Exposure to which component of solar-radiation could be a health hazard?

1 Explain why only a part of the solar energy that strikes the upper regions of atmosphere, reaches the surface of the earth. 3 With the help of a diagram, explain the construction and working of a box type solar cooker. 4 Mention any two differences between a box type and a concentrator type solar heater. 5 Why is it not possible to make use of solar cells to meet all our energy needs? State atleast two reasons to support your answer.