Chapter 11 – Human Eye and Colourful World

1) For someone with myopia, the farthest point is 80 cm in front of the eye. What kind of lens and how much power does it need to fix the problem?

Ans – 80 cm is the farthest point for shortsighted people.

Because the person has myopia, the lens will be curved & the focal length will be short.

=> f = -80cm = -0.8m

We are aware that a lens’s power equals the focal length times itself.

=> P = 1/f

=> P = -0.8/-1 = -1.25D

The power of the lens is -1.25D & it is concave.

2) Choose the natural event that causes the stars to twinkle. Will a moon watcher notice it?

Ans – The process behind the twinkling of stars is air refraction. The viewer on the moon cannot notice the twinkling of stars as the moon has no atmosphere.

3) Why cannot a regular eye see things properly closer to 25 cm?

Ans – The average eye cannot visualize things closer to 25 cm due to the burned power of adaption at a distance of 25 cm. Hence, the eye cannot focus the light rays on the retina, when the object is less than 25 cm.

4) Why does the Sun look orange early in the morning?

Ans – The sun is far from the earth’s surface at sunrise. There is a massive moment of light rays in the Earth’s atmosphere before you see. In the atmosphere, the light rays with fewer wavelengths get spread by the Earth’s atmosphere & the red light with higher wavelengths can catch our eyes. Hence, the Sun looks reddish in the early mornings.

5) What’s the reason to check random wavering of the items close to fire or on a hot day?

Ans – We see random wavering or flicking of the items close to fire or on hot days due to air refraction. The above fire area is hot & lighter than the cold air above due to which its refractive index is low & has a change in density. Hence, the desired location of the item blinks.

6) Why do we emerge from a darkroom with blurry vision?

Ans – The iris widens the pupil in a dark room to let more light into the eye. A lot of light floods our eyes when we emerge from a dark place, and the glare sensation prevents us from seeing properly.

7) Explain Presbyopia! Indicate the reasons for this flaw & how it can be treated.

Ans – A person with presbyopia, a visual impairment mainly brought on by aging, cannot easily perceive close and distant things.

The following are the primary reasons for presbyopia:

1. The eye lens stiffness
2. The power of accommodation of the eye may decline with age.
3. There is weakness in the ciliary muscles.

A bifocal lens with a concave upper half and a convex bottom half corrects presbyopia.

8) Why do stars twinkle?

Ans – The reason for the twinkling of stars is light refraction in the atmosphere. Being distant objects, stars function as point sources of light. The star’s location might fluctuate as air refraction occurs along its light beam route. The amount of light that enters the eye varies turning the star to look brighter at one moment and fainter at another.

9) Describe the absence of planet Twinkle.

Ans – The planets are far closer to Earth than the stars are. One may see a planet as an assembly of many few light sources. The overall amount of light entering our eye from the individual point sources will be the same although light from each one flickers. Planetary blinks are therefore absent and the planets seem to be equally brighter.

10) One person can visualize at a 150 cm distance & wants to read 25 cm away. Which focal-length glass should be used? And what’s the eye defect mentioned here?

Ans – Given,

v = – 150cm
u = – 25cm

From the lens formula, we got:

=>  1/f = 1/v – 1/u

=> 1/f = 1/- 150 – 1/- 25

=>  1/f = 1/30

=>  f = 30cm

As the focal length is positive, the convex lens will be used & eye problem is Hypermetropia.