A person uses spectacles of power −2.5 D. What defect do they have and what is the focal length?

Negative power → concave lens → corrects Myopia (short-sightedness). Focal length f = 1/P = 1/(−2.5) = −0.4 m = −40 cm. The person cannot see objects clearly beyond ~40 cm without glasses.

Why does a convex lens form both real and virtual images?

When the object is beyond the focal point, refracted rays converge to form a real image on the other side. When the object is between the focal point and the lens, refracted rays diverge and appear to come from a point on the same side as the object — a virtual image. The simple magnifying glass uses a convex lens with the object inside the focal length.

Lens Formula, Magnification, and Power of Lenses

Ray Optics and Optical Instruments • Class 12 Physics • NCERT • CBSE

Lens formula: 1/f = 1/v − 1/u. Power P = 1/f (D). Combination of lenses in contact: P = P₁ + P₂ + P₃. Linear magnification m = v/u = h'/h.

Key Formulas

Lens formula: 1/f = 1/v − 1/u
Magnification: m = v/u
Power: P = 1/f (f in metres, P in Dioptre)
Combined power: P = P₁ + P₂

Frequently Asked Questions

A person uses spectacles of power −2.5 D. What defect do they have and what is the focal length?: Negative power → concave lens → corrects Myopia (short-sightedness). Focal length f = 1/P = 1/(−2.5) = −0.4 m = −40 cm. The person cannot see objects clearly beyond ~40 cm without glasses.
Why does a convex lens form both real and virtual images?: When the object is beyond the focal point, refracted rays converge to form a real image on the other side. When the object is between the focal point and the lens, refracted rays diverge and appear to come from a point on the same side as the object — a virtual image. The simple magnifying glass uses a convex lens with the object inside the focal length.

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