When a shoe is polished  we are applying some mechanical force on it,and removes the dirt in it.due to the effect of that force it shines
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When the polish is applied with the brush, all the small pits, kinks, and uneven surfaces are filled with the polish. The polish solidifies.  At this point the surface of the shoe appears uneven and dull.  

When the shoe is polished a second time with dry cloth or a soft brush, the polish melts again and spreads uniformly. The surface becomes very smooth. The polish layer becomes highly reflective. It reflects all the light incident on it. So it appears bright, brilliant and shiny.

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