Question

A student has to project a 3 times magnified image of the candle flame on a wall.name the type of lens required for this purpose.If the candle flame is at a distance of 6m from the wall,find the focal length of the lens.

Answer 1

Converging lens or Convex lens should be used by the user to project the image of candle on the wall as he wants it to be three times magnified.

Reason:
In Convex lens, when object is placed between F and C , the image will be formed beyond C .
Nature of Image :
--> magnified, real and inverted.

2) hi=3ho
Image distance= object distance + 6
v=6+u
we know that magnification=m=v/u
m=v/u
-3=v/u[since image is magnifed three times ]
-3=6+u/u
-3u=6+u
4u=-6
u=-6/4
u=-1.5 m
v=6-(-1.5)
v=7.5m
by using lens formula,
1/f =1/v-1/u
1/7.5 +1/1.5=1/f
10/75+10/15=1/f
10+50/ 75=1/f
∴ f=75/60=1.25m
Thus, Focal length of a given convex lens is 1.25m