Question

A 10mm long awl pin is placed vertically in front of a concave mirror.A 5mm long awl pin is formed at a distance of 30cm in front of mirror.Find the focal length of the mirror?​

Answer 1

Hey!

Here, size of object = O = 10.0 mm = 1.0 cm (as, 1 cm = 10 mm)
Size of Image size = I = 5.0 mm = 0.5 cm                                                                             
Image distance(v) = −30 cm  
Let the object distance be u.

Focal length = ??

Magnification (m)= -v/u
I/O = -v/u
0.5/1 = -30/u
= -60cm

By mirror formula 1/f = 1/v + 1/u
= 1/-30 +1/-60
= -2-1/60
= -3/60
= 20cm

focal lengt(f) = 30cm.