Question

A 4 cm long awl pin is placed vertically in front of a concave mirror,a 2 cm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is

Answer 1

ʜ0=10ᴍᴍ ʜɪ=5ᴍᴍ ᴠ=−30ᴄᴍ

ʜᴇʀᴇ ɢɪᴠᴇɴ ,

ɴᴏᴡ ᴡᴇ ᴋɴᴏᴡ

ᴍ=−ᴠ ᴜ =ʜɪ ʜᴏ ⇒ᴜ=−ᴠʜᴏ ʜɪ =−30×10 5 =−60 ᴄᴍ ɴᴏᴡ ᴜꜱɪɴɢ ᴍɪʀʀᴏʀ ꜰᴏʀᴍᴜʟᴀ 1 ꜰ =1 ᴠ +1 ᴜ =1 −30 +1 −60 =−1 20 ʜᴇɴᴄᴇ ꜰ=−20 ᴄᴍ