Key Concepts: Optical Instruments

The Human Eye

• The eye uses a converging lens to focus light on the retina.
• Near point: ~25 cm (closest clear focus). Far point: infinity (for normal vision).
• Myopia (nearsightedness): Corrected with diverging lens. Hyperopia (farsightedness): Corrected with converging lens.

Magnifying Glass

• A converging lens held closer than its focal length produces a magnified, virtual, upright image.
• Angular magnification ≈ 25 cm / f.

Microscope

• Uses two converging lenses: objective (short f, near object) and eyepiece (longer f).
• Total magnification = M_objective × M_eyepiece.

Telescope

• Uses two lenses to view distant objects: objective (large f) creates an image at the focal point of the eyepiece.
• Angular magnification ≈ f_objective / f_eyepiece.