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

What is the time difference between the actual sunset and the apparent sunset?

Study the diagram given below and answer the questions that follow it.
$(i)$ Identify the defect of vision represented in this case. Give a reason for your answer.
$(ii)$ Mention the two causes of this defect.

$A$ 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.

When a light ray travels from an optically denser medium to an optically rarer medium, what is the angle of incidence called at which the angle of refraction becomes $90^{\circ}$?

Which of the following colors are primary colors?