Momento do Fóton (p=E/c)

Photon momentum: p = h/λ = E/c

A photon has no mass, yet it still carries momentum: p = h/λ = E/c, where h = 6.626·10⁻³⁴ J·s and c = 3·10⁸ m/s. Take a green photon with E = 3.6·10⁻¹⁹ J, and you get p ≈ 1.2·10⁻²⁷ kg·m/s. Hit a surface with light and that momentum gets transferred, which is what we call radiation pressure. Any single photon does almost nothing, but it adds up. That is why a comet shows two tails at once: an ion tail, blown straight back by the solar wind, and a curved dust tail that gets nudged by the radiation pressure of sunlight. The same idea drives solar sails. JAXA's IKAROS (2010) was the first one flown between planets, and the Planetary Society's LightSail 2 (2019) raised its orbit around Earth on nothing but sunlight.

Applications

Solar sails for deep-space propulsion. Absolute radiometers, where the Nichols radiometer reads radiation pressure directly. Optical tweezers (Arthur Ashkin, Nobel 2018), which trap and move single cells, viruses and DNA. Laser cooling of atoms. And the Yarkovsky effect, which slowly shifts asteroid orbits over time.

FAQ

How can a photon have momentum without mass? Relativity gives E² = (pc)² + (mc²)². Set m = 0 and that collapses to E = pc, so p = E/c. The momentum traces back to energy, not to rest mass.

Why two comet tails? The charged solar wind drives the ion tail, which points straight away from the Sun. Sunlight's radiation pressure drives the dust tail, which lags behind and curves along the comet's orbit.

Do solar sails really work? They do. IKAROS (JAXA, 2010) reached Venus on sunlight, and LightSail 2 (2019) kept raising its orbit around Earth the same way.