Optics Camera obscura Optical lessons Commentary

Abu-‘Ali Al-Hasan ibn Al-Haytham (Alhazen) optics

Optics

• Alhazen was familiar with the work
• of Aristotle, Ptolemy, and Euclid
• and worked to correct the errors
• in their understanding of optics.
• Alhazen stressed the need for experimentation.
• Euclid was wrong; Aristotle was right;
• we see not from light emitted by our eyes
• but from light entering them.
• Alhazen held that light is emitted or reflected
• from objects at all angles
• but he struggled to understand
• how we see images clearly
• when a profusion of rays from every point
• hits the eyes at different angles.
• It must be, he said, that our eyes see
• only rays that hit them perpendicularly.

Camera obscura

• Translation into Latin gave us our spelling
• “Alhazen,” which today
• would not be transliterated the same way.
• Alhazen wrote over two hundred books in Arabic,
• but only fifty-five have survived
• and not all of these have been translated into English.
• An image projected through a pinhole lens
• is dim and rotated
• one hundred and eighty degrees.

Optical lessons

• Some objects emit light, some reflect it,
• some refract or defract it, and some absorb it.
• Some are opaque; others are transparent
• or partly transparent to all or only some colors.
• The angle of light that hits a mirror
• is equal to the angle of light that bounces off.
• Prisms, lenses, and small openings refract light,
• bending it toward the perpendicular when entering a lens
• and away from the perpendicular when leaving.
• Thomas Young’s famous double-slit experiment
• demonstrated diffraction and proved light is a wave.
• Heisenberg and others showed it’s also a beam of particles.
• Light travels in an elliptical wave
• and many wavelengths can travel together incoherently,
• but a polarized filter blocks waves in all but one direction.
• So here I am this rainy morning, thinking
• there ought to be many analogies in optics
• to intellectual and spiritual enlightenment,
• whether I am a source of light or reflect it,
• whether, being optically dense, I refract light,
• and whether I am incoherent or polarized.
• I sit in my living room on the couch,
• enjoying the gurgling and pitter-patting of rain.
• If the sun were to peek from the clouds,
• I would be willing to go outside
• to look for the promised rainbow.