Ray optics and optical instruments form a high‑yield portion of Class 12 Physics: they combine ray diagrams, algebraic imaging relations and practical instrument designs (microscope, telescope, prism, lenses) which are frequently tested in CBSE board problems and appear in JEE/NEET numericals. Mastery of sign conventions, lens‑maker relations, combination of lenses and dispersion effects builds strong problem‑solving skills required in competitive exams.
Practising a mix of numerical, graph/data interpretation and assertion‑reason problems sharpens conceptual understanding (e.g., immersion effects, chromatic and spherical aberrations, minimum deviation) and trains the strategy to convert diagrams and formulas into correct algebraic steps under exam time pressure.
15
Minutes
10
Questions
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Marking Scheme
Q1. A thin equiconvex lens has both surfaces of radius and is made of glass with refractive index . Using the lens‑maker formula and , the focal length of the lens is:
Q2. For a thin convex lens the image distance and object distance (measured from the lens) satisfy . If the graph of versus (for ) has a vertical asymptote at and passes through , the image distance when equals:
Q3. A thin converging lens made of glass () has focal length in air. It is immersed in a liquid of refractive index . Using the relation for a lens in a medium and eliminating the curvature terms with the known , the new focal length (in cm) is approximately:
Q4. Two thin lenses in contact have focal lengths (convex) and (concave). The equivalent focal length of the combination is given by . The value of (in cm) is:
Q5. Assertion (A): For a glass–air interface the critical angle for blue light is smaller than that for red light.
Reason (R): This follows because the refractive index of typical optical glass is larger for blue than for red (normal dispersion).
Q6. A thin convex lens has focal lengths (red) and (blue). An object at is sharply imaged on a screen for red light. When illuminated with blue light without moving lens or screen, the blue image forms at a different position. Approximately how far and in which direction is the blue image relative to the screen set for red light?
Q7. In a compound microscope the objective has focal length and the eyepiece has focal length . The tube length is . For the relaxed‑eye case (final image at infinity) and taking the near point , the approximate total angular magnification . The value of is:
Q8. Assertion (A): Reducing the aperture (stopping down) of a lens reduces spherical aberration but does not eliminate chromatic aberration.
Reason (R): Spherical aberration arises from marginal rays focusing at different points and stopping the aperture removes marginal rays; chromatic aberration is due to wavelength‑dependent refractive index and cannot be removed by reducing aperture alone.
Q9. In an experiment with an equilateral prism () the measured minimum deviation is . Using , the refractive index of the prism material is approximately:
Q10. For a thin lens the image distance as a function of object distance satisfies and . For which of the following object distances (measured in multiples of ) does a small change in produce the largest magnitude change in ?