Phy...........

a) This is similar to effect of a green house. The visible light and short wavelength infrared radiation from the sun pass through the glass of the car and are absorbed by the object inside the car, but the long wavelength infrared radiation which is re-radiation from the heated objects inside the car is unable to pass through the glass of the car. The trapping of the long wavelength infrared radiation leads to more heating and a higher resultant temperature inside the car.
b) The radiations from the sun pass through the glass of the car, and most of the radiations are immediately reflected by the shiny shield. The reflected radiations leave the car through the glass, and cause much less heating effect than the case without a shiny shield.