Eye, defects, dispersion, scattering MCQs

Q1. Why is the colour of the clear sky overhead a brighter blue at high altitudes (e.g., from a mountain top) than at sea level?

• A.Less atmosphere above causes less multiple scattering and absorption, preserving the dominant Rayleigh-scattered blue
• B.Air is colder, so blue is brighter
• C.Sunlight intensity is higher
• D.The pupil dilates at high altitude

Answer: A. Less atmosphere above causes less multiple scattering and absorption, preserving the dominant Rayleigh-scattered blue

NCERT fact (physics, chapter 'The Human Eye and the Colourful World'): At high altitudes, less air column means fewer particles to cause secondary scattering and absorption that would whiten the sky. The Rayleigh-scattered blue dominates more purely, giving deeper blue. Sunlight intensity is similar (slightly higher) but the colour change is from atmospheric path effect. Final answer - A) Less atmosphere above causes less multiple scattering and absorption, preserving the dominant Rayleigh-scattered blue.

Q2. In a primary rainbow, which colour appears on the OUTER edge of the arc?

• A.Green
• B.Red
• C.Violet
• D.Blue

Answer: B. Red

In a primary rainbow light undergoes one internal reflection; red emerges at a larger angle (about 42 deg) than violet (about 40 deg), so red is on the outer edge and violet on the inner.

Q3. Holding a 1 cm pencil at 25 cm (the near point), what angle does a healthy eye perceive it to subtend? (in degrees)?

• A.0.57 deg
• B.2.29 deg
• C.0.040 deg
• D.25.0 deg

Answer: B. 2.29 deg

Angle (rad) = height/distance = 1/25 = 0.040 rad. In degrees: 0.040*180/pi = 2.29 deg.