OPTICS was defined by Sir David Brewster as that branch of knowledge which treats of the properties of light and vision as performed by the human eye. The modern divisions of this science may be stated to be as follows :—(i) Light and its sources ; (2) transmission, velocity, and intensity of light ; (3) reflection and mirrors ; (4) single reflection and lenses ; (5) dispersion and achromatism ; (6) optical instruments ; (7) the eye as an optical instrument ; (8) phosphorescence and fluorescence, and (9) double refraction interference and polarization.
Light and its is the agent which by its action on the retina excites in us the sensation of vision. Various hypotheses have been made as to the origin of light, the two most important being the emission or corpuscular theory and the undulating theory. These will be found more clearly de fined under the heading Light (q.v.) Transmission, Velocity and Intensity of ous bodies are those which emit light such as the sun, and ignited bodies such as a candle. Transparent or diaphan ous bodies are those which readily transmit light. Trans lucent bodies transmit light but partially, as objects cannot be seen through them as in the case with transparent bodies. Opaque bodies do not transmit light.* It is a well-known fact that light always travels in straight lines, and cannot turn a corner. The term ray of light is used to indicate the straight line along which light progresses. If the luminous body be central, rays of light are emitted from all its points and in all directions. A collection of rays emitted from a luminous point, and circumferentially so limited in their passage as to form a conical outline, is called a diverging pencil of rays, the apex of the cone from which they proceed being called the focus of the pencil. See Fig. 305.
If the rays proceed from so distant a point that any divergency is inappreciable, the rays are termed parallel rays.
See Fig. 307.
Every luminous body emits divergent rectilinear rays from all its points and in all direc.
tions. As light travels in straight lines it will be easily understood that if any opaque body be interposed between the source of light and the eye it will prevent it from being seen. The light cannot penetrate into the space behind it, and this space is termed the shadow.
When, however, they are made to converge to a common point they are described as a converg ing pencil of rays, and the point at which they meet is termed the focus. See Fig. 306.
If a small aperture be made in an opaque substance, such as, for instance, a piece of tinfoil, and luminous rays made to pass through this aper ture into a dark chamber and thrown on to a screen, they will form images of the external objects.
See Fig. 308.
These images are always inverted owing to the fact that the luminous rays proceeding from the external objects across one another on their passage, as shown in the diagram Light moves at a velocity of about 190,000 miles in a second. M. Foucalt constructed an apparatus for measuring the velocity of light based on the use of the rotating mirror adopted by Wheatstone in measuring electricity. With this arrangement he calculated the speed to be 185,157 miles per second. The intensity of illumination of given surface is subject to the two following optical laws: The intensity of illumination on a given surface is inversely as the square of its distance from the source of light.
intensity of illumination which is received obliquely is proportional to the cosine of the angle which the luminous rays make with the normal to the illuminated surface.
The first law is one that should be carefully noted by the photographer on exposing sensi tive material to the action of light rays.
Reflection and a ray of light meets a polished surface it is reflected according to the two following laws: • No body can be said to be absolutely opaque, as all are mere or less translucent cut cr formed into sufficiently thin sheets.