[borachio54 (OP)]

if an atom is 99% space ..and light has no mass (so is not affected my mass ) why is the planet and everything in the universe not transparent? (light just passes through whatever object it encounters)
what am i not considering as its quite obvious i am wrong!!! in writing this.but i cant think what iam missing ,

[jeffreyH]

This is to do with the wavelength of photons and the size of the atoms in the material being penetrated by the EM radiation. Longer wavelengths will penetrate more easily than shorter ones.
https://en.m.wikipedia.org/wiki/Penetration_depth

[Bigjoemonger]

Well it's a bit more complicated than you're thinking it is.

First, light is energy, and massless. But according to E=mc^2 energy is mass. So a photon of light with specific energy does have a relativistic mass. But that's not really important here.

Second, light doesn't pass through everything, it gets absorbed.

Sure materials would be "relatively" transparent if they were a single atom layer thick, but they're not. A piece of paper is several hundred thousand layers thick. These atom layers are bound together in a lattice like a crystal. How tightly compacted the layers are is dependant on the size of the atoms and the charges binding them. Then you need to consider that an electron isn't a distinct solid particle orbiting the atom like a planet around the sun. An electron is more of a cloud of charge with the "particle" existing somewhere in that cloud basically jumping around blinking in and out of existance.

So now a piece of paper, hundreds of thousands of atoms thick, each atom has electrons. Hydrogen has 1. Carbon has 6. Oxygen has 8. So hundreds of thousands of atom nuclei and millions of electrons. That's just in thickness at any point.

Now consider that a photon acts as a wave, so it doesn't just travel in a straight line. It oscellates up and down as it moves forward. So imagine a material an inch thick. If it travelled straight through it's distance travelled in the material would be an inch. But it's not travelling straight through it's moving up and down so the distance travelled would be greater than an inch.

All of this drastically increases the likelihood of an interaction occurring between the photon and some part of an atom causing the photon to be absorbed and decreasing the number that passes through.

Smaller wavelength photons such as gammas, are so small they can basically drill their way through without being absorbed, appearing to travel in a straight line because their oscillation is so small. Whereas larger wavelength photons, like am radio, are so large that when passing through materials they actually do travel in a straight line and can make it without being absorbed. Which is why am radio is so reliable.

[evan_au]

Electrons can be considered as waves which fill the space around the nucleus.

An incoming visible-light  photon that matches the energy gap to an unfilled electron shell is likely to be absorbed, and turned into heat, or re-radiated as lower-frequency photons in random directions.

In a mixed material like dirt, rock or living things, photons of all visible frequencies are likely to be scattered or turned into heat before they travel many millimetres.

[jeffreyH]

You could also include the Rydberg formula where the wavenumber determines the spectral lines of atoms. It can get very complicated and over most layman's heads.
https://en.m.wikipedia.org/wiki/Rydberg_formula