Question
A student can distinctly see the object upto a distance 15 cm. He wants to see the blackboard at a distance of 3m. Focal length and power of lens used respectively will be
-4.8cm, -3.3D
-5.8cm, -4.3D
-7.5cm, -6.3D
-15.8cm, -6.33D
Solution
D.
-15.8cm, -6.33D
The rays emanating from a point actually meet at another point after reflection and/or refraction. That point is called the image of the first point. The image is real if the rays actually converge to the point, it is virtual if the rays do not actually meet but appear to diverge from the point from the point when produces backwards.
The student should use a lens which forms image at a distance of 15 cm of the object placed at 3m i.e object distance u= -3 m =-3000 cm, image distance v =-15 cm
