Geometrical optics / by Archibald Stanley Percival.

  • Percival, A. S. (Archibald Stanley), 1862-
Date:
1913
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Licence: In copyright

Credit: Geometrical optics / by Archibald Stanley Percival. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    ILLUMINATION—PINHOLES—SHADOWS We shall here confine ourselves to the study of some of the simplest properties and laws of Light. When we say that we see the sun or a tree we merely mean that we see the light that comes from them; the sun, of course, sends out light of its own, whereas the tree passes on or reflects the light that it receives from something else. It is obvious that in neither case do we see the thing itself; we are only conscious of a certain sense-impression derived from it. The nature of this sense-impression and the way in which it is developed from a physical stimulation of the retina, are problems that are still engaging the attention of physio- logists and psychologists; with questions such as these the science of Optics does not deal. We shall commence our study by the consideration of the following laws:— Law 1. In a homogeneous medium light is propagated in every direction in straight lines. Law 2. The intensity of illumination varies inversely as the square of the distance from the source of light, and it is greatest when the angle of incidence is 0. Law 3. When the incident rays of light are parallel, the intensity of illumination varies as the cosine of the angle of incidence. Our experience of shadows confirms the truth of Law 1. A luminous point in space sends out light in all directions,