A long sighted person cannot see objects clearly at a distance less than 40 cm from his eyes. the power of the lens needed by him to read a paper at 25 cm is ________?
Question: A long sighted person cannot see objects clearly at a distance less than 40 cm from his eyes. the power of the lens needed by him to read a paper at 25 cm is ________?
To find the power of the lens needed by the long-sighted person to read a paper at 25 cm, we can use the lens formula:
1/f = 1/v - 1/u
Where:
f = focal length of the lens
v = image distance
u = object distance
In this case, the person cannot see objects clearly at a distance less than 40 cm, so the object distance (u) is -40 cm (negative sign indicates that the object is placed on the same side as the observer).
We need to find the focal length (f) that allows the person to see clearly at 25 cm, so the image distance (v) is -25 cm.
Substituting these values into the lens formula:
1/f = 1/-25 - 1/-40
Simplifying the equation:
1/f = -1/25 + 1/40
1/f = (-40 + 25) / (40 * 25)
1/f = -15 / (40 * 25)
1/f = -3 / (8 * 25)
Multiplying both sides by (8 * 25):
8 * 25 / f = -3
f = -8 * 25 / 3
f ≈ -66.67 cm
The focal length of the lens needed by the long-sighted person to read a paper at 25 cm is approximately -66.67 cm.
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