Class 12 Physics
CBSE and UP Board
Ray Optics and Optical Instruments
Chapter-9 Exercise 9.24
NCERT Solutions for Class 12 Physics Chapter 9 Question-24
For a normal eye, the far point is at infinity and the near point of distinct vision is about 25cm in front of the eye. The cornea of the eye provides a converging power of about 40 dioptres, and the least converging power of the eye-lens behind the cornea is about 20 dioptres. From this rough data estimate the range of accommodation (i.e., the range of converging power of the eye-lens) of a normal eye.
Share
Least distance of distinct vision, d = 25 cm
Far point of a normal eye, d’ = ∝
Converging power of the cornea, Pc = 40 D
Least converging power of the eye-lens, Pe = 20 D
To see the objects at infinity, the eye uses its least converging power.
Power of the eye-lens, P = Pc + Pe = 40 + 20 = 60 D Power of the eye-lens is given as:
P= 1/Focal length of the eye lens (f)
f =1/P = 1/60D = 100/60 = 5/3 cm
To focus an object at the near point ,object distance (u) = -d = -25 cm
Focal length of the eye lens = Distance between the cornea and the retina = Image distance
Hence, image distance ,v = 5/3 cm
According to the lens formula ,we can write:
1/f” =1/v + 1/u
Where ,f” = focal length
1/f” = 3/5 + 1/25 = (15+1)/25 = 16/25 cm⁻¹
Power, P’ = 1/f ‘ x 100
= 16/25 x 100 = 64D
Therefor power of the eye-lens = 64-40=24D
Hence ,the range of accommodation of the eye-lens is from 20D to 24D