A person’s Near point is 50cm, what is the focal length of the lens he should use?
Answers
Answer:
A far sighted person has his near point 50cm,
To find the power of lens he should use to see at 25cm, clearly.
Solution:
Distant objects need to be imaged at most 50 cm from the eye.
According to the given criteria,
u=25cm,v=−50cm
Applying lens formula, we get
f
1
=
v
1
+
u
1
⟹
f
1
=
−50
1
+
25
1
⟹
f
1
=
50
−1+2
⟹f=50cm=0.5m
The power of the lens he should use is,
P=
f
1
=
0.5
1
=+2D
Answer:
he should used a lens having focal length equal to 0.5m.
Explanation:
A person suffering from nearsightedness can see distinct objects clearly but faces difficulty in seeing nearby objects clearly. It happens because the eye lens focuses the incoming divergent rays beyond the retina. Convex lens is used for the solution of this problem because A convex lens of suitable power converges the incoming light.
The given person will be able to clearly see the object kept at 25 cm (near point of the normal eye), if the image of the object is formed at his near point, which is given as 1 m.
Object distance, u = -25 cm
Image distance, v = -50cm
Focal length, f
Using the lens formula,
1/f = 1/v - 1/u
here u = -25 cm
v = -50cm
calculating for f, we get
f = +50cm = +0.5 m