what makes an object transparent and opaque like in case of carbon , graphite is a black opaque substance but when heated to a high temp. it converts to diamond which is transparent in spite of it is denser than graphite ?

Structure of crystal.

If there is a bandgap, or transitions, that correspond to visible light, the material will be opaque. That depends on the atoms involved and (as Sensei notes) the structure of the material.

Materials are often opaque at one photon frequency range, and transparent at other frequency range.

 

As a result we can take photos of our body internals using x-rays. X-rays are differently reflected or refracted by bones than other organs, making easy to distinguish them, or external unwanted objects that were f.e. shoot at body.

Edited November 18, 201312 yr by Sensei

Materials are often opaque at one photon frequency range, and transparent at other frequency range.

 

As a result we can take photos of our body internals using x-rays. X-rays are differently reflected or refracted by bones than other organs, making easy to distinguish them, or external unwanted objects that were f.e. shoot at body.

 

That's not quite the same thing. Items are not inherently transparent to x-rays, since it's ionizing radiation. It's more that the absorption depends on density (some combination of mass, number, and charge density), and tissue is less dense than bone.

 

OTOH, for non-ionizing radiation, you can have two items of similar density where one is transparent and one is opaque.

It's more that the absorption [of X rays] depends on density (some combination of mass, number, and charge density), and tissue is less dense than bone.

X rays absorption depends fundamentally on the atomic number. In part through the density, very much through the absorption by deep electron levels.

Would anybody explain it further !

If a photon's frequency imparts sufficient energy to an electron, the electron will be excited enough to raise to the next energy level and the photon is absorbed, rendering the material opaque. If a photon's frequency is too low, the photon will momentarily be excited but then drops back down to its base energy level and the photon is emitted to carry on through the material or until it meets another electron whereupon the same will happen again...this is transparency.

 

If you think of energy levels as a series of rising steps, in a transparent material the distance between each step is greater than for opaque materials and thus needs more energetic photons to lift the electron to the next step. Because all of the visible light spectrum is not energetic enough, materials like glass appear transparent.

X rays absorption depends fundamentally on the atomic number.

Which dictates charge density

If a photon's frequency imparts sufficient energy to an electron, the electron will be excited enough to raise to the next energy level and the photon is absorbed, rendering the material opaque. If a photon's frequency is too low, the photon will momentarily be excited but then drops back down to its base energy level and the photon is emitted to carry on through the material or until it meets another electron whereupon the same will happen again...this is transparency.

 

If you think of energy levels as a series of rising steps, in a transparent material the distance between each step is greater than for opaque materials and thus needs more energetic photons to lift the electron to the next step. Because all of the visible light spectrum is not energetic enough, materials like glass appear transparent.

if electron is excited More or less it will come to its ground state then what should we say about it like in metal if we heat them their electron excite but after ward come to rest but they are not transparent

In a metal there are electrons in the conduction band, which are free to respond to EM fields of a wide range of frequencies, so metals will tend to be opaque even outside of the visible spectrum.