What is light? What are sources of light?

Light is a form of electromagnetic radiation visible to the human eye. It travels in waves and also behaves as particles called photons (wave-particle duality).

Properties:

• Speed: 299,792,458 m/s (≈3 × 10⁸ m/s) in vacuum — the fastest speed in the universe
• Visible wavelength range: 380 nm (violet) to 700 nm (red)
• Travels in straight lines (rectilinear propagation)
• Can be reflected, refracted, absorbed, and diffracted

The visible spectrum (VIBGYOR):

Violet – Indigo – Blue – Green – Yellow – Orange – Red

(Beyond: ultraviolet on one end, infrared on the other — both invisible to the human eye)

Sources of light:

Natural sources:

• The Sun primary source of light for Earth
• Stars (including our Sun)
• Fire and flames chemical combustion releasing light
• Bioluminescent organisms — fireflies, deep-sea fish, jellyfish
• Lightning electrical discharge in the atmosphere

Artificial sources:

• Incandescent bulbs electric current heats a filament until it glows
• Fluorescent tubes mercury vapor + phosphor coating converts UV to visible light
• LED electric current through a semiconductor releases photons; most energy-efficient
• Neon lights electric current through noble gases
• Lasers coherent, single-wavelength focused beams