By signing in you confirm you are 18 or older, or have your parent’s or guardian’s consent.

NDA 2 2024 GAT, question 121

Topic: Physics

Q121

Which one among the following is the correct focal length of a combination of lenses of power 2.5 D2.5\text{ D} and −2.0 D-2.0\text{ D} ?

  1. (a) +0.5 m+0.5\text{ m}
  2. (b) −0.5 m-0.5\text{ m}
  3. (c) +2.0 m+2.0\text{ m}Correct answer
  4. (d) −2.0 m-2.0\text{ m}

Solution.

Answer: (c) +2.0 m+2.0\text{ m}

  1. For thin lenses in contact, powers add algebraically.
  2. The stated powers give P=0.5 D, so focal length f=1/P=2 m.
  3. The sign of each lens's power must be retained while adding.