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Because they do not absorb any part of radiation energy from the light source

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Transparent materials reflect light waves and do not absorb much of the light incident on them.  The opaque materials absorb most of the radiation (light) incident on them.  The absorbed energy goes into heating of the substance. 

Opaque materials reflect from the surface only that color (frequency) in the light spectrum corresponding to its own color.  Transparent substances reflect all the frequencies received.  So appear colorless.
All materials reflect, transmit (refract), scatter the radiation from the light waves, when light is incident on the surface of the material.

In materials like glass or water (transparent), the light waves (photons with energy) falls on the surface, and causes the electrons in the atoms to change energy levels .  These electrons give back energy and cause radiation to progress continuously inwards the material.  So all over the transparent material, the electrons are continuously absorbing energy, and emitting the energy.   They also reflect radiation from the surface. 

   It so happens that in transparent materials, the incoming radiation and radiation (waves) from the electrons are in phase (additive).  So the waves propagate every where.  We see the light from every part of the transparent medium and from the surface.

   In opaque materials, the incoming radiation and the radiation from the electrons in the atoms of the substance are in opposite phase and hence the waves nullify each other at the surface level.  So the energy goes in to heating of the substance.  Only a small part of the light from the surface is reflected, that tells us the color of material.

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