What actually whitens
One ingredient does the work: peroxide. Hydrogen or carbamide, strength × time, and the methods honestly ranked.
Every product that works has the same ingredient. Everything else is marketing.
The one ingredient
Peroxide — hydrogen or carbamide. It breaks into oxygen molecules that pass through enamel and break apart stain inside dentin. No peroxide, no real whitening. 10% carbamide is roughly 3.5% hydrogen; carbamide is slower and gentler.
Strength × time
Low and slow gets you to the same place as strong and fast. High strength: short wear, faster, more zing. Low strength: longer wear, a week slower, far less sensitivity. There is no shortcut around total peroxide exposure — pick the version your teeth can tolerate.
The light myth
The blue light does nothing. Studies haven't shown LED lights add whitening beyond what the gel does alone. The light warms the gel; you're paying for the box and the glow. Buy the gel. Skip the light.
Do this now
Flip the box over and find the peroxide percentage. If it isn't listed, put it back. Crowded or rotated teeth: trays. Straight teeth, mild stain: strips are a fine start.