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Indian Navy 10+2 B.Tech Cadet Ray Optics and Optical Instruments — practice questions

Explore 33 free Ray Optics and Optical Instruments questions in Indian Navy 10+2 B.Tech Cadet Physics. Open each question or exercise to read its content, reference answers and available explanation.

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Ray Optics and Optical Instruments questions and exercises with answers

Showing questions 1–30 of 33.

A convex lens forms a real image at $60$ cm on one side of an object placed $20$ cm in front of it. Find the focal length of the lens.Read the answer and worked solution →Refractive index of water is 1.33. Critical angle for water-air interface:Read the answer and worked solution →Two thin lenses of powers P₁ and P₂ in contact give combined power:Read the answer and worked solution →Speed of light in a medium of refractive index n:Read the answer and worked solution →Object at focus of a convex lens. Image is:Read the answer and worked solution →Critical angle for diamond (n = 2.42):Read the answer and worked solution →Light passes from air (n=1) into glass (n=1.5) at an incidence angle of 60°. Using Snell's law, the angle of refraction isRead the answer and worked solution →Cartesian sign convention: positive or negative?Read the answer and worked solution →True or false: the sign conventionRead the answer and worked solution →Complete the condition for total internal reflectionRead the answer and worked solution →Tap every example of total internal reflectionRead the answer and worked solution →Critical angleRead the answer and worked solution →True or false: optical fibresRead the answer and worked solution →Match each formula to what it describesRead the answer and worked solution →Converging or diverging?Read the answer and worked solution →Complete the definition of lens powerRead the answer and worked solution →Lenses in contactRead the answer and worked solution →Order the steps: solve a mirror problemRead the answer and worked solution →Assertion and reasonRead the answer and worked solution →Match each instrument to its magnifying powerRead the answer and worked solution →True or false: optical instrumentsRead the answer and worked solution →Which instrument does this belong to?Read the answer and worked solution →Two are true, one is notRead the answer and worked solution →Three of these are wrong. Find the one that holds.Read the answer and worked solution →Tap every statement true of refractionRead the answer and worked solution →Complete the prism relationRead the answer and worked solution →Flip and pair: symbol with meaningRead the answer and worked solution →Spot the one that does not belongRead the answer and worked solution →Reading the laws backRead the answer and worked solution →Order the reasoning: why an optical fibre worksRead the answer and worked solution →

Worked practice questions

A convex lens forms a real image at $60$ cm on one side of an object placed $20$ cm in front of it. Find the focal length of the lens.

  1. $10$ cm
  2. $12$ cm
  3. $15$ cm
  4. $20$ cm
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Correct answer: C. $15$ cm

Lens formula: $\dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$. Sign convention: $u = -20$ cm (object on the left), $v = +60$ cm (real image on the right). We have $\dfrac{1}{f} = \dfrac{1}{60} - \dfrac{1}{-20}$. That is $\dfrac{1}{f} = \dfrac{1}{60} + \dfrac{3}{60}$. That is $\dfrac{1}{f} = \dfrac{4}{60}$. So $\dfrac{1}{f} = \dfrac{1}{15}$. So $f = 15$ cm. **Trap.** Forgetting the sign on $u$ gives $\dfrac{1}{f} = \dfrac{1}{60} - \dfrac{1}{20} = -\dfrac{1}{30}$, leading to a negative $f$ — that's how option B (12) or A (10) creeps in.

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Refractive index of water is 1.33. Critical angle for water-air interface:

  1. 30°
  2. sin⁻¹(1/1.33) ≈ 48.75°
  3. 90°
  4. 60°
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Correct answer: B. sin⁻¹(1/1.33) ≈ 48.75°

sin θ_c = 1/n = 1/1.33 ≈ 0.752. θ_c ≈ 48.75°. Beyond this, light striking water-air interface from inside undergoes TIR.

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Two thin lenses of powers P₁ and P₂ in contact give combined power:

  1. P₁ - P₂
  2. P₁ × P₂
  3. P₁ + P₂
  4. sqrt(P₁ P₂)
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Correct answer: C. P₁ + P₂

**In contact:** powers add (P = P₁ + P₂). **Equivalent focal length:** 1/f = 1/f₁ + 1/f₂. Used to design corrective lens systems.

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