A convex lens of focal length 25cm and a concave lens of focal length 10cm are placed in close focal contact with each other. Calculate the lens power of this combination.
Download our app for free and get started
Focal length: the distance between the centre of a lens or curved mirror and its focus. The equivalent distance in a compound lens or telescope. Optical power (also referred to as dioptric power, refractive power, focusing power, or convergence power) is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device, $\text{P}=\frac{1}{\text{f}}$$\text{f}=25\text{cm (Convex lens)}$
$\text{f}=-10\text{cm (Concave lens)}$
Power of convex lens $=\frac{1}{\text{f}}$$=\frac{1}{0.25\text{d}}=4.0\text{d}$
Power of concave lens $=\frac{1}{\text{f}}$$=\frac{1}{-0.10\text{d}}=-10\text{d}$
Total Power $=4.0\text{d}-10.0\text{d}$ Total Power $=-6.0\text{d}$
Download our app
and get started for free
Experience the future of education. Simply download our apps or reach out to us for more information. Let's shape the future of learning together!No signup needed.*
The refractive indices of three media are given below :
Medium
Refractive Index
A
1.6
B
1.8
C
1.5
A ray of light is travelling from $A$ to $B$ and another ray is travelling from $B$ to $C$. (a) In which of the two cases the refracted ray bends towards the normal? (b) In which case does the speed of light increase in the second medium? Give reasons for your answer.
What is meant by:
Draw diagram to show the action of convex mirror on a beam of parallel light rays. Mark on this diagram principal axis, focus F, centre of curvature C, pole P and focal length ƒ, of the convex mirror.
To find-the image-distance for varying object-distances in case of a convex lens, a student obtains on a screen a sharp image of a bright object placed very far from the lens. After that he gradually moves the object towards the lens and each time focuses its image of the screen.
In which direction - towards or away from the lens, does he move the screen to focus the object?
What happens to the size of the image - does it increase or decrease?
What happen when he moves the object very close to the lens?