Sunscreen is often described as a barrier, which suggests a shield sitting on top of skin. The actual mechanism is more interesting and explains most of a product's behaviour.
Absorption is the dominant mechanism
Most filters are organic molecules that absorb ultraviolet energy directly. The molecule takes in the photon, becomes briefly excited, then releases the energy as a small amount of heat.
This happens at the molecular level rather than at the surface, which is why a well-formulated sunscreen can be nearly invisible while still doing substantial work.
Different filters absorb different portions of the ultraviolet range. No single one covers the whole span well, so broad protection normally requires several working together.
Mineral filters scatter as well as absorb
Zinc oxide and titanium dioxide are often called physical blockers. They do reflect and scatter some light, but a large share of their protection also comes from absorption.
Because they are solid particles suspended in a base, they sit more visibly on skin. That is the origin of the white cast associated with mineral formulas.
Grinding the particles finer reduces the cast but changes how they interact with light, so formulators trade cosmetic elegance against protection across parts of the spectrum.
Why films need to be even
Protection depends on a continuous layer. Ultraviolet light passing through a thin patch is not stopped, and the skin underneath receives close to full exposure.
This is why application amount matters so much. A thin smear of a high-rated product performs far below its label because the film it forms is patchy rather than uniform.
Ratings are measured under a standardised thick application in laboratory conditions, which is considerably more product than most people apply in ordinary use.
What degrades the film during the day
Some filters become less effective after prolonged light exposure as their molecules break down. Stabilising ingredients are added specifically to slow that process.
Mechanical loss matters more in practice. Sweat, water, towels, clothing and simple face-touching remove the film unevenly across the course of a few hours.
Reapplication is a response to that physical removal rather than to the chemistry wearing out, which is why it is advised after swimming and heavy sweating in particular.
Why texture varies so much between products
Organic filters are often oils or solids that must be dissolved into a carrier. The choice of carrier drives whether a formula feels light, greasy or heavy on skin.
Mineral particles need dispersing agents to stay evenly suspended, and those agents affect finish and how the product behaves under makeup.
Regional rules differ on which filters are permitted, which is one reason a sunscreen bought in one country can feel entirely unlike one bought in another.