It may seem like a bit of rocket science but the way your room heats on a sunny day is relatively simple
The diagram above gives a very basic explanation of how your room heats through sunlight falling on its windows.
The blue line represents the window glass. The yellow arrows are the sunlight. Sunlight moves very fast and has a lot of energy in small packets of short wavelength.
Some slight is reflected back (the little yellow arrow on the left) while most of it passes through the glass. Light has a short wavelength so it can pass through the glass.
The big brown blob represents the solid objects in your room. Let’s imagine you’re in sitting in the lounge and this object is your favourite chair.
The sunlight that has passed through the glass and hits your chair. Suddenly that light is not moving fast anymore.
All the energy packed into that sunlight moving very fast has to go somewhere. As the sunlight slows down, or even stops, that energy is changed to heat energy.
The heat energy produced radiates out from your chair and you can even feel it warming up your chair and the air around it.
The chair and air in the room warm up. Now comes the interesting bit…
Eventually this heat gets absorbed by the objects in the room, the floor, the walls, the air in the room, even you.
Eventually, all these things reach the maximum amount of heat they can hold and begin radiating it our into the air of the room, warming it even further
When the heat reaches the glass, it can’t pass through. It’s wavelength is too long and it is trapped in the room (well, to tell the truth, some does get out).
So now the heat is building up in the room and more is coming in as sunlight passes through the glass. Things can get pretty hot like this.