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Chapter 12: Light Physics - Complete Theory and Formulas
Introduction to Light
Light is a form of electromagnetic wave that allows us to see objects around us. Objects can be classified as:
- Luminous objects: Produce their own light (e.g., lamp, fire)
- Non-luminous objects: Reflect light from other sources (e.g., wall, picture)
12.1 Reflection of Light
The Law of Reflection
Two fundamental principles:
- The angle of incidence (i) equals the angle of reflection (r)
- The incident ray, reflected ray, and normal all lie in the same plane
i = r
(where i = angle of incidence, r = angle of reflection)
Reflection Diagram
Properties of Mirror Images
Images formed by plane mirrors have these characteristics:
- Same size as the object
- Laterally inverted (left appears right, right appears left)
- Upright (not upside down)
- Virtual (cannot be captured on screen)
- Same distance from mirror as object
12.2 Refraction of Light
Refraction is the bending of light as it passes from one transparent medium to another. This occurs because light travels at different speeds in different materials.
Snell's Law (Law of Refraction)
For two given media, the ratio of sine of angle of incidence to sine of angle of refraction is constant.
sin i / sin r = constant = n
(where n = refractive index)
n = c / v
(where c = speed of light in vacuum, v = speed of light in medium)
n = real depth / apparent depth
Refraction Diagram
12.3 Total Internal Reflection
Total Internal Reflection (TIR) occurs when light traveling from a denser medium to a less dense medium is completely reflected back into the denser medium.
Conditions for Total Internal Reflection
- Light must travel from optically denser to optically less dense medium
- Angle of incidence must be greater than the critical angle
sin c = 1 / n
(where c = critical angle, n = refractive index)
i > c
(where i = angle of incidence, c = critical angle)
12.4 Refraction by Thin Lenses
Types of Lenses
- Converging Lens: Thicker at center, converges parallel rays
- Diverging Lens: Thinner at center, diverges parallel rays
Key Terms for Lenses
- Principal axis: Horizontal line through lens center
- Optical center (C): Midpoint of lens on principal axis
- Principal focus (F): Point where parallel rays converge
- Focal length (f): Distance from optical center to focus
Image Formation by Converging Lens
| Object Distance (u) | Image Characteristics | Image Distance (v) | Applications |
|---|---|---|---|
| u = ∞ | Inverted, Real, Diminished | v = f | Telescope objective |
| u > 2f | Inverted, Real, Diminished | f < v < 2f | Camera, Human eye |
| u = 2f | Inverted, Real, Same size | v = 2f | Photocopier |
| f < u < 2f | Inverted, Real, Magnified | v > 2f | Projector |
| u = f | Image at infinity | v = ∞ | Telescope eyepiece |
| u < f | Upright, Virtual, Magnified | Same side as object | Magnifying glass |
Converging Lens Ray Diagram
12.5 Dispersion of Light
Dispersion is the phenomenon where white light splits into its constituent colors when passing through a medium like a prism. This occurs because different wavelengths of light have different refractive indices.
Visible Spectrum (ROYGBIV)
- Red - Longest wavelength, lowest frequency, least refraction
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet - Shortest wavelength, highest frequency, most refraction
Dispersion Through Prism
Summary of Key Formulas
Applications Summary
Reflection Applications
- Blind corner mirrors for safety
- Periscopes for submarine navigation
- Instrument scales to avoid parallax error
- Vision testing in optical shops
Total Internal Reflection Applications
- Optical fibers for telecommunications
- Endoscopes for medical examinations
- Prisms in binoculars and periscopes
- SLR camera viewfinders
Lens Applications
- Magnifying glasses for close examination
- Spectacle lenses for vision correction
- Camera lenses for photography
- Telescope and microscope lenses
Study Tip
Remember the key principle: Light behavior changes when it moves between different media. Whether it's reflection, refraction, or total internal reflection, understanding the interaction between light and different materials is fundamental to mastering optics.
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