Bending light on purpose
How Do Lenses Work?
Curved glass slows light unevenly, and uneven slowing makes it turn.
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Step 1 of 5
Light slows down inside glass.
In air light moves at 300,000 km per second. In glass it drops to about 200,000. Slower, but still very fast.
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Step 2 of 5
Entering at an angle makes it turn.
If one edge of the beam reaches the glass first, that edge slows first and the beam pivots — like a trolley with one wheel hitting grass.
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Step 3 of 5
A curved surface bends each ray differently.
In a convex lens the outer rays meet the glass at a steeper angle than the middle ones, so they turn more. All of them end up heading for the same point.
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Step 4 of 5
Fatter lens, shorter focus.
A strongly curved lens bends light hard and brings it to a point quickly. A gentle curve focuses far away. That number is the focal length.
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Step 5 of 5
Concave lenses do the opposite.
Thinner in the middle, they spread light apart instead of gathering it. Short-sighted glasses use these to move your eye's focus back onto the retina.
Your eye has a lens made of living tissue, and muscles squeeze it fatter or thinner to focus at different distances.
The short version
A lens works by slowing light unevenly across a curved surface, so the beam bends toward or away from a point.
Try it yourself
Put a drop of water on a phone screen and look through it. The curved drop is a lens, and the text jumps in size.
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