Question

You are observing a 20 cm-tall object in a pinhole camera with a screen at the end. The screen is 25 cm from the pinhole. The image on the screen is 5 cm tall.

(1) What can you say about the image?

a. Upright, Reduced
b. Inverted, Enlarged
c. Inverted, Reduced
d. Upright, Enlarged

Answer 1

Answer:

Upright, Reduced                    

Explanation:

Given that,

Height of the object, h = 20 cm

Image distance, v = 25 cm

Height of the image, h' = 5 cm

We know that the magnification of the mirror is given by :

$m=\dfrac{h'}{h}=\dfrac{v}{u}$

$\dfrac{5}{20}=\dfrac{25}{u}$

u = -100 cm (negative due to sign conventions)

Let f is the focal length of the lens. It can be calculated as :

$\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}$

$\dfrac{1}{f}=\dfrac{1}{25}-\dfrac{1}{(-100)}$

f = 20 cm

It is seen that the focal length of the lens is positive. So, it is a convex lens. The image formed by the convex lens is Upright, Reduced. Hence, the correct option is (a).