Calculate the focal length of a reading glass of a person if his distance of distinct vision is 75 cm.

Options

A. `37.5 `mm
B. `100.4` cm
C. `25.6 ` cm
D. `75.2 ` cm

Answer

A

Explanation

The distance of normal vision is 25 cm . So if book to be read is at u = -25 cm , its image should be formed at v = = - 75 cm . Therefore , the required focal length (f) is
`1/f = 1/v - 1/u `
` = 1/(-75 cm) - 1/(-25 cm)`
`(-1)/(75 cm) + 1/(25 cm ) = (-1+3)/(75 cm ) = 2/(75 cm ) `
` f = (75cm)/2 = 37.5 cm `