Add information about mirrors and guides (See #36)

doc/model.md

@@ -1,9 +1,54 @@
 # Geometric Optics - Model Description
 
-The simulation describes light propagation through a lens in
-terms of rays. The rays in 
-geometric optics are useful abstraction useful for approximating the paths 
-along which light propagates.
+The simulation describes light propagation through a lens in terms of rays. The
+rays in geometric optics are useful abstraction useful for approximating the
+paths along which light propagates.
+
+The magnitude of the focal length of the lens is given by f= R/(2(1-n)))
+where n and R are respectively the index of refraction and the radius of
+curvature of the lens.
+
+In the case of the mirror, the magnitude of the focal length is determined in
+the paraxial approximation, i.e. given by f = R/2.
+
+As for the sign, the focal length f is positive for converging optical
+elements ( convex lenses and concave mirrors) and negative for diverging optical
+elements ( concave lenses and convex mirrors)
+
+For the lens, the position of the image is determined by the lens law which
+states that 1/d_o + 1/d_i = 1/f where d_o is the distance to the object, d_i is
+the distance to the image and f is the focal length of the optical element.
+
+For lenses, the image distance d_i is positive for images on the far side of the
+object (real images). If the image and object are on the same side of the lens,
+the image distance is negative (and the image virtual).
+
+A similar equation holds for mirrors, butw where the sign convention of the
+distance is reversed. The image distance d_i is positive for images on the same
+side of the object (real images). If the image is on the far side of the object,
+the image distance is negative (and the image virtual).
+
+The principal rays mode denote rays that help identify the position of the
+image. The principal rays are not physical rays and at times fail to go through
+a lens/mirror. Nevertheless, the method remains useful to find the location of
+the image. The principal rays are defined by three rays:
+
+- A ray from the top of the object proceeding parallel to the optical axis of
+  the lens. Beyond the lens, it will pass through the principal focal point. For
+  a negative lens, it will proceed from the lens as if it emanated from the
+  focal point on the near side of the lens.
+- A ray through the center of the lens, which will be undeflected.
+- A ray through the principal focal point on the near side of the lens. It will
+  proceed parallel to the centerline upon exit from the lens.
+
+The marginal rays, also called rim rays, are the rays that start at the object
+and touch the edge of the lens. The marginal rays emphasizes the limiting
+aperture associated with an optical element.
+
+The guides of the lens are meant to highlight the ability of lenses to bend
+light. The internal angle of the guides is a property of the focal length of a
+lens and its diameter. The guides are a useful approximation that allows to
+approximately determine the position of the image without performing raytracing.