ELI5: Does a photon have a physical size?

No, a photon does not have a physical size in the way everyday objects do; it is a point particle with zero width, height, or volume.

To understand this, think about how we measure things in everyday life. If you hold a marble, you can measure it with a ruler because it has hard edges. A photon, which is a single tiny packet of light, is completely different because it has no solid surface or edges at all.

Even though a photon has no physical size, it can still act like it takes up space in two everyday ways:

• The ripples in a puddle: When you toss a pebble into water, the splash creates waves. The ripple spreads out, but the water itself is not a solid object. A photon acts like a wave, and its wavelength is like the distance between those ripples. For the light you see, these ripples are extremely tiny—far thinner than a single strand of hair.
• The swinging jump rope: Imagine a jump rope spinning fast. The rope itself is very thin, but because it is swinging in a wide loop, it creates a large danger zone where nobody can walk. The photon is like the thin rope, while its wavelength is the wide swinging area that affects everything around it.

• Microwave doors: The metal mesh on your microwave door has holes that let you see inside, but the holes are too small for the microwave photons to squeeze through. The invisible light waves get trapped because their wavelength is bigger than the holes.
• Eyeglasses and dust: Light photons bounce off walls, cars, and dust motes, not because the photon is physically fat, but because its wave is the right size to notice those objects.