An optical instrument employed for angular magnification contains an objective of 25 D and an eyepiece of 20 D . When the eye is least strained, the tube length is 25 cm . What type of instrument is this?

Step-by-step solution

The focal length of the objective lens is f_o = 100 P_o = 100 25 = 4 cm . The focal length of the eyepiece is f_e = 100 P_e = 100 20 = 5 cm . Since both the objective and the eyepiece have small focal lengths, the instrument is a compound microscope. In a telescope, the objective lens has a much larger focal length compared to the eyepiece. For a telescope in normal adjustment (least strained eye), the tube length would be L = f_o + f_e = 4 + 5 = 9 cm , which contradicts the given tube length of 25 cm . For a micro