If the focal length of a lens is -25 cm, the power of the lens will be: 

The correct answer is -4D.

Key Points

• The power of a lens (P) is calculated using the formula P = 1/f, where f is the focal length in meters.
• Given the focal length (f) is -25 cm, which is -0.25 meters (since 1 cm = 0.01 meters).
• Substituting the value into the formula: P = 1 / -0.25 = -4 Diopters (D).
• A negative power indicates that the lens is a diverging (concave) lens.
• Therefore, the power of the lens is -4D, making option 3 the correct answer.

Additional Information

• Diopter (D): A unit of measurement for the optical power of a lens, defined as the reciprocal of the focal length in meters.
• Focal Length (f): The distance between the center of a lens and its focus, where parallel rays of light converge or appear to diverge.
• Concave Lens: A lens that diverges light rays passing through it, typically used to correct myopia (nearsightedness).
• Convex Lens: A lens that converges light rays passing through it, typically used to correct hyperopia (farsightedness).
• Lens Formula: The relationship between the object distance (u), image distance (v), and focal length (f) of a lens, given by 1/f = 1/v + 1/u.