Lenses

Thicker at the center, Can converge light rays, Can form real images

A convex lens is thicker at the center and thinner at the edges. It can converge parallel light rays to a focus point. It can form both real images (when object is beyond focus) and virtual images (when object is between focus and lens).

Principal focus

The principal focus (F) is the point on the optic axis where light rays parallel to the axis converge after refraction through a convex lens. This is a fundamental definition in lens optics.

They diverge light rays, They have negative focal length, They always form virtual images

Concave lenses are diverging lenses that spread out light rays. They have negative focal length and negative power. They always form virtual, erect, and diminished images regardless of object position.

8 cm

Given: h₀ = 4 cm, u = -15 cm, f = 10 cm. First find v: 1/10 = 1/v - 1/(-15), so 1/v = 1/10 - 1/15 = 1/30, v = 30 cm. Magnification m = v/u = 30/(-15) = -2. Image height hᵢ = m × h₀ = (-2) × 4 = -8 cm. The magnitude is 8 cm (negative indicates inverted).