Geometrical Optics: Feynman's Physics Explained
Using least time we can understand lenses and mirrors. A converging lens is thicker in the middle, so light through the center travels a shorter distance in glass but longer in air; the shape is tuned so every ray from an object point takes the same total time to reach the image point. That is how an image forms. The limits of this simple picture — blur, aberrations, and the ultimate sharpness of a lens — appear because light is really a wave, and 'least time' is only an approximation.
The big idea
A lens works by making every light path take the same time.
Think about it
Why can't even a perfect lens show you details smaller than the wavelength of the light it uses?
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