Various optical apparatus provide a source of parallel light (7, 75). The parallel light (7, 75) is generally achieved by directing an incident beam at the apex of a prism (1, 22, 24, 26, 28). The prism may have varying configurations. One configuration has a forward conical face (24). Another configuration has a pyramidal forward end (22). Other configurations arc also disclosed. The application also discloses the use of reflectors (20, 78, 216, 316, 400) having internal reflective surfaces shaped as three-dimensional figures of revolution, for example paraboloid or ellipsoid. The reflectors (20, 78, 216, 316) focus light incident onto the reflectors at one or more foci (F, 220, 320, 420). The reflectors may be used in combination with the optical apparatus including the prisms (1, 22, 24, 26, 28). The reflectors (20, 78, 216, 316) may be used in flow cytometers for focussing light at a sample stream (237, 337) passing through the focus (F, 220, 320, 420) of the reflector (20, 78, 216, 316). The collection of scattered and/or fluorescent light from an illuminated sample stream (237, 337) in a flow cytometer may be achieved with the use of a collector shaped as a figure of revolution e.g. paraboloid or ellipsoid. Various optical methods and methods for flow cytometry are also disclosed.