Advertisements
Advertisements
प्रश्न
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?
Advertisements
उत्तर
(a) Power of convex lens P1 = + 5 D
Power of concave lens P2 = - 7.5 D
The combined power is the algebraic sum of the individual powers of the lenses.
∴ Power of the combination P = P1 + P2
P = 5 D - 7.5 D = - 2.5 D
Therefore, the power of the combination of the lenses is - 2.5 D.
(b) The power and focal length are related as:
`p=1/f`
`-2.5=1/f`
∴ f=`-1/2.5=-0.4m=-40 cm`
Therefore, the focal length of the combination of the lenses is 40 cm. The negative sign shows that the combination of the two lenses causes it to behave like a concave lens.
APPEARS IN
संबंधित प्रश्न
Find the power of a concave lens of focal length 2m.
(a) At what distance should the lens be held from the figure in order to view the squares distinctly with the maximum possible magnifying power?
(b) What is the magnification in this case?
(c) Is the magnification equal to the magnifying power in this case? Explain.
The lens mentioned in 6(b) above is of focal length 25cm. Calculate the power of the lens.
Consider two statements A and B given below:
A: real image is always inverted
B: virtual image is always erect
Out of these two statements:
How is the power of a lens related to its focal length?
A diverging lens has focal length of 3 cm. Calculate the power.
A lens has a focal length of, −10 cm. What is the power of the lens and what is its nature?
An object of height 4 cm is placed at a distance of 15 cm in front of a concave lens of power, −10 dioptres. Find the size of the image.
State the condition for the following a lens has both its focal lengths equal .
How is the sign (+ or -) of power of a lens related to its divergent or convergent action?
On reducing the focal length of a lens, its power ______.
Mohan obtained a sharp inverted image of a distant tree on the screen placed behind the lens. He then moved the screen and tried to look through the lens in the direction of the object. He would see:
(1) a blurred image on the wall of the laboratory.
(2) an erect image of the tree on the lens.
(3) no image as the screen has been removed
(4) an inverted image of the tree at the focus of the lens.
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
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.
The power of a lens is +2.0 D. Find its focal length and state the kind of lens.
If the lens is of focal length 25 cm. Calculate the power of the lens.
If there is a convex lens of focal length 75 cm and a concave lens of focal length 40 cm, then calculate their combined power and combined focal length.
Focal length : metre : : power of 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.
