Light
1. What is Light?
Strictly speaking, "light" usually refers to visible light—the tiny part of the electromagnetic spectrum that the human eye can detect. However, in physics, the term often encompasses the entire spectrum of electromagnetic radiation.
The Electromagnetic Spectrum: Visible light is sandwiched between infrared (IR) and ultraviolet (UV) radiation. The spectrum ranges from radio waves (long wavelength, low energy) to gamma rays (short wavelength, high energy).
Color: What we perceive as color is actually different wavelengths of light. Red has the longest wavelength in the visible spectrum, while violet has the shortest.
2. The Physics of Light
Light has two very famous and unique characteristics that distinguish it from matter.
Wave-Particle Duality: Light behaves like a wave (it can bend around corners and interfere with itself) and also like a particle (it travels in discrete packets of energy called photons).
The Speed Limit: In a vacuum, light travels at approximately 299,792,458 meters per second (roughly $3 \times 10^8$ m/s). According to Einstein’s theory of relativity, nothing in the universe can travel faster than this speed.
3. How Light Behaves
When light interacts with objects, it generally does one of three things: reflection, refraction, or absorption.
Reflection: When light hits a smooth surface (like a mirror) and bounces off. The angle at which it hits is equal to the angle at which it bounces off.
Refraction: When light passes from one medium to another (e.g., from air into water), it changes speed and bends. This is why a straw looks "broken" in a glass of water and how lenses (like in glasses or cameras) focus images.
Absorption: When light is taken in by an object and converted to heat. The color of an object is determined by which wavelengths it absorbs and which it reflects. For example, a red apple absorbs all colors except red, which it reflects back to your eye.
4. Sources of Light
Natural Sources: The Sun (our primary source), stars, lightning, and fire. Some animals produce their own light through chemical reactions, a process called bioluminescence (e.g., fireflies, deep-sea jellyfish).
Artificial Sources: Incandescent bulbs (heating a filament), LEDs (moving electrons through a semiconductor), and Lasers (highly focused, coherent beams of light).