In order to image (see) the different parts of the Sun's atmosphere, we must look with different kinds of light.

The PHOTOSPHERE is best seen with normal visible light. This is because it acts like a very hot (5800 K) black body radiator, which has its PEAK in the optical.

The CHROMOSPHERE is tenuous and not so hot. However, it is bright in certain atomic lines, such as the main RED line of Hydrogen, called H-Alpha.

The CORONA is extremely tenuous. However, it is also extremely hot. The Corona is so hot, it excites atoms to emit light at X-Ray wavelengths. Thus the Corona is best seen when looking at X-Ray light.

The Corona can be seen with visible light during an eclipse, when the Sun's bright photosphere is blocked out by the Moon.