Ray Optics & Optical Instruments

11 NEET-UG practice questions on Ray Optics & Optical Instruments, part of Physics. The first 11 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   An object stands 30 cm in front of a concave mirror of focal length 10 cm. Where is the image, and what is its nature?

   1. A15 cm behind the mirror, virtual
   2. B15 cm in front of the mirror, real
   3. C7.5 cm in front of the mirror, real
   4. D30 cm in front of the mirror, real

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   Answer: B · 15 cm in front of the mirror, real

   With u = -30 cm and f = -10 cm, 1/v = 1/f - 1/u = -1/10 + 1/30 = -1/15. So v = -15 cm, on the object's side, and the image is real.

   • A: The sign of v was misread; a negative v means in front of the mirror.
   • C: The reciprocals were added, giving 1/10 + 1/30.
   • D: The image was placed at the object's position, which happens only for an object at C.

   Remember this

   An object beyond the centre of curvature of a concave mirror gives a real, diminished image between C and F.

2. Question 2 · Hard

   An object is 30 cm to the left of a thin convex lens of focal length 20 cm. A plane mirror stands perpendicular to the axis, 40 cm to the right of the lens. What leaves the lens after the light has reflected from the mirror and passed through the lens again?

   1. AA beam converging to a point 60 cm left of the lens
   2. BA beam converging to a point 30 cm left of the lens
   3. CA beam parallel to the axis, travelling left
   4. DA beam converging to a point 20 cm left of the lens

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   Answer: C · A beam parallel to the axis, travelling left

   The lens alone would form an image at v: 1/v = 1/20 - 1/30 = 1/60, so 60 cm to its right, which is 20 cm behind the mirror. This acts as a virtual object, so the mirror forms a real image 20 cm in front of itself, which is 20 cm right of the lens. That point is the lens's focus, so the light leaves parallel to the axis.

   • A: The second object distance was wrongly taken as 30 cm.
   • B: The rays would retrace their path only if the mirror were at the first image point.
   • D: The focal distance was used as the image distance on the far side.

   Remember this

   A point at a lens's focus sends a parallel beam; treat each element's image as the next object.

3. Question 3 · Medium

   Assertion A: In normal adjustment, the separation between the objective and eyepiece of an astronomical refracting telescope equals f_o - f_e. Reason R: In normal adjustment, the magnifying power of such a telescope has magnitude f_o/f_e. Choose the correct answer.

   1. AA is not correct but R is correct
   2. BBoth A and R are correct and R is the correct explanation of A
   3. CBoth A and R are correct but R is not the correct explanation of A
   4. DA is correct but R is not correct

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   Answer: A · A is not correct but R is correct

   In normal adjustment the second focal point of the objective coincides with the first focal point of the eyepiece, so the separation is f_o + f_e and A is wrong. The final image is at infinity with angular magnification f_o/f_e, so R is right.

   • B: The separation is f_o + f_e, so A fails.
   • C: A fails, since the focal lengths add.
   • D: A is the wrong statement here, and R is the correct one.

   Remember this

   Telescope in normal adjustment: tube length f_o + f_e, magnifying power f_o/f_e.

4. Question 4 · Medium

   In a minimum deviation trial with a prism of angle 60°, the angle of incidence is found to be 45°. What are the angle of minimum deviation and the refractive index of the prism?

   1. A30° and √3
   2. B15° and √2
   3. C30° and √2
   4. D45° and √2

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   Answer: C · 30° and √2

   At minimum deviation A + D = 2i, so D = 90° - 60° = 30°, and r = A/2 = 30°. Then n = sin 45°/sin 30° = (1/√2)/(1/2) = √2.

   • A: sin 60° was used in the numerator instead of sin 45°.
   • B: i - A/2 was used for D, not 2i - A.
   • D: The angle of incidence was taken as the deviation.

   Remember this

   At minimum deviation i equals e, r = A/2, and D = 2i - A.

5. Question 5 · Medium

   A thin convex lens of focal length 20 cm is placed in contact with a thin concave lens of focal length 30 cm. What is the focal length of the combination?

   1. A+60 cm
   2. B-10 cm
   3. C-60 cm
   4. D+12 cm

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   Answer: A · +60 cm

   For lenses in contact, 1/F = 1/f₁ + 1/f₂ = 1/20 + 1/(-30) = 1/60. So F = +60 cm and the combination is converging.

   • B: The focal lengths were added directly, not their reciprocals.
   • C: The sign was misjudged; the stronger convex lens dominates.
   • D: The concave lens was treated as convex.

   Remember this

   Thin lenses in contact add their powers with signs, so the stronger lens sets the nature of the combination.

6. Question 6 · Medium

   Which feature of an optical fibre keeps light inside it over long distances?

   1. AThe core and cladding have equal refractive indices, so no reflection occurs
   2. BThe core's outer surface is silvered, giving ordinary reflection
   3. CThe cladding has a higher refractive index than the core, giving total internal reflection
   4. DThe core has a higher refractive index than the cladding, giving total internal reflection

   Answer and explanation in LearnFree account

7. Question 7 · Easy

   A coin rests at the bottom of a tank filled with water of refractive index 4/3 to a depth of 24 cm. Seen from nearly straight above, by how much does the coin appear raised?

   1. A6 cm
   2. B32 cm
   3. C8 cm
   4. D18 cm

   Answer and explanation in LearnFree account

8. Question 8 · Medium

   A coin lies at the bottom of a beaker holding the two liquid layers shown. Looking straight down, how far below the top surface of the oil does the coin appear?

   

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   1. A21 cm
   2. B15 cm
   3. C6 cm
   4. D14.75 cm

   Answer and explanation in LearnFree account

9. Question 9 · Medium

   A ray inside a transparent block meets the top surface and just grazes along it, as drawn. What is the speed of light in the block? Take c = 3 × 10⁸ m/s.

   

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   1. A1.73 × 10⁸ m/s
   2. B6 × 10⁸ m/s
   3. C2.6 × 10⁸ m/s
   4. D1.5 × 10⁸ m/s

   Answer and explanation in LearnFree account

10. Question 10 · Easy

    Where is the image formed by the mirror for the object O placed as shown?

    

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    1. A60 cm behind the mirror
    2. B12 cm in front of the mirror
    3. C12 cm behind the mirror
    4. D60 cm in front of the mirror

    Answer and explanation in LearnFree account

11. Question 11 · Hard

    Two thin convex lenses of the focal lengths marked share an axis. Find the position of the final image of O and the total magnification.

    

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    1. A60 cm beyond L₂, magnification +2
    2. B12 cm beyond L₂, magnification +0.4
    3. C20 cm beyond L₂, magnification −2
    4. D20 cm beyond L₂, magnification +2

    Answer and explanation in LearnFree account