The objective is to demonstrate that the energy from sunlight can evaporate water. Sunlight is made up of a range of wave lengths that the human eyes see as different colours. Different wave lengths have different amount of energy. The bulk of the Sun's energy received by our Earth has a band of short wave length and the energy they carry is strong enough to evaporate water.
Water vapour is the most important greenhouse gas.
Like other greenhouse gases, increase of water vapour in the atmcosphere traps more heat and raises the Earth's temperature, then more W ater will evaporate into the atmosphere.