A far sighted person cannot see objects that are less than 100cm away. If this person wants to read a book at 25cm, what type and focal length of lens does he need?
convex, 20cm
concave, 20cm
convex, 33cm
concave, 33cm
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The selected answer is wrong:
We can infer from the explanation of long sight that a long sighted person need a convex lens....read new school physics page 320.the least distance of distinct vision is the object distance(u) and is positive.... The near point of the observer is the image distance(v)
1/f=(1/v)+(1/u)
1/f=(1/-100)+(1/25)
1/f=33.3
Hence we need a convex lens of focal length 33.3
REF: Check new school physics page 320&321, example 14.1
C is correct
concave lense is used to correct short sightedness
convex lense is used to correct Long sightedness
To help a farsighted person read a book at 25 cm, we need to determine the type of lens and its focal length.
A farsighted person (hyperopic) can see distant objects clearly but struggles with close objects. We use the lens formula:
\[
\frac{1}{f} = \frac{1}{v} - \frac{1}{u}
\]
Where:
- \( f \) = focal length of the lens
- \( v \) = image distance (the distance at which the person can clearly see the image, here it’s 100 cm)
- \( u \) = object distance (the distance of the book, which is -25 cm since it's in front of the lens)
Now, substituting the values:
\[
\frac{1}{f} = \frac{1}{100} - \frac{1}{-25}
\]
\[
\frac{1}{f} = \frac{1}{100} + \frac{1}{25}
\]
\[
\frac{1}{f} = \frac{1}{100} + \frac{4}{100} = \frac{5}{100} = \frac{1}{20}
\]
Thus, the focal length \( f = 20 \) cm.
Since the focal length is positive, this means we need a **convex lens**.
The answer is **A. convex, 20 cm**.
someone long sighted suffers from hypermetropia and it can be corrected,with the use of a covex lens not a concave. the ans should be C not D
The answer isn't correct. long sightedness is managed with convex lens not concave lens
I'll be honest with you guys.
Firstly it can't be a concave because concave is used in treating short sightedness not long sightedness 😁...
