Greenhouse effect:

The warming results when solar radiation is trapped by the atmosphere, caused by atmospheric gases that allow sunshine to pass through but absorb heat that is radiated back from the warmed surface of the earth.

Explanation:

Light from the Sun contains thermal radiations of long wavelengths as well as light and ultraviolet radiations of short wavelengths. Glass and transparent polythene sheets allow radiations of short wavelengths to pass through easily but not long wavelengths of thermal radiations. Thus, a greenhouse becomes a heat trap.

Radiations from the Sun pass easily through the glass and warms up the objects in a greenhouse. These objects and plants give out radiations of much longer wavelengths. Glass and transparent polythene sheets do not allow them to escape out easily and are reflected in the greenhouse. Thus, maintaining the inside temperature of the greenhouse. The greenhouse effect promises better growth of some plants.