Which optical phenomenon is responsible for the twinkling of stars?

Which part of the eye is responsible for changing the focal length of the eye lens by adjusting its thickness?

What is the complementary color of red?

What is meant by scattering of light? Use this phenomenon to explain why the clear sky appears blue or the sun appears reddish at sunrise.

Which colour of white light suffers $(i)$ least deviation and $(ii)$ maximum deviation when a beam of white light passes through a glass prism?

$(a)$ 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.