1) Why LEDs and other artificial lights “pulsate” (flicker)

Most modern artificial lights are not powered by perfectly steady current, so their light output is often modulated in time:

• Mains-powered lamps (AC): In many countries the line frequency is 50 or 60 Hz. If the lamp’s light output follows the instantaneous power, you can get modulation at 100/120 Hz (because power goes like (V^2), giving two peaks per cycle).
• LEDs specifically: LEDs respond very fast to current changes, so any ripple in the driver current becomes light flicker. Common causes:
  • Cheap/simple drivers that rectify AC with minimal smoothing → strong 100/120 Hz flicker.
  • LED drivers using PWM (pulse-width modulation) for dimming → flicker at anywhere from tens of Hz to many kHz.
  • Switching power supplies → can add higher-frequency ripple.

Is it visible?

• Many people won’t perceive 100/120 Hz flicker as obvious “blinking” when staring steadily, but it can still be noticeable:
  • in peripheral vision
  • as a stroboscopic effect on moving objects (fans, spinning wheels)
  • on phone cameras (rolling shutter makes it very obvious)
  • as eye strain/headache for some individuals (sensitivity varies)
• Lower-frequency PWM (e.g., <200 Hz) is more likely to be consciously seen.

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2) Does the Sun pulsate, and does it change its brightness?

The Sun is not perfectly steady, but it does not pulsate like a lamp on the timescales and amplitudes you’re describing.

There are two relevant effects:

• Solar oscillations (helioseismology): The Sun has many acoustic modes with periods around ~5 minutes. These are real “pulsations” of the surface, but the associated brightness variations are tiny (parts per million to parts per thousand depending on what you measure and where). They are not perceptible by eye.
• Solar activity (sunspots, faculae, flares):
  • Over the 11-year cycle, total solar irradiance varies by about ~0.1%.
  • Solar flares can be dramatic in X-rays/UV, but the change in visible light at Earth is usually small and brief.

So: yes, the Sun varies, but not in a way that produces visible flicker or a “pulsing halo.”

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3) Do all light sources have some oscillation, or can any be truly constant?

At a fundamental level, light emission is quantized (photons) and often produced by random microscopic events (spontaneous emission, thermal radiation). That implies:

• There is always some statistical “photon noise” (shot noise) if you look with sufficiently sensitive, fast detectors.
• Thermal sources also have microscopic fluctuations.

However, for everyday purposes many sources are effectively constant because the fluctuations are extremely small relative to the average when many photons are involved.

Whether you get macroscopic flicker depends mostly on the power supply / driving electronics (for LEDs, fluorescents) or on mechanical instabilities (some arc lamps, candles, etc.), not on an intrinsic “pulsation of light itself.”

Incandescent bulbs, for example, can have 100/120 Hz modulation from AC power, but the filament’s thermal inertia smooths it, so flicker is usually much less than in poorly driven LEDs.

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4) Why you perceive pulsation/halos around bright lights

What you describe (“light radiating outward,” halo-like effects, sometimes seeming to pulse) is very often not the light source changing, but optics + physiology:

Optical causes (real light redistribution)

• Scattering in the eye: Light scatters in the cornea/lens/vitreous humor, producing glare and halos—worse with dry eyes, cataracts, or dirty/greasy surfaces.
• Diffraction: The pupil and eyelashes can create starbursts or rings.
• Tear film dynamics: A slightly uneven tear film can change from moment to moment (especially as you blink), making halos appear to shimmer or pulse.
• Glasses/contact lenses/windows: Smudges and micro-scratches strongly enhance halos.

Neural/perceptual causes (your visual system)

• Microsaccades (tiny involuntary eye movements) and adaptation can make a steady light seem to “shimmer.”
• Peripheral sensitivity to flicker: Even if you don’t see direct flicker, your peripheral vision may detect modulation.
• Afterimages/adaptation: Bright lights saturate photoreceptors; as they recover, you can perceive changing “aura” effects.

How to tell which it is

• Record it with a phone camera:
  • If the camera shows banding/flicker, it’s likely real intensity modulation (driver/mains/PWM).
  • If the camera looks steady but your eyes see halos/pulsation, it’s more likely eye optics/physiology.
• Try cleaning the bulb cover, any lampshade, and your glasses; blink deliberately; compare with another lamp.

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Bottom line

• LEDs often flicker because of their drivers and AC/PWM modulation; it can be visible or subtly perceptible.
• The Sun varies, but not with visible flicker.
• No light source is perfectly constant at microscopic scales, but many are effectively constant macroscopically.
• The halo/pulsation you perceive is frequently dominated by scattering/diffraction in your eye and visual processing, sometimes combined with real LED flicker.

If you tell me the bulb type (dimmable/non-dimmable, any dimmer switch, country mains frequency) and whether your phone camera shows flicker bands, I can help narrow down the cause.