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DSSSB TGT Science — practice by topic

Browse 371 questions across 78 Science topics. Choose what you want to revise, then open a question to check its answer and worked solution.

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Two lenses of power +5 D and −2 D are placed in contact. The focal length of the combination isRead the answer and worked solution →A 10 mm pin in front of a concave mirror forms a real, inverted 5 mm image at 30 cm in front of the mirror. The focal length of the mirror isRead the answer and worked solution →The refractive index of a medium is 1.5. The speed of light in that medium is (c = 3×10⁸ m/s)Read the answer and worked solution →The magnification produced by a plane mirror isRead the answer and worked solution →

Worked practice questions

Two lenses of power +5 D and −2 D are placed in contact. The focal length of the combination is

  1. 50 cm
  2. −33.3 cm
  3. 33.3 cm
  4. 20 cm
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Correct answer: C. 33.3 cm

Powers add in contact: $P=+5-2=+3$ D. $f=1/P=1/3\,\text{m}=33.3$ cm (converging).

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A 10 mm pin in front of a concave mirror forms a real, inverted 5 mm image at 30 cm in front of the mirror. The focal length of the mirror is

  1. 30 cm
  2. 10 cm
  3. 15 cm
  4. 20 cm
Show answer and solution

Correct answer: D. 20 cm

$m=h'/h=-5/10=-0.5=-v/u$ with $v=-30$, so $u=-60$ cm. $\frac1f=\frac1v+\frac1u=-\frac1{30}-\frac1{60}=-\frac{3}{60}\Rightarrow f=-20$ cm. Magnitude 20 cm.

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The refractive index of a medium is 1.5. The speed of light in that medium is (c = 3×10⁸ m/s)

  1. $4.5 \times 10^8$ m/s
  2. $3 \times 10^8$ m/s
  3. $2 \times 10^8$ m/s
  4. $1.5 \times 10^8$ m/s
Show answer and solution

Correct answer: C. $2 \times 10^8$ m/s

$n=c/v \Rightarrow v=c/n=(3\times10^8)/1.5=2\times10^8$ m/s.

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