1. Why do LEDs and artificial lights flicker or pulsate? Is this visible to the human eye?

LEDs and most artificial lights flicker due to their power supply:

• AC mains frequency: Household electricity (50/60 Hz) causes LEDs to turn on/off rapidly unless perfectly filtered. Cheap drivers produce 100–120 Hz ripple.
• Pulse-width modulation (PWM): Many LEDs dim via PWM (rapid on/off cycles, often 100–2000 Hz) to control brightness.
• Other causes: Faulty drivers, overheating, or poor capacitors.

Visibility:

• Human eye flicker fusion threshold is ~50–90 Hz for most; above 100 Hz, it's imperceptible to steady gaze but shows as strobing on cameras or fast motion.
• Sensitive people (e.g., with migraines) or low frequencies see it as pulsation. Your observation might blend flicker with visual effects (see Q4).

High-quality LEDs (DC-driven) minimize this.

2. Does the Sun actually pulsate, and if so, does this affect its light output in any perceptible way?

The Sun does pulsate, but imperceptibly:

• Solar oscillations: Acoustic p-modes (pressure waves) cause surface vibrations every ~5 minutes, varying brightness by ~0.001% (10 parts per million).
• Longer cycles: 11-year solar cycle modulates output by ~0.1%, plus tiny granulation (cellular convection).

These are detected by satellites (e.g., SOHO), not naked eye. No perceptible pulsation—Sunlight appears constant due to averaging over vast scale.

3. Do all light sources have some form of pulsation or oscillation, or are some truly constant?

No, not all pulsate macroscopically:

• Inherently pulsating:

  Source	Pulsation Cause	Frequency/Notes
  Fluorescent/CFL	AC ballast	100–120 Hz
  Cheap LEDs	AC ripple/PWM	100 Hz–kHz
  Stars (many)	Pulsation instability	Hours–days

• Truly constant (steady output):
  • Incandescent/halogen bulbs: Thermal inertia of filament smooths AC to near-DC.
  • Continuous-wave (CW) lasers: Stable output via steady pumping.
  • High-end LEDs/strobes: DC power or high-frequency (>20 kHz) PWM.
  • Ideal blackbodies (e.g., cavity radiators): Steady if input is constant.

Quantum mechanically, all light has photon shot noise (Poisson fluctuations), but it's negligible for bright sources.

4. What causes the visual perception of pulsation or halos around bright lights—is it the light source itself or something about how our eyes/brain process light?

Your "pulsating with light radiating outward" is likely perceptual, not the source:

• Source-independent causes (eye/brain):
  • Diffraction/aberrations: Pupil acts as aperture; point sources (e.g., LED) produce rings/rays (Airy disk). Common at night.
  • Glare/halos: Scattering in cornea/lens (worse with cataracts, dry eyes, dirty glasses). "Radiating outward" matches starbursts from spherical aberration.
  • Phosphenes/neural effects: Bright light overstimulates retina, causing perceived flicker/pulsation (entoptic phenomena).
  • Foveal processing: Brain fills gaps in peripheral vision.
• Source-related: If flickering (Q1), it amplifies perception, especially in low light.
• Test: View through pinhole (reduces aberrations) or phone camera (reveals true flicker).

Bottom line: Halos/pulsation around LEDs is usually optical artifacts in your eye, not bulb instability. Consult optometrist if persistent.