Question

Less than half of the time remains. Remaining Time: 21 minutes, 42 seconds. * Question Completion Status: 48 A Moving to the next question prevents changes to this answer. Question 18 of 19 Question 18 12 points sum M Save Answer 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 hy ken OB. 13/2 O C. 28/5 OD. 30/4 O E. 25/3 OF. 15/4 AF ase A Moving to the next question prevents changes to this answer. Question 18 of 19

Answer 1

Given the height of the object is ho = 2cm and that of the image is hi = 7cm. Also the image distance u is 4cm. By sign convention u = -4cm. The magnification of the image is given by,

$m=\frac{h_{i}}{h_{o}}=\frac{7}{2}$

If f is the focal length, then the magnification is given by

$m=\frac{f}{f-u}$

$\frac{7}{2}=\frac{f}{f-(-4)}$

$\frac{7}{2}=\frac{f}{f+4}$

$7(f+4)=2f$

$7f+28=2f$

$7f-2f=-28$

$5f=-28$

$f=-\frac{28}{5}$

So the focal length of the concave mirror is 28/5. The negative sign indicate the mirror is concave.

Third option is the correct answer.