CBSE Class 10 Science 2018 · Paper 31/1/C · Question 20

(a) Presbyopia is the defect of vision in which the power of accommodation of the eye decreases with age: the near point gradually recedes, so the person cannot see nearby objects comfortably and distinctly without corrective glasses. Cause: gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Correction: spectacles with a convex lens of suitable power; a person who also has myopia uses bifocal lenses, whose upper part is a concave lens (distant vision) and lower part a convex lens (near vision). (b) Early in the morning the Sun is near the horizon, so its light passes through a much thicker layer of air, a longer distance in the atmosphere, before reaching our eyes. Air molecules scatter blue light (shorter wavelength) much more than red light (longer wavelength), so most of the blue is scattered away on the way and mainly red light reaches our eyes: the Sun appears reddish. At noon the light travels a short path through the air, little blue is scattered, and the Sun appears white. The labelled diagram (the figure at step 5) shows the Sun near the horizon with its long path through the atmosphere, the Sun overhead with its short path, the observer on the Earth, and blue light scattered away along the long path.