Human Eye And Colourful World

The ability of an eye to focus the distant objects as well as the nearby objects and forming the images on the retina by changing the focal length of the eye lens is called accommodation.

The person can use concave lens (negative power) to improve the vision.

The far point of human eye with normal vision is at infinity and the near point is 25 cm distance from the eye.

Since the child cannot see the blackboard writing clearly, he is suffering from 'myopia' or 'short-sightedness' i.e. he can see closer things with naked eye but not distant things. Myopia can be corrected by using spectacles with concave lenses of appropriate power.

Accommodation is the property of human eye to adjust the focal length of our eyes at different distances. Thus, option (b) is correct.

The image formation spot in our eye is retina. Thus, option (d) is correct.

25 cm is the least distance of distinct vision for a young adult with normal vision.

ciliary muscles cause the change in focal length in our eyes.

(i) For distant vision:

Power of lens, P = -5.5 D

Thus, the focal length of the lens required for correcting distant vision is, -18.2 cm.
Note: The negative sign of the length tells us that it is a concave lens.

(ii) For near vision:

Power of lens, P = + 1.5 D

So, Focal length, f = + 66.66 cm (or + 66.7 cm)

Thus, the focal length of the lens required for correcting near vision is + 66.7 cm.

Note: The positive sign of focal length tells us that it is a convex lens.

The defect called myopia is corrected by using a concave lens. We will now calculate the focal length of the concave lens required in this case. The far point of the myopic person is 80 cm. This means that this person can see the distant object (kept at infinity) clearly if the image of this distant object is formed at his far point.

Object distance, u = ∞ (infinity)

Image distance, v = -80 cm (Far point, in front of lens)

And, Focal length, f=? (To be calculated)

Putting these values in the lens formula:

We get

or

or f = -80 cm

Thus, the focal length of the required concave lens is 80 cm. We will now calculate its power. Please note that the focal length, f= -80 cm = -0.8 m.

So, the power of concave lens required is -1.25D

(a) The hyper-metropic correction is shown below by using convex lens correction.

The eye defect called Hypermetropia is corrected by using convex lens spectacles.

We will first calculate the focal length of the convex lens required in this case. For hypermetropic eye can see the nearby object kept at 25 cm (at the near point if normal eye) clearly if the image of this object is formed at its own near the point which is 1 meter here. So, in this case :

Image distance, v = -1 m (Near point of this defective eye) = -100 cm

Putting these values in the lens formula,

We get :

Or,

Power of Lens needs to be calculated,
Given by P=
∴ We convert 33.3 cm into meter i.e 0.33m
P=
= +3.0 D
Thus, the power of the convex lens required is +3.0 diopters.

A normal eye sees objects with the help of converging eye lens. The ciliary muscles contact and expand thereby changing the converging power of the lens. The near point of our eyes is 25 cm i.e. our eyes can see clearly with minimum distance of 25 cm. But, when the distance between the object and our eyes is less than 25 cm, the ciliary muscles are unable to focus sharply. Thus, we cannot see with naked eye and need the support of any external device to see the object with distance less than 25 cm

The sky appears dark instead of blue to an astronaut because, there is no atmosphere in the outer space so there is no scattering of light. And because there is no scattering of light so no color is scattered. Thus, the sky appears dark to the astronaut.