Shedding Light on Light*

There are four books describing early studies and views on light, optics and vision that are considered to be landmarks in this branch of science. The first of these is Euclid’s Optica, published about 300BC, in which he postulated that light travelled in straight lines; he described the laws of reflection and described them mathematically. The second is by Ptolemy. He is best known for his 15 volume treatise on astronomy, Almagest, but he also wrote five books on optics around 100AD (only one of which survives to modern days). Ptolemy is credited with stressing the importance of basing theory on experiment, and in his Optics he studies colour, reflection and refraction, and mirrors of various shapes. The third is by Ibn al-Haitham, known later as Alhazen, who wrote a book on optics around 1000AD, which was later translated into Latin as Opticae Thesaurus. This describes experiments with mirrors and lenses, but is most famous for refuting a widely held view of Greek scholars (but not a unanimous view), that vision was due to rays emanating from the eyes, and hitting the object being viewed. He had several arguments against this view, the most telling being that it could not explain why some objects shone brightly (the Sun), and others dimly. He also noted the significance of the camera obscura, or pinhole camera, in which light from an object comes through a small hole and produces and inverted image. The last book, and the one which expresses views most consistent with modern science, is Newton’s Opticks, of which more later.