What is the difference between the mirror formula and the lens formula?

Mirror formula: 1/f = 1/v + 1/u (plus sign between 1/v and 1/u). Lens formula: 1/f = 1/v − 1/u (minus sign). Magnification: mirrors use m = −v/u; lenses use m = v/u (no minus sign). Easy memory: Mirror = Plus, Lens = Minus.

What is the power of a lens and what unit is it measured in?

Power of a lens = 1/focal length (in metres). Unit: Dioptre (D). Convex lenses have positive power; concave lenses have negative power. Example: f = +50 cm = +0.5 m → P = +2D. Power is used by opticians when prescribing spectacles — '+2.5D' means a convex lens of 40 cm focal length.

When does a concave mirror form a virtual image?

A concave mirror forms a virtual, erect, and magnified image only when the object is placed between the pole (P) and the focus (F) — i.e., closer than the focal length. In all other positions, the concave mirror forms real, inverted images. This is why concave mirrors are used as shaving/makeup mirrors — placed close to the face, they magnify.

Mirror Formula and Lens Formula

Light — Reflection and Refraction • Class 10 Science • NCERT • CBSE

The mirror formula is 1/f = 1/v + 1/u, where f is focal length, v is image distance, and u is object distance. The lens formula is 1/f = 1/v − 1/u. Magnification m = −v/u for mirrors and m = v/u for lenses.

Key Formulas

Mirror formula: 1/f = 1/v + 1/u
Lens formula: 1/f = 1/v − 1/u
Magnification (mirror): m = −v/u
Magnification (lens): m = v/u
Power of lens: P = 1/f(m) [in Dioptres]
Focal length: f = R/2

Frequently Asked Questions

What is the difference between the mirror formula and the lens formula?: Mirror formula: 1/f = 1/v + 1/u (plus sign between 1/v and 1/u). Lens formula: 1/f = 1/v − 1/u (minus sign). Magnification: mirrors use m = −v/u; lenses use m = v/u (no minus sign). Easy memory: Mirror = Plus, Lens = Minus.
What is the power of a lens and what unit is it measured in?: Power of a lens = 1/focal length (in metres). Unit: Dioptre (D). Convex lenses have positive power; concave lenses have negative power. Example: f = +50 cm = +0.5 m → P = +2D. Power is used by opticians when prescribing spectacles — '+2.5D' means a convex lens of 40 cm focal length.
When does a concave mirror form a virtual image?: A concave mirror forms a virtual, erect, and magnified image only when the object is placed between the pole (P) and the focus (F) — i.e., closer than the focal length. In all other positions, the concave mirror forms real, inverted images. This is why concave mirrors are used as shaving/makeup mirrors — placed close to the face, they magnify.

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