Hi Shani, good question! It is because of electronic limitations at the sensor level and you can find more here: 

Hi Carly, I noticed that the editing of that section of the class was wrong and I understand the confusion! Slide 3 should have appeared around minute 17 and slide 4 around minute 19 (when you see me clicking). Hopefully these will then make more sense :) But basically: 

1) Some EM radiation easily passes the atmosphere and reaches the Earth

2) Some other EM radiation is absorbed and is not reaching the Earth – thus satellites operating at those wavelengths will not receive any information. Atmospheric components are the cause of the energy absorption, and are noted in the figure. 

3)  You do not see this in the figure, but the absorbed solar shortwave radiation is emitted back to outer space in thermal infrared or longwave infrared to maintain the Earth's heat energy balance and global warming is caused by the imbalance of the energy. The reason for this are greenhouse gases (GHG) that block / absorb the thermal infrared energy.

In the next slide, you can see instead how different land cover have different signatures in the electromagnetic spectrum (that slide should have appeared at 19:18). 

Hope it is clearer!

Hi Carly - not an answer to your question - but how do I post a question this discussion page for the course? I can't figure it out.

If you click on the 'reply to thread' button, you can post your own question!