A myopic person has a power of dioptre what is the focal length and nature of his lens

The living content of cell is called protoplasm. It is composed of :

Which one of the following islands is of volcanic origin

A beam of light converges at a point P. Now a lens is placed in the path of the convergent beam 12 cm from P. At what point does the beam converge if the lens is (a) a convex lens of a focal length 20 cm, and (b) a concave lens of focal length 16 cm?

Here, the point P on the right of the lens acts as a virtual object.

Object distance, u = 12 cm

∴           1v =120+112      =3+560 =860

i.e.,           v = 60/8 = 7.5 cm. 

Image is at a distance of 7.5 cm to the right of the lens, where the beam converges. (b)Now,Focal length of concave lens, f = –16 cmObject distance, u = 12 cm 

⇒      v = 48 cm 

Hence, the image is at a distance of 48 cm to the right of the lens, where the beam would converge.

Text Solution

Solution : Here, `P = -1.5 D, f = (100)/(P) cm = (100)/(-1.5) cm = -66.7 cm` <br> This is the focal length of concave lens used by the person for distant vision. This is the distance upto which the person can see clearly without specs.

A myopic person has a power of -1.25 Dioptre. What is the focal length and nature of his lens?

1. 50 cm and convex lens
2. 80 cm and convex lens
3. 50 cm and concave lens
4. 80 cm and concave lens

Answer

P = 1/f

f = 1/P = 1/1.25 = 0.8 m or 80 cm

Convex lenses, or converging lenses, have positive values, as they magnify objects. A diverging lens or concave lens, on the other hand, have a negative dioptre, since it makes objects appear smaller than they are.

The correct option is D.

Answer (Detailed Solution Below)

Option 4 : 80 cm and concave lens

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10 Qs. 10 Marks 10 Mins

• Myopia is an eye condition in which you can see objects near to you clearly but far ones are blurry. It is also called near-sightedness.
• Myopia can be corrected by Concave lens.
• A Dioptre is a unit of measurement of the optical power of a lens.
• It is equal to the reciprocal of the focal length.
• D = 1/F 🡪 -1.25 = 1/F 🡪 Focal length is in centimetres. 🡪 F = -100/1.25
• F = -80.

Focal Length is – 80 cm.

Hence, the concave lens is used.

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