The Human Eye & the Colorful World - 2 - UPSC MCQ

When light enters the eye, most of the refraction takes place at the outer surface of the cornea. The crystalline lens only makes some finer adjustment.

Cilliary muscles modify the curvature of eye lens. When eye muscles are relaxed, eye lens becomes thinner thereby are relaxed, eye lens becomes thinner thereby increase in the focal length of eye lens.

The correct answer is a) iris.

The iris is the colored part of the eye that controls the size of the pupil, which is the opening that allows light to enter the eye. By adjusting the size of the pupil, the iris regulates the amount of light entering the eye, making it larger (dilating) in dim light and smaller (constricting) in bright light.

The correct option is the eye is not able to decrease the focal length beyond a limit.
A normal eye cannot focus on objects closer than 25 cm away because the eye cannot reduce its focal length any farther since the ciliary muscles are most stretched up to that point.
After that, the muscles are not strained at all. This means that the focal length cannot be smaller.
Therefore, at distances of less than 25 cm, the eye cannot see objects clearly.

When the light is very bright the iris makes the pupil contract so as to control the quantity of light entering the eye.

The fine water molecules mainly scatter blue light due to its shorter wavelength.

The image of an object situated in front of eye is formed on the retina. The image formed is a real, inverted and diminished image

Red colour has longer wavelength so least scattered by smoke or fog.

Range of vision for a normal human eye is between its near point and far point i.e, from 25cm to infinity.

Violet and blue colour have shorter wavelength.
So, they scattered more than lights of other colour by the molecules present in the atmosphere.

To focus the objects, situated at different distances from the eye, distinctly at the retina the eye lens adjusts its focal length. This adjustment of focal length of eye lens is achieved by the contraction/relaxation of the ciliaiy muscles.

Twinkling of star is due to atmospheric refraction of starlight caused by the gradual change in refractive index of different air layers at different height, the apparent position of star keeps on changing.

At noon, the sun is at top and the light rays coming from the sun have to travel lens distance hence, all colours get scattered very less even blue and violet.

A student, who cannot read words written on blackboard from the last bench of his class room, is suffering from the myopia or short-sightedness defect of vision of eye.

In figure (a) base BC of the prism is at the bottom, then violet colour lies at the bottom but in figure (b), the base BC is at the top, then violet would be at the top after dispersion, and third colour would be blue.

A person, who cannot see objects placed beyond 80 cm distinctly, is suffering from the myopia or short-sightedness defect of vision of eye.

A girl who can see distant objects clearly but is unable to see clearly the objects situated close to her eye, is suffering from hypermetropia or long-sightedness defect.

A student sitting on the last bench of class can read the letters written on the blackboard. It means his farsight is normal. However, he cannot read the letters written in his textbook distinctly. It means he is suffering from long-sightedness (hypermetropia) and the near point of his eye has receded away

Person cannot see distant objects clearly. So he is suffering from myopia. The defect is corrected by using concave lens of power

Objects are not visible through the fog because droplets scatter the light rays.