Name the defects of vision of the human eye.

Why does the sky appear dark and black to an astronaut instead of blue?

Which scientist established that white light is composed of seven constituent colors?

What would have been the colour of the earth's sky if there were no atmosphere?

$(a)$ Explain how a normal human eye is able to see distinctly objects placed at a distance as well as those placed at a nearer distance.
$(b)$ What are the far point and near point of a normal human eye?

How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw the diagram.