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प्रश्न
A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be:
(a) +10 cm
(b) +20 cm
(c) −10 cm
(d) −20 cm
उत्तर
+20 cm
We know that:
Power (P) = `1/"focal length"`
Therefore
Power of convex lens `p_1=1/(10cm)=1/(0.1m)=+10D`
Power of convave lens `P_2=1/(-20cm)=-1/(0.2m)=-5D`
We know that power of lens adds up when lenses comes in contact. Therefore, we have:
Net power (P) = P1 + P2
Net power (P) = 10 D-5 D = 5 D
Net focal length = `1/"power"=1/5=0.2m=20cm`
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संबंधित प्रश्न
Analyse the following observation table showing variation of image-distance (v) with object-distance (u) in case of a convex lens and answer the questions that follow without doing any calculations :
| S. No. | Object-Distance u (cm) |
Image-Distance v (cm) |
| 1 | –100 | +25 |
| 2 | –60 | +30 |
| 3 | –40 | +40 |
| 4 | –30 | +60 |
| 5 | –25 | +100 |
| 6 | –15 | +120 |
(a) What is the focal length of the convex lens? Give reason to justify your answer.
(b) Write the serial number of the observation which is not correct. On what basis have you arrived at this conclusion?
(c) Select an appropriate scale and draw a ray diagram for the observation at S.No. 2. Also find the approximate value of magnification.
A ray of light travelling in water emerges into air. Draw a ray-diagram indicating the change in its path.
Describe with the help of a ray-diagram, the size, nature and position of the image formed by a convex lens when an object is placed beyond 2f in front of the lens.
An object 2 cm tall is placed on the axis of a convex lens of focal length 5 cm at a distance of 10 m from the optical centre of the lens. Find the nature, position and size of the image formed. Which case of image formation by convex lenses is illustrated by this example?
What type of lens is used to correct
hypermetropia
A lens always forms an image between the object and the lens.
A lens always forms an image between the object and the lens.
If an object is placed in front of a convex lens beyond 2F1, then what will be the position, relative size, and nature of an image which is formed? Explain with a ray diagram.
In sunglasses, both of its surfaces are curved, yet their behaviour is neither like a convex lens nor like a concave lens. State the reason.
Can a normal convex lens behave like a concave lens and vice-versa?
We can burn a piece of paper by focussing the sun rays by using a particular type of lens. Name the type of lens used for the above purpose. Draw a ray diagram to support your answer.
