A transparent photographic slide is placed in front of a converging lens with a

Question

A transparent photographic slide is placed in front of a converging lens with a focal length of 2.19 cm. An image of the slide is formed 15.2 cm from the slide. (a) How far is the lens from the slide if the image is real? (Enter your answers from smallest to largest starting with the first answer blank. Enter NONE in any remaining answer blanks.) cm cm (b) How far is the lens from the slide if the image is virtual? (Enter your answers from smallest to largest starting with the first answer blank. Enter NONE in any remaining answer blanks.) cm cm

Explanation / Answer

Focal length of the lens, f = 2.19 cm
Image distance, v = 15.2 cm.
Take the object distance as u

Using the lens equation,
1/f = 1/u + 1/v
1/u = 1/f - 1/v
= 1/2.19 - 1/15.2 = 13.01 / (2.19 x 15.2)
u = 2.56 cm

a)
For a converging lens, real images are formed from the object placed at infinity to the focal length.
Smallest = f = 2.19 cm
Largest = infinity (If the slide can not be moved more than its starting point of 2.56 cm, then the answer is 2.56 cm)

b)
Virtual images are formed if the object is placed between f and the center of the lens
Smallest distance = 0
Largest distance = f = 2.19 cm

Note: If the object is exactly at the focal point, no image is formed and also, we can not keep an object at the center of the lens.

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