Which of the following colors are primary colors?

Why does the clear sky appear blue? How would the sky appear in the absence of Earth's atmosphere?

$A$ person suffering from an eye defect uses lenses of power $-1 \,D$. Identify the eye defect. Draw a suitable diagram to illustrate how this type of lens corrects the defect. Give the nature of the corrective lens to be used. Also,calculate the focal length of the corrective lens.

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.

$A$ person is advised to wear spectacles with concave lenses. What type of defect of vision is he suffering from?

When white light is incident on an object and it absorbs all the component colors and reflects none of them,what color does the object appear to be?