Question

A 7·cm tall candle is placed 175·cm from a lens with a focal length of +35·cm. Note: enter the absolute value of your answers for all distances and heights ... use the signs to determine the type and orientation of the images.

(a) How far from the lens will the image be?   cm.
(b) How tall will the image be?   cm

Now, imagine that the candle is moved closer, so that it is only 14·cm from the lens.

(d) How far from the lens will the image be?  cm.
(e) How tall will the image be?  cm
(f) Which of the following describes the image? Choose all that apply.

real

virtual

upright

inverted

Answer 1

a)
Object distance, u = 175 cm
Focal length, f = 35 cm
Consider the image distance as v.

Using the relation, 1/f = 1/u + /v,
1/v = 1/f - 1/u
= 1/35 - 1/175
= 0.023
v = 1/0.023
= 43.75 cm

b)
Consider the object height as ho and the image height as hi.
hi/ho = v/u
hi = (v/u) x ho
= (43.5/175) x 7
= 1.75 cm

d)
New object distance, u = 14 cm
1/v = 1/f - 1/u
= 1/35 - 1/14
= - 0.0428
v = - 1/0.0428
= - 23.33 cm

e)
hi/ho = v/u
hi = (v/u) x ho
= (23.33/14) x 7
= 11.67 cm

f)
The image is virtual, up right and on the same side of the object.