If everything is mostly "empty space" than why can't we see through it?

The human eye isn't equipped to see between the electrons.
They are whirling around so fast they create a nice blur that looks solid.
If you and I condensed to no space we would basically be such a tiny dot as to be undetectable. Supposedly the entire universe (the one we know about) started as a dot.

The problem is that the light gets absorbed or deflected and can't travel through. So you can't see through most substances.

Indeed, the electromagnetic waves get absorbed or reflected by the electrons whirling around the nucleus of the atom, though sometimes, depending on the substance, they may let some light go through.

The previous answers are all correct, but the biggest factor is that the wavelength of visible light is much, much larger than an atom. You can think of it kind of like trying to drive a car on one of those pedestrian-only walkways where they put steel or concrete posts spaced a few feet apart. Pedestrians can easily walk between the posts, but cars and other large vehicles can't fit. Light is similar. Even though the atoms are very small, they act like narrow posts spaced a few Ångströms apart that block the "wide" light beams. It wouldn't matter if you had the largest magnifying glass in the whole world; you can't resolve things as small as atoms with light in the visible spectrum. However, when you go to higher frequency light waves, outside of the visible spectrum, (the higher the frequency, the shorter the wavelength), the waves become narrow enough to go through certain materials. X-rays, for example, can "squeeze between the posts", so to speak, of most liquids and low-density solids such as muscle and organ tissue. Dense solids like bone are still opaque to those frequencies, so x-rays can be used to take images of the your skeleton right through your body. As the frequency goes up even higher the light can squeeze through more and more materials. At frequencies above gamma rays, the waves can travel essentially unimpeded through anything. Such rays pass through the earth mostly undetected.

Thanks for this explanation, you made me gather information correctly, great teacher you are .

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Originally Posted by jcaron2

The previous answers are all correct, but the biggest factor is that the wavelength of visible light is much, much larger than an atom. You can think of it kind of like trying to drive a car on one of those pedestrian-only walkways where they put steel or concrete posts spaced a few feet apart. Pedestrians can easily walk between the posts, but cars and other large vehicles can't fit. Light is similar. Even though the atoms are very small, they act like narrow posts spaced a few angstroms apart that block the "wide" light beams. It wouldn't matter if you had the largest magnifying glass in the whole world; you can't resolve things as small as atoms with light in the visible spectrum. However, when you go to higher frequency light waves, outside of the visible spectrum, (the higher the frequency, the shorter the wavelength), the waves become narrow enough to go through certain materials. X-rays, for example, can "squeeze between the posts", so to speak, of most liquids and low-density solids such as muscle and organ tissue. Dense solids like bone are still opaque to those frequencies, so x-rays can be used to take images of the your skeleton right through your body. As the frequency goes up even higher the light can squeeze through more and more materials. At frequencies above gamma rays, the waves can travel essentially unimpeded through anything. Such rays pass through the earth mostly undetected.

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