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
Focal length, f = 20 cm (a) Using the lens formula, 1v =1f+1u∴ 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
∴
1v=1f+1u =-116+112 = -3+448 =148
⇒ 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?
- 50 cm and convex lens
- 80 cm and convex lens
- 50 cm and concave lens
- 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
Free
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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