Answer:
They can be penetrated only with longer wavelengths such as radio and infrared.
Explanation:
A dark nebula can be defined as an interstellar cloud that is so dense as a result of high concentration of gas and dust and as such it obscures the visible wavelengths of light from stars behind it, thus appearing completely opaque (dark patch) in front of a bright emission nebula or in regions having plenty stars.
<em>Hence, dark nebulae can be penetrated only with longer wavelengths such as radio and infrared light because they cannot be absorbed by the dust layers or particles coated with frozen carbon monoxide and nitrogen residing in the dark nebulae</em>.
