Ray Optics and Optical Instruments

This chapter has

Ray Optics and Optical Instruments — solved practice questions

8 NEET Physics questions with step-by-step solutions. Attempt each, then reveal the answer.

1. Q1easy

   The power of a thin lens of focal length 25 cm is:

   • A2.5 D
   • B4 D
   • C5 D
   • D0.25 D
   Show answer & solution

   Correct answer: (B) 4 D

   Power P = 1/f (in metres) = 100/25 cm = 4 dioptres.

2. Q2medium

   An object is placed 30 cm in front of a convex lens of focal length 20 cm. The image is formed at a distance of:

   • A12 cm
   • B20 cm
   • C30 cm
   • D60 cm
   Show answer & solution

   Correct answer: (D) 60 cm

   Using 1/v − 1/u = 1/f with u = −30, f = +20: 1/v = 1/20 − 1/30 = 1/60, so v = 60 cm (real, on the other side).

3. Q3medium

   An object is placed 30 cm from a concave mirror of focal length 15 cm. The magnification of the image is:

   • A−1
   • B−2
   • C+1
   • D+2
   Show answer & solution

   Correct answer: (A) −1

   1/v + 1/u = 1/f with u = −30, f = −15 gives v = −30 cm. Magnification m = −v/u = −(−30)/(−30) = −1 (real, inverted, same size).

4. Q4medium

   The critical angle for a medium of refractive index 1.5 (with respect to air) is closest to:

   • A30°
   • B36°
   • C42°
   • D48°
   Show answer & solution

   Correct answer: (C) 42°

   sin C = 1/n = 1/1.5 = 0.667, so C ≈ 42°.

5. Q5medium

   A ray travels from a denser medium of refractive index √2 into air. The critical angle for total internal reflection is:

   • A30°
   • B45°
   • C60°
   • D90°
   Show answer & solution

   Correct answer: (B) 45°

   sin C = 1/n = 1/√2, so C = 45°.

6. Q6easy

   The speed of light in a medium of refractive index 1.5 is (c = 3 × 10^8 m/s):

   • A3 × 10^8 m/s
   • B4.5 × 10^8 m/s
   • C1.5 × 10^8 m/s
   • D2 × 10^8 m/s
   Show answer & solution

   Correct answer: (D) 2 × 10^8 m/s

   v = c/n = 3 × 10^8 / 1.5 = 2 × 10^8 m/s.

7. Q7medium

   Two thin lenses of powers +5 D and −2.5 D are placed in contact. The power of the combination is:

   • A+2.5 D
   • B+7.5 D
   • C−2.5 D
   • D+1.25 D
   Show answer & solution

   Correct answer: (A) +2.5 D

   For lenses in contact powers add: P = P1 + P2 = +5 + (−2.5) = +2.5 D.

8. Q8hard

   An equiconvex lens is made of glass of refractive index 1.5, each surface having radius of curvature 20 cm. Its focal length is:

   • A10 cm
   • B15 cm
   • C20 cm
   • D40 cm
   Show answer & solution

   Correct answer: (C) 20 cm

   Lensmaker: 1/f = (n−1)(1/R1 − 1/R2) = (0.5)(1/20 − 1/(−20)) = (0.5)(2/20) = 1/20, so f = 20 cm.

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