Why does the sky appear dark instead of blue to an astronaut?

Why do planets not twinkle?

Why are normal eyes not able to see clearly objects placed closer than 25 cm?

Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision, he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting : (i) distant vision, and (ii) near vision?

The least distance of distinct vision for a young adult with normal vision is about (a) 25 m (b) 2.5 cm (c) 25 cm (d) 2.5 m

The human eye forms the image of an object at its : (a) cornea (b) iris (c) pupil (d) retina

The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to : (a) presbyopia (b) accommodation (c) near-sightedness (d) far-sightedness

A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?

What is the far point and near point of the human eye with normal vision?

A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?

What is meant by power of accommodation of the eye?

Name the substance which on treatment with chlorine yields bleaching powder.