Question

A 2cm high tooth is placed 4cm in front of a concave mirror. if the image is 7cm high, it is virtual and straight, the focal length of the mirror is:

Un diente de 2 cm de altura se coloca a 4 cm enfrente de un espejo cóncavo. Si la imagen tiene 7 cm de alto, es virtual y derecha, la longitud focal del espejo es: O A. 16/3 OB. 13/2 OC. 28/5 OD. 30/4 O E. 25/3 OF. 15/4

Answer 1

object distance, u = 4 cm
Consider v as the image distance.
Consider that ho and hi are the heights of the object and the image.
ho / hi = u / v
v = u * (hi / ho)
= 4 * (7 / 2)
= 14 cm
Consider f as the focal length
-------------------------------------
1 / f = 1 / u + 1 / v
= 1 / 4 + 1 / (-14) = 10 / 56 [Here v is negative since the image is virutal]
= 5 / 28

f = 28 / 5