Physics, asked by avnigoyal, 1 year ago

the near point of a hypermetropic person is 40 cm. calculate the focal length and power of a convex lens used in his spectacles.

avnigoyal: yes I hv the answer but the method is not mentioned

Bunti360: if you are sure that the answer is correct , I will write the answer !

avnigoyal: thank u sooo much!

Bunti360: is the focal length -2/3 and power -3/2?

avnigoyal: yup

Bunti360: ok !

Bunti360: did you understand ?

avnigoyal: yeah!

glenjohnymj: But the focal length you have written is in negative .... Which is not possible coz convex lens has a positive focal length

Bunti360: yeah, what you told was correct, The focal length is positive, and i have done a mistake at image and object distance ! I have mistakenly reversed them, Take image distance as object's and object's distance as image distance, Now you will get +ve focal length ! Thanks for informing me glenjohnmyj !

Answers

Answered by Bunti360

104

So to find the focal length, We need to use lens makers formula,

We know that , the formula is ,

1/f = 1/v - 1/u,

Normal person's Near point is 25cm,

Hypermetropic person usually can't see near objects, So what they have to do is, Use convex lens and make the object's image fall at 25 cm and let the lens of eye to make the image as object,

So what we have to do here is, Make the near point of Hypermetropic person to 25cm,

So here object distance = Near point of Hypermetropic person,

Image distance = -25cm(- indicates direction),

=> Object distance = u = -40cm (Direction),
Focal length of Convex lens is always positive !,

Now Inputting all the values to find focal length in cm,

=> 1/f = -(1/25) -(-1/40),
=> 1/f = 1/40 - 1/25,
=> 1/f = (5-8)/200
=> 1/f = -3/200 cm,
=> f = -200/3 cm, Converting it into m,
=> f = -200/3/100 = -2/3 m,

So therefore focal length = -2/3 m,

Now calculating power,

Power = 1/Focal length in m ,
Units = D , Dioptre,

=> Power = 1/-2/3 = -3/2D,

Therefore power = -3/2 D, and focal length = -2/3 m,

Hope you understand, Have a great day !

avnigoyal: thanks

Bunti360: no problem !

Bunti360: Thank you for the Brainliest Answer !

Answered by sarvika2407

Answer:

Power = -3/2 D, and Focal length = -2/3 m,

Explanation:

So to find the focal length, We need to use lens makers formula,

We know that , the formula is ,

1/f = 1/v - 1/u,

Normal person's Near point is 25cm,

Hypermetropic person usually can't see near objects, So what they have to do is, Use convex lens and make the object's image fall at 25 cm and let the lens of eye to make the image as object,

So what we have to do here is, Make the near point of Hypermetropic person to 25cm,

So here object distance = Near point of Hypermetropic person,

Image distance = -25cm(- indicates direction),

=> Object distance = u = -40cm (Direction),

Focal length of Convex lens is always positive !,

Now Inputting all the values to find focal length in cm,

=> 1/f = -(1/25) -(-1/40),

=> 1/f = 1/40 - 1/25,

=> 1/f = (5-8)/200

=> 1/f = -3/200 cm,

=> f = -200/3 cm, Converting it into m,

=> f = -200/3/100 = -2/3 m,

So therefore focal length = -2/3 m,

Now calculating power,

Power = 1/Focal length in m ,

Units = D , Dioptre,

=> Power = 1/-2/3 = -3/2D,

Therefore power = -3/2 D, and focal length = -2/3 m,

Previous Question

Next Question