1. M. Born and E. Wolf, Principles of Optics (Pergamon, Oxford, 1969).
2. J. W. Bruce and P. J. Giblin, Curves and Singularities: A Geometrical Introduction to Singularity Theory (Cambridge Univ. Press, Cambridge, 1992).
3. R. P. Feynman, R. B. Leighton and M. Sands, The Feynman Lectures on Physics. Vol. 3: Quantum Mechanics (Addison-Wesley, Boston, MA, 1963).
4. Yu. A. Kravtsov and Yu. I. Orlov, Geometric Optics of Inhomogeneous Media (Nauka, Moscow, 1980) [in Russian].
5. V. I. Arnold, Singularities of Caustics and Wave Fronts (Springer, Berlin, 2013).