Ray Optics and Optical Instruments – JEE Mains Physics
1. Introduction
Ray optics (or geometrical optics) is a branch of optics that describes the behavior of light in terms of rays. It deals with reflection, refraction, and the formation of images by mirrors and lenses.
2. Reflection of Light
- Law of Reflection: Angle of incidence = Angle of reflection
- Plane mirror: Image is virtual, erect, and laterally inverted
- Spherical mirrors (concave & convex): Use the mirror equation
1/f = 1/v + 1/u
3. Refraction of Light
Refraction is the bending of light when it passes from one medium to another due to a change in speed. Described by Snell's Law: n₁ sinθ₁ = n₂ sinθ₂
4. Total Internal Reflection (TIR)
- Occurs when light travels from a denser to rarer medium
- Condition: Angle of incidence > critical angle
- Applications: Optical fibers, mirage, diamond sparkle
5. Lenses and Image Formation
- Convex lens: Can form real/inverted or virtual/erect images
- Concave lens: Always forms virtual, erect, and diminished images
- Lens formula:
1/f = 1/v - 1/u - Magnification:
m = h'/h = v/u
6. Power of a Lens
Power is defined as the reciprocal of focal length in meters. P = 100/f (in cm)
Unit: Diopter (D)
7. Combination of Lenses
When two or more lenses are placed in contact: 1/F = 1/f₁ + 1/f₂ + ...
Total magnification: Product of individual magnifications.
8. Optical Instruments
- Simple Microscope: Uses convex lens to enlarge close objects
- Compound Microscope: Two convex lenses for higher magnification
- Telescope: Used to view distant objects, uses objective and eyepiece lenses
- Human Eye: Natural optical instrument with adjustable lens
- Defects of Vision: Myopia, hypermetropia, presbyopia, corrected using lenses
9. Important Diagrams to Practice
- Ray diagrams for concave and convex mirrors
- Ray diagrams for convex and concave lenses
- Diagrams for microscope and telescope working