SIQ Science 10th

Q. 2)  

(a) Two lenses have power of (i) +2D (ii) –4D. 

What is the nature and focal length of each lens?

(b) An object is kept at a distance of 100 cm for a lens of power –4D. Calculate image distance.

Ans.

(a) 

(i) P₁ =+ 2D

f1 = 1/P₁

     = 1/2

     = 0.50m

_f1 = 50 cm

Lens is convex lens.

(ii) P₂ = –4D

_f2 = 1/P1

     = 1/–4

     = –0.25m

f₂ = –25 cm

Lens is convex lens.

b. For the lens of power P 

D = – 4D , we have

f = –25 cm, 

u = –100 cm

Q. 2) One half of a convex lens is covered with a black paper.

a. Show the formation of image of an object placed at 2F₁ of such covered lens with the help of ray diagram. Mention the position and nature of 

image.

b. Draw the ray diagram for same object at same position in front of the same lens, but now uncovered. Will there be any difference in the 

image obtained in the two cases? Give reason for your answer.

c. A 4 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm.

The distance of the object from the lens is 15 cm. Find the nature, position and size of the image.

Ans.

a. If the lower half of the lens is covered with anything even then it will form a complete real and inverted image of same size at C₁(2F₂) with reduced intensity of image.

b. There will be no change in the nature and position of the object except in later case the image will 

be brighter after uncovering it.

c.