Question details is given in picture below?
1 Answer
Explanation:
It is easy to see that for the needle to coincide with the image, the object must be placed at the focus of the lens - so that after refraction through the lens, light emerges as a parallel beam, falls normally on the mirror and is reflected right back, converging back at the focus on its return journey. So, the distance between the lens and the object must be equal to the focal length of the lens in magnitude.
Thus, the effective focal length of the glass lens + liquid lens combination is
Now, using the lens-maker's formula, we have
and again, for the liquid lens
Thus