A method for generating light is provided. The method further includes measuring a period of time during which one of targets from a fuel target generator passes through two detection positions. The method also includes exciting the targets with a laser generator so as to generate plasma that emits light. In addition, the operation of exciting the targets with the laser generator includes: irradiating a pre-pulse laser on the targets to expand the targets; detecting conditions of expanded targets; and adjusting at least one parameter of the laser generator according to the measured period of time and the conditions when the measured period of time is different from a predetermined value. The parameter of the laser generator which is adjusted according to the measured period of time includes a frequency for generating a laser for illuminating the targets.