Advertisements
Advertisements
Question
Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of –50 cm. What is the nature of the lens and its power used by each of them?
Advertisements
Solution
Power of a lens indicates the converging effect of the lens. It is the reciprocal of the focal length of the lens.
Mathematically, `"Power" ("P") = 1/"f"`
The unit of power is dioptre (D).
The nature and power of the lens in each case are:
When the focal length of the lens is 50 cm,
`("P") = 1/"f"`
`= 1/0.5`
= 2D
This lens has positive value of focal length. So, it is convex lens.
When the focal length of the lens is -50 cm,
`("P") = 1/"f"`
`= 1/(-0.5)`
= -2 D
This lens has negative value of focal length. So, it is concave lens.
APPEARS IN
RELATED QUESTIONS
Define the power of a lens.
Find the power of a concave lens of focal length 2m.
Which type of lens has (a) a positive power, and (b) a negative power?
How is the power of a lens related to its focal length?
A diverging lens has focal length of 3 cm. Calculate the power.
The optician's prescription for a spectacle lens is marked +0.5 D. What is the:
(a) nature of spectacle lens?
(b) focal length of spectacle lens?
The focal length of a lens is +150 mm. What kind of lens is it and what is its power?
A convex lens of power 5 D and a concave lens of power 7.5 D are placed in contact with each other. What is the :
(a) power of this combination of lenses?
(b) focal length of this combination of lenses?
A combination of lenses for a camera contains two converging lenses of focal lengths 20 cm and 40 cm and a diverging lens of focal length 50 cm. Find the power and focal length of the combination.A optical instrument in which the above arrangement of convex lens is used is a convex lens used to burn paper by focusing sunlight.
A converging lens has a focal length of 50 cm. The power of this lens is:
Express the power (with sign) of a concave lens of focal length 20 cm.
Name the type of lens whose power is positive.
The image of an object formed by a lens is real, inverted and of the same size as the object. If the image is at a distance of 40 cm from the lens, what is the nature and power of the lens ? Draw ray diagram to justify your answer.
How does focal length of a lens change when red light incident on it is replaced by violet light? Give reason for your answer.
A double convex lens has two surfaces of equal radii R and refractive index \[m = 1 \cdot 5\]
A pin of length 2.00 cm is placed perpendicular to the principal axis of a converging lens. An inverted image of size 1.00 cm is formed at a distance of 40.0 cm from the pin. Find the focal length of the lens and its distance from the pin.
A diverging lens of focal length 20 cm and a converging lens of focal length 30 cm are placed 15 cm apart with their principal axes coinciding. Where should an object be placed on the principal axis so that its image is formed at infinity?
The following diagram shows the object O and the image I formed by a lens. Copy the diagram and on it mark the positions of the lens LL’ and focus (F). Name the lens.
Assertion and reasoning type
- Assertion: Myopia is due to the increase in the converging power of eye lens.
- Reason: Myopia can be corrected with the help of concave lens.
In what unit is the power of a lens expressed?
