Geometric Optics – Mirrors, Lenses & Image Formation Explained | Chapter 34 of University Physics

Chapter 34 covers how light forms images by reflection and refraction, using the ray model and simple geometry. You’ll learn the mirror and lens equations, magnification rules, and see applications in cameras, the human eye, microscopes, and telescopes.

Watch the full video summary on YouTube for step-by-step ray diagram demonstrations.

Plane Mirrors

Plane mirrors produce virtual, erect images behind the mirror. Key properties:

• Image distance equals object distance: s′ = –s
• Lateral magnification: m = +1
• Images are same size and orientation as the object

Spherical Mirrors

Spherical mirrors follow the mirror equation:

1/s + 1/s′ = 1/f, where f = R/2. Magnification is m = –s′/s. Behavior differs by mirror type:

• Concave (f>0):
  • Object beyond f → real, inverted image
  • Object inside f → virtual, erect, magnified image
• Convex (f<0):
  • Always produces virtual, erect, reduced images

Use principal rays to locate images; parabolic mirrors reduce spherical aberration.

Curved Refracting Surfaces & Thin Lenses

Refraction at a curved interface obeys:

n₁/s + n₂/s′ = (n₂–n₁)/R, with magnification m = –(n₁ s′)/(n₂ s).

Thin Lenses

• Lens equation: 1/s + 1/s′ = 1/f
• Magnification: m = –s′/s
• Converging (f>0):
  • Object beyond f → real, inverted image
  • Object inside f → virtual, erect, magnified image
• Diverging (f<0):
  • Always virtual, erect, reduced image
• Lensmaker’s equation: 1/f = (n–1)(1/R₁ – 1/R₂)

Optical Instruments

Cameras

Converging lenses form real, inverted images on a sensor. Changing the lens–sensor distance focuses the image. Focal length controls zoom and field of view; aperture diameter (D) affects brightness with f-number = f/D.

Human Eye & Vision Correction

The eye accommodates by changing lens shape. Common corrections:

• Myopia (nearsighted): diverging lens
• Hyperopia (farsighted): converging lens
• Astigmatism: cylindrical lens

Lens power P (in diopters) = 1/f (in meters).

Microscopes & Telescopes

• Compound Microscope: objective forms a real image; eyepiece magnifies. Total M ≈ (s′₁ / f₁) × (25 cm / f₂).
• Refracting Telescope: objective (long f) creates a real image; eyepiece magnifies. M = –f₁/f₂. Reflecting telescopes use a concave mirror as the objective.

Conclusion

Mastering mirror and lens equations, magnification, and ray diagrams empowers you to analyze and design diverse optical systems—from simple magnifiers to advanced telescopes and imaging devices.

For detailed examples and visual guides, watch the full video summary.

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