A student uses spectacles of focal length $-2.5 \text{ m}$.

(a) Name the defect of vision he is suffering from.

(b) Which lens is used for the correction of this defect?

(c) List two main causes of developing this defect.

(d) Compute the power of this lens.

(a) Red colour is selected for danger signals because red light has a longer wavelength and is scattered the least by atmospheric particles like smoke, dust, or fog. Consequently, it can travel longer distances without losing intensity and remains clearly visible from far away.

(b) The sky appears dark in space because there is no atmosphere in outer space to scatter sunlight. In the absence of atmospheric particles, no scattering of light occurs, and hence no light reaches the observer's eyes from other directions, making the sky appear black/dark.

(c) The time difference between actual sunset and apparent sunset is about 2 minutes due to atmospheric refraction. As sunlight enters the Earth's atmosphere, it passes through layers of gradually increasing refractive index and continuously bends towards the normal. This causes the Sun to appear slightly higher than its actual position, making it visible to us even after it has actually crossed below the horizon.