Which two colors of light combine to form the color yellow?

What is the complementary color of blue?

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

$(a)$ State two main causes of a person developing near-sightedness (myopia). With the help of a ray diagram, suggest how this disability can be corrected.
$(b)$ The far point of a myopic person is $150 \ cm$ in front of the eye. Calculate the focal length and power of the lens required to enable him to see distant objects clearly.

When a beam of white light is passed through a prism,violet colour bends the most and red colour the least.

Every colour travels with the same speed in a transparent medium.