questions that ask you to predict what would happen if a parameter is changed (e.g., 'What happens to the focal length if the lens is immersed in water?').
Interlinking Topics: Be ready for questions that connect concepts, such as using the prism formula and then applying Snell's law.
8. What are some common mistakes students make in Ray Optics numericals that lead to a loss of marks?
Students often lose marks in Ray Optics numericals due to careless errors, not a lack of knowledge. The most common mistakes to avoid are:
Incorrect Sign Convention: Not applying the Cartesian sign convention correctly for u, v, f, and R is the most frequent error.
Unit Conversion: Forgetting to convert all units to a consistent system (e.g., cm to m) before calculation, especially when calculating power.
Forgetting the Final Statement: Failing to write the final characteristics of the image (real/virtual, inverted/erect, magnified/diminished) after finding the image distance.
Calculation Errors: Simple mistakes in handling fractions and decimals during calculations for the mirror or lens formula.
