Question

(a) Give reasons for the following:
(i) Colour of the clear sky is blue.
(ii) The sun can be seen about two minutes before the actual sunrise.
(iii) We cannot see an object clearly if it is placed very close to the eyes.
​(b) What is Presbyopia? Write two causes of this defect.

Answer 1

(a) (i) The sky appears blue when viewed from Earth, because of the scattering of the light. When sunlight strikes molecules in our atmosphere, the light is redirected in many directions. The blue light is scattered more than the red light because blue color has the smallest wavelength i.e. comparable to the size of the particles present in our atmosphere. So, it is scattered the most, causing the sky to be blue.

(ii) The Sun is visible to us two minutes before the sunrise and two minutes after the sunset because of the bending of the light due to atmospheric refraction. The light from the Sun gets refracted by the atmospheric layers and reaches the observer. This causes the observer to think that the light is coming straight from the Sun.

(iii) If an object is placed very close to the eyes, the light reflected from it does not fall on the retina to form a clear image. The minimum distance for seeing an object clearly is 25 cm for the human eye.

(b) Presbyopia is a common defect of vision, which generally occurs at old age. A person suffering from this type of defect of vision cannot see the nearby objects clearly and distinctively. A presbyopic eye has its near point greater than 25 cm and it gradually increases as the eye becomes older.

Presbyopia is caused by the:

1. The weakening of the ciliary muscles.

2. Reduction in the flexibility of the eye lens.