Note- Air is a bad conductor of heat.
1. In deserts, days are very hot and night are very cold- This is because of the very low specific heat of sand which, therefore absorbs the heat rapidly and hence, its temperature rises by a large degree during the day. At nights the sand radiates the heat with equal rate, making the temperature very low.
* Specific Heat- In easy words, it is the amount of heat that a substance required to raise its temperature by 1degree celsius. it is very high for water, It means water needs more heat to increase its temperature by 1degree celsius. So we use it as a car radiator cooler in our car.
2. Two thin Blanket is warmer than one thick Blanket- This is because the air of enclosed in between the two blankets is a bad conductor of heat. For exactly the same reason, A new quilt is warmer than an old one. This is because the new quilt encloses a large amount of air which is a bad conductor of heat.
3. In winter, birds often swell their feathers- Air enclosed between their body and feathers does not allow the flow of heat from the body of birds to the cold surroundings. So, it saves their body heat and they feel warm.
Black Body and perfect Black body- A body which can absorb all radiation of each wavelength at any temperature, which is incident on it and emits the full radiation spectrum on being heated, is known as a body.
Such a body neither reflects nor transmits any part of the incident heat radiation and hence, appears black irrespective of the colour of the incident radiation. This implies that a perfectly black body has unit absorbance. Also, a perfectly black body when heated emits radiation of all possible wavelength at that temperature. Materials like black velvet or lamp black come close to being ideal black bodies but the best practical realisation of an ideal black body is a small hole in a cavity, as this absorbs 98% of the radiation incident on them.
4. The silvered surface of a thermos flask is bad absorber- It does not absorb much heat from the surroundings. That is why ice inside the flask does not melt. Also, the silver surface is a bad emitter/radiator, therefore hot liquids inside the flask do not cool.
5. When a green glass is heated in a furnace and taken out, it is found to glow with red light- This is because red and green are complementary colours. At ordinary temperatures, a green glass appears green, because it transmits green colour and absorbs red colour strongly. According to Kirchoff's law, this green glass, on heated, must emit the red colour, which it absorbed strongly.
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