(a) Upper half of a convex lens is covered with a black paper. Draw a ray diagram to show the formation of…
(a) Upper half of a convex lens is covered with a black paper. Draw a ray diagram to show the formation of image of an object placed at a distance of 2F from such a lens. Mention the position and nature of the image formed. State the observable difference in the image obtained if the lens is uncovered. Give reason to justify your answer.
(b) An object is placed at a distance of 30 cm from the optical centre of a concave lens of focal length 15 cm. Use lens formula to determine the distance of the image from the optical centre of the lens.
Solution
(a)
[Diagram: two ray diagrams for a convex lens forming a real, inverted image between F2 and 2F2, for an object placed beyond 2F1]
Note: Any one of the above drawn ray diagrams should be marked.
When the upper half of lens is covered:
Position of image: at 2F on the other side of the lens
Nature of image: Real and inverted
Observable difference in the image, if the lens is uncovered
The brightness of the image will increase
Reason: More number of rays will pass through the lens to form the image.
(b)
Here u = – 30 cm, f = –15 cm, v = ?
$\dfrac{1}{v}$ − $\dfrac{1}{u}$ = $\dfrac{1}{f} \dfrac{1}{v}$ = $\dfrac{1}{f}$ + $\dfrac{1}{u}$
= 1/−15 + 1/−30
v = – 10 cm