All questions
8
Q1
Describe the function of the iris and pupil in the human eye.
The iris controls the size of the pupil, which regulates the amount of light entering the eye.
Q2
Explain the term 'power of accommodation' of the human eye and describe how it helps us see objects at different distances.
Power of accommodation is the ability of the eye lens to adjust its focal length with the help of ciliary muscles, allowing us to see objects clearly at varying distances.
Q3
Differentiate between myopia and hypermetropia based on their causes and the type of corrective lenses used.
Myopia (near-sightedness): distant objects appear indistinct because the image forms in front of the retina; corrected with a concave lens. Hypermetropia (far-sightedness): nearby objects appear indistinct because the image forms behind the retina; corrected with a convex lens.
Q4
Explain the refraction of light through a triangular glass prism. Describe the path of the incident ray, the refracted ray, and the emergent ray, and define the angle of deviation.
When light passes through a triangular glass prism, it bends twice — towards the normal on entering the prism and away from the normal on exiting. The emergent ray is deviated from the incident ray's direction. The angle between the direction of the incident ray extended and the emergent ray is called the angle of deviation.
Q5
What is dispersion of white light? Explain why different colours bend through different angles when passing through a prism.
Dispersion is the splitting of white light into its seven constituent colours (VIBGYOR — Violet, Indigo, Blue, Green, Yellow, Orange, Red) when it passes through a glass prism. Different colours bend by different angles because each wavelength has a different refractive index in the prism material.
Q6
Briefly explain the phenomenon of atmospheric refraction. How does it lead to advance sunrise and delayed sunset?
Atmospheric refraction is the bending of light by Earth's atmosphere, which has varying refractive indices due to temperature and density differences. This makes the Sun visible about 2 minutes before it actually rises above the horizon and about 2 minutes after it sets below the horizon.
Q7
Why do stars twinkle, but planets do not?
Stars twinkle because they appear as point-sized sources of light, and continuous atmospheric refraction causes fluctuations in their apparent position and brightness. Planets appear as extended sources, so the total variation in light from multiple points averages out, nullifying the twinkling effect.
Q8
Explain why the sky appears blue on a clear day and why danger signals are often red.
The sky appears blue because shorter-wavelength blue light is scattered more strongly by fine atmospheric particles (air molecules) than longer-wavelength red light. Danger signals are red because red light is scattered the least by fog or smoke, making it visible from greater distances.