The Total solution for NCERT class 6-12
Fora normal eye, the far point is at infinity and the near point of distinctvision is about 25cm in front of the eye. The cornea of the eye provides aconverging power of about 40 dioptres, and the least converging power of theeye-lens behind the cornea is about 20 dioptres. From this rough data estimatethe range of accommodation (i.e., the range of converging power of theeye-lens) of a normal eye.
Least distance of distinct vision, d =25 cm
To see the objects atinfinity, the eye uses its least converging power.
Power of the eye-lens, P = Pc + Pe = 40 + 20 = 60 D
Power of the eye-lens isgiven as:
To focus an object at the near point, objectdistance (u) = −d = −25 cm
Focal length of theeye-lens = Distance between the cornea and the retina
= Image distance
Hence,image distance,
According to the lensformula, we can write:
Where,
= Focal length
∴Powerof the eye-lens = 64 − 40 = 24 D
Hence,the range of accommodation of the eye-lens is from 20 D to 24 D.