In an experiment to determine the focal length $(f)$ of a concave mirror by the $u-v$ method, a student places the object pin $A$ on the principal axis at a distance $x$ from the pole $P$. The student looks at the pin and its inverted image from a distance keeping his/her eye in line with PA. When the student shifts his/her eye towards left, the image appears to the right of the object pin. Then

• A

  $x < f$
• Correct answer
  B

  $f < x < 2 f$
• C

  $x=2 f$
• D

  $x>2 f$

Solution

Since object and image move in opposite directions, the positioning should be as shown in the figure. Object lies between focus and centre of curvature $$ f < x < 2 f \text {. } $$ $\therefore$ Correct option is (b).

Hence, the correct answer is option: 'B'.