a truck driver has 2 pay rs 1900 as challan bcoz his truck was emitting black smoke . Is it justified

Yes, the challan for the emission of smoke is justified. When an automobile emits smoke, it negatively affects the surroundings in the form of pollution. That is , it is said to create a negative externality. A negative externality is said to occur when the actions of one entity adversely affects the other entities. Sometimes, it is argued that a tax/challan must be imposed on the entities creating negative externality. This is because, the challan would encourage reduction of the negative externality. For example, if, a challan is imposed on the truck driver for the emission of smoke, then it would force him to adopt appropriate pollution control measures (such as regular pollution checks). Thus, on this ground the challan is justified as it would help in reducing the level of smoke emission and also to keep the pollution (environmental degradation) within limits.