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DSSSB TGT Light — practice questions

Explore 13 free Light questions in DSSSB TGT Science. Open each question or exercise to read its content, reference answers and available explanation.

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Light questions and exercises with answers

Showing questions 1–13 of 13.

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 →A convex lens has power +4 D. Its focal length isRead the answer and worked solution →An object is placed at the centre of curvature of a concave mirror. The image formed isRead the answer and worked solution →A convex lens forms a real image three times the size of the object on a screen. If the focal length is 20 cm, the object distance isRead the answer and worked solution →The laws of reflection hold good forRead the answer and worked solution →A ray of light travelling in air enters water (n = 4/3). It bendsRead the answer and worked solution →An object 4 cm tall is placed 15 cm from a convex lens of focal length 10 cm. The image height isRead the answer and worked solution →A small bulb is placed at the focus of a concave mirror of focal length 12 cm. The reflected beam isRead the answer and worked solution →The power of a lens is −2.5 D. The lens isRead the answer and worked solution →For a concave mirror, as an object moves from infinity towards the focus, the real imageRead 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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