How many times is the wavelength of red light compared to the wavelength of blue light (in $\times$)?

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

Which of the following is true for the eye defect known as hypermetropia (farsightedness)?

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

$A$ 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.

Which phenomenon is responsible for the blue color of the clear sky?