Question

If a spherical lens has a power of, −0.25 D, the focal length of this lens will be (a) −4 cm
(b) −400 mm
(c) −4 m
(d) −40 cm

Answer 1

P = 1/f

-0.25 = 1/f

f = -1/0.25 m

f = -4m

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Answer 2

If a spherical lens has a power of, −0.25 D, the focal length of this lens will be −4 m.

Explanation:

When the spherical lens has -0.25 D as the power, then the lens’ focal length will be -4 m. It is shown as Focal length = 1 Power = 1− 0.25 = −4 m.

• An optical system’s focal length is a measure to show how strongly the system diverges or converges light.

• It is the optical power of the system and it is inverse.

• The indication of a positive focal length is by a system converging light, whereas the system diverging light is indicated by the negative focal length.

Therefore the correct answer is Option(c) -4 m.