Ray Optics and Optical Instruments — Class 12 MCQs with Answers

Ray Optics and Optical Instruments — important questions & MCQs with answers (Class 12 Physics)

14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 76 are timed and scored when you sign in.

1. Q1Easy

   Snell's law:

   A.None

   B.n₁ + n₂ = θ

   C.n = c/v only

   D.n₁ sinθ₁ = n₂ sinθ₂✓ Correct

   Answer: D. n₁ sinθ₁ = n₂ sinθ₂

   Explanation: Snell's law governs refraction: n₁sinθ₁ = n₂sinθ₂, relating the incident and refracted angles through each medium's refractive index — it is not a sum of indices or n=c/v alone.

2. Q2Easy

   A simple microscope uses:

   A.A single convex lens of short focal length✓ Correct

   B.Two concave lenses

   C.A plane mirror

   D.A prism

   Answer: A. A single convex lens of short focal length

   Explanation: A simple microscope is just one convex lens of short focal length held close to the eye, magnifying without the second lens a compound microscope needs.

3. Q3Easy

   Power of a lens unit:

   A.Watt

   B.Dioptre (1/m)✓ Correct

   C.Tesla

   D.Newton

   Answer: B. Dioptre (1/m)

   Explanation: Power of a lens P = 1/f with f in metres, so it's measured in dioptres (D), not watts or newtons — mixing up f in cm forgets the metre conversion.

4. Q4Easy

   The objective of an astronomical telescope has:

   A.Negative power

   B.Small focal length

   C.Small aperture

   D.Large focal length and large aperture✓ Correct

   Answer: D. Large focal length and large aperture

   Explanation: A telescope objective needs large focal length for magnification and large aperture to gather more light and improve resolving power — a small aperture would blur faint objects.

5. Q5Easy

   Mirror formula is:

   A.v + u = f

   B.1/v − 1/u = 1/f

   C.1/v + 1/u = 1/f✓ Correct

   D.u/v = f

   Answer: C. 1/v + 1/u = 1/f

   Explanation: For spherical mirrors under the Cartesian sign convention, the mirror formula is 1/v + 1/u = 1/f, linking image distance, object distance and focal length.

6. Q6Easy

   Eyepiece of a microscope or telescope has:

   A.Infinite focal length

   B.Very long focal length

   C.Short focal length✓ Correct

   D.Zero focal length

   Answer: C. Short focal length

   Explanation: The eyepiece has a short focal length so it strongly magnifies the real image already formed by the objective, unlike the objective which needs a long focal length.

7. Q7Medium

   Total internal reflection occurs when:

   A.Light passes from denser to rarer beyond critical angle✓ Correct

   B.At normal incidence

   C.Always

   D.In vacuum

   Answer: A. Light passes from denser to rarer beyond critical angle

   Explanation: Total internal reflection needs light going from a denser to a rarer medium at an angle beyond the critical angle — normal incidence or a rarer-to-denser path never triggers it.

8. Q8Medium

   Magnification of compound microscope ≈

   A.L·D

   B.f_o/f_e

   C.(L/f_o)(1 + D/f_e)✓ Correct

   D.f_o + f_e

   Answer: C. (L/f_o)(1 + D/f_e)

   Explanation: Compound microscope magnification ≈ (L/f_o)(1 + D/f_e), where L is the tube length between the lenses and D the near-point distance — objective and eyepiece contributions multiply.

9. Q9Medium

   Lens formula:

   A.1/v + 1/u = 1/f

   B.1/v − 1/u = 1/f✓ Correct

   C.u + v = f

   D.u·v = f²

   Answer: B. 1/v − 1/u = 1/f

   Explanation: Using the Cartesian sign convention, the thin-lens formula is 1/v − 1/u = 1/f, not 1/v + 1/u = 1/f (that's the mirror formula) — the sign flip is the easy mix-up.

10. Q10Medium

    Telescope length in normal adjustment:

    A.f_o + f_e✓ Correct

    B.f_o − f_e

    C.f_o·f_e

    D.f_o/f_e

    Answer: A. f_o + f_e

    Explanation: In normal adjustment the objective's real image forms exactly at the eyepiece's focal point, so the tube length is L = f_o + f_e, not f_o − f_e or their product.

11. Q11Medium

    An object is 30 cm from a concave mirror of f = 15 cm. The image distance is:

    A.−30 cm✓ Correct

    B.−15 cm

    C.+30 cm

    D.+15 cm

    Answer: A. −30 cm

    Explanation: With f = −15 cm and u = −30 cm (object at the centre of curvature, 2f), 1/v = 1/f − 1/u = −1/15 + 1/30 = −1/30, giving v = −30 cm — the image forms back at 2f.

12. Q12Hard

    A prism splits white light because refractive index:

    A.Is zero

    B.Is constant

    C.Equals 1

    D.Varies with wavelength (dispersion)✓ Correct

    Answer: D. Varies with wavelength (dispersion)

    Explanation: A prism splits white light because its refractive index varies with wavelength (dispersion): violet bends more than red since n is higher at shorter wavelengths.

13. Q13Hard

    Compound microscope uses:

    A.Two concave

    B.Concave + convex

    C.Two convex lenses (objective + eyepiece)✓ Correct

    D.One mirror

    Answer: C. Two convex lenses (objective + eyepiece)

    Explanation: A compound microscope uses two convex lenses — an objective and an eyepiece — with total magnification M = M_o × M_e, the product of each lens's own magnifying power.

14. Q14Hard

    A prism of A = 60° in minimum deviation has δm = 30°. n =

    A.2

    B.1.5

    C.√2✓ Correct

    D.√3

    Answer: C. √2

    Explanation: At minimum deviation, n = sin((A+δm)/2)/sin(A/2) = sin45°/sin30° = (√2/2)/(1/2) = √2 — forgetting to halve A and δm before taking the sine is the usual mistake.

Ray Optics and Optical Instruments — FAQs

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