Advertisements
Advertisements
Question
What is meant by a power of a lens? Define its SI unit.
Advertisements
Solution 1
The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P. The power P of a lens of focal length f is given as
P = 1/f
The SI unit of power is dioptre (D).
Solution 2
Power of lens: Power of lens is defined as its ability to converge or diverge the rays of light falling on it. SI unit of power is Dioptre.
APPEARS IN
RELATED QUESTIONS
What is the focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm? Is the system a converging or a diverging lens? Ignore thickness of the lenses.
The lens mentioned in 6(b) above is of focal length 25cm. Calculate the power of the lens.
A student uses a lens of focal length 40 cm and another of –20 cm. Write the nature and power of each lens
Give the usual name for the following:
A point inside a lens through which the light passes undeviated.
Which of the two has a greater power: a lens of short focal length or a lens of large focal length?
A converging lens has focal length of 50 mm. What is the power of the lens?
The focal length of a lens is +150 mm. What kind of lens is it and what is its power?
Fill in the following blank with suitable words :
The reciprocal of the focal length in metres gives you the _________ of the lens, which is measured in ___________.
How is the sign (+ or -) of power of a lens related to its divergent or convergent action?
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 symmetric double convex lens in cut in two equal parts by a plane perpendicular to the principal axis. If the power of the original lens was 4 D, the power a cut-lens will be
Consider three converging lenses L1, L2 and L3 having identical geometrical construction. The index of refraction of L1 and L2 are \[\mu_1 \text{ and } \mu_2\] respectively. The upper half of the lens L3 has a refractive index \[\mu_1\] and the lower half has \[\mu_2\] following figure . A point object O is imaged at O1 by the lens L1 and at O2 by the lens L2placed in same position. If L3 is placed at the same place,
(a) there will be an image at O1
(b) there will be an image at O2.
(c) the only image will form somewhere between O1 and O2
(d) the only image will form away from O2.
A 5.0 diopter lens forms a virtual image which is 4 times the object placed perpendicularly on the principal axis of the lens. Find the distance of the object from the lens.
Consider the situation described in the previous problem. Where should a point source be placed on the principal axis so that the two images form at the same place?
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.
The above lens has a focal length of 10 cm. The object of height 2 mm is placed at a distance of 5 cm from the pole. Find the height of the image.
An object is kept at a distance of 1m from a lens of power +2D:
- Identify the type of lens.
- Calculate its focal length and distance of the image formed.
The focal length of a double convex lens is equal to the radius of curvature of either surface. What is the refractive index of its material?
