Question 3: A thermometer is placed in direct sun light. Will it read the temperature of air or of the sun or something else?
A thermometer is a temperature measuring instrument. We know that almost all substances expand by heating and thermometer is making use of this property of thermal expansion of matter.
In order to do so, the thermometer, and hence the thermometric substance, is brought in thermal contact with the substance whose temperature is intended to be measure.
When thermometer is placed in direct sun light, it becomes in thermal contact with the surrounding air. The process of heat exchange is established and after some time the temperatures of the thermometer and the surrounding air become same. The thermometer will record this temperature.
Thus the thermometer placed in direct sun light will record the temperature of the surrounding air.