Answer : As a result of overlapping two light waves emanating from a coherent source, the luminosity of a point increases and the intensity of another point decreases . This results in periodic light and dark conditions on any surface. This periodic variation in the intensity of light is called light anomaly There can be two types of anomalies. A. Constructive anomalies b. Destructive anomalies a. Constructive anomaly: When two light waves of approximately equal wavelength and equal amplitude emitted from two sources meet, the brightness of a point increases when the light intensity of a point increases , this phenomenon is called constructive anomaly. B. Catastrophic anomalies: When two wavelengths of approximately equal wavelength and equal amplitude emitted from two sources meet in opposite directions, the light intensity of a point decreases and becomes dark. This incident is called destructive perversion. Conditions of exception: 1. The two light sources must be compatible. 2. The spread of the two sources must be equal or almost equal. 3. The two sources must be close together. 4. The two sources must be very fine.