Question

One half a slab of ice is covered with black cloth and the other half with white cloth. This is then placed in sunlight. After some time the pieces of cloth are removed. Then
(A) Ice has melted equally under both pieces.
(B) More ice has melted under white cloth.
(C) More ice has melted under black cloth.
(D) It will depend on the medium in which ice is placed.

Answer 1

Hint: Like a perfect black color absorbs radiation incident on it, the black cloth also absorbs more heat when compared to the white cloth. Since black cloth absorbs more heat, this heat is used for melting of ice.

Complete step-by-step solution

Colors are seen because the bodies reflect/re-emit/bounce off the light of particular color from them. So if you incident say purely blue light on a body and it doesn’t show any color, which means that the blue light gets absorbed. This will make the body look dark/black.

Perfect black color absorbs the radiation incident on it completely. It neither reflects or transmits any radiation. A perfect ideal black body doesn’t exist but a black object behaves in the similar manner, it absorbs the radiation more than any other color.

     


Since the ice block is covered with black cloth it absorbs more heat from the sun rays while the slab covered with white cloth behaves exactly opposite to it, it bounces back the radiation and avoids absorption of heat.

Hence, the slab covered with black cloth gets hotter and melts more.

The correct option is C.

Note: The same theory explains why people carry a black umbrella on a sunny day and why it’s suggested to wear white or light colored clothes on hot days. The reason is that dark colors closer to a black body absorb more heat. Now we can also guess why all the airplanes are usually light colored.