Optical instruments
A magnifier is a convex lens with a small focal length (1–10 cm); with the object inside the focus it gives a virtual, upright, enlarged image. Since the eye’s best viewing distance is D₀ = 25 cm, the magnification is K = D₀/F, i.e. 2.5–25 times. In a camera the lens (converging) projects a real, inverted, reduced image of the object onto a light-sensitive element inside; in a digital camera the image is divided into pixels (pixel – picture element), and more pixels give a sharper image. In a microscope the objective forms a real, inverted, enlarged image and the eyepiece enlarges it again like a magnifier; K ≈ l·D₀/(F₁F₂) (l – the distance between the lenses). A telescope increases the viewing angle of distant objects; a refractor has an objective lens, a reflector a concave mirror, and K = F_objective/F_eyepiece. Never look at the Sun through any optical instrument: it damages the eye.
Observation: look at newsprint or a salt crystal through a magnifier. How does the image change as you hold the lens closer to or farther from your eye? At what distance is it sharpest? Measure the magnifier’s focal length roughly (image of a distant window on paper) and estimate K = 25/F.