Beruni and Ibn Sina on light phenomena
In “Monuments of Past Generations” Beruni wrote that sunlight carries heat in itself, and he correctly explained a lunar eclipse as the Moon entering the Earth’s shadow and a solar eclipse as the Moon passing between the Sun and Earth. On the speed of light he wrote that the speed of a ray cannot be sensed. Beruni asked Ibn Sina why a transparent glass sphere filled with water can focus sunlight and burn, while one filled with air cannot; Ibn Sina explained it by refraction of light in water — we will see this clearly in the lesson on lenses. Rejecting the view of the ancient Greek Plato, Ibn Sina correctly concluded that rays do not leave the eye to light objects (otherwise we would see at night too), but rays coming from objects enter the eye, are refracted in the lens of the eye and form an image on the retina. He explained the rainbow as sunlight splitting into coloured rays in raindrops — mostly right; in today’s science the arc shape follows from the geometry of refraction and reflection in drops (at about 42°). Importantly, the scholars explained phenomena by experiment and observation rather than legend.
Observation (safe): put a transparent plastic bottle full of water on a page and watch the text look magnified; compare with an air-filled bottle. Safety: do not focus sunlight onto paper or skin with a water sphere or magnifier — risk of fire and burns. Do not look at the Sun.