Walk into a greenhouse during the day, even a dull day and you will certainly notice how much warmer it is inside the greenhouse than outside. The greenhouse acts as a radiation trap. The Sun is very hot, about 6000 °C and emits short wave radiation at a temperature corresponding to this. This wavelength is able to pass through the greenhouse glass and is absorbed by plants and objects inside. It is then re – radiated, but the objects inside the greenhouse are much cooler - maybe 30 ° C - than the Sun, and the objects re – radiate at a longer wavelength in the infra red (IR) part of the electromagnetic spectrum.
The longer wavelength radiation is much less able to pass through the glass of the greenhouse. It bounces around inside the greenhouse, being repeatedly absorbed and re - radiated before it eventually escapes. This extended stay inside the greenhouse heats it up.