Question

10 mm long pin is placed vertically in front of concave mirror. 5 mm long image of pin is formed at 30 cm in front of the mirror the focal length of the mirror is is?

Answer 1

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⎟⎟ ✪✪ ＱＵＥＳＴＩＯＮ ✪✪⎟⎟

ᴀ 10 ᴍᴍ ʟᴏɴɢ ᴘɪɴ ɪs ᴘʟᴀᴄᴇᴅ ᴠᴇʀᴛɪᴄᴀʟʟʏ ɪɴ ғʀᴏɴᴛ ᴏғ ᴄᴏɴᴄᴀᴠᴇ ᴍɪʀʀᴏʀ. 5 ᴍᴍ ʟᴏɴɢ ɪᴍᴀɢᴇ ᴏғ ᴘɪɴ ɪs ғᴏʀᴍᴇᴅ ᴀᴛ 30 ᴄᴍ ɪɴ ғʀᴏɴᴛ ᴏғ ᴛʜᴇ ᴍɪʀʀᴏʀ ᴛʜᴇ ғᴏᴄᴀʟ ʟᴇɴɢᴛʜ ᴏғ ᴛʜᴇ ᴍɪʀʀᴏʀ ɪs ?

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✿✿ ＡＮＳＷＥＲ ✿✿

h = 10 mm

h2 = -5 mm

v = -30 cm

m = h2 / h1 = -5 / 10

m = -1 / 2

-v / u = -1 / 2

➡ -30 / u = 1 / 2

.°. u = -60 cm

ᴍɪʀʀᴏʀ ғᴏʀᴍᴜʟᴀ :-

➡ 1/v + 1/u = 1/f

➡ -1 / 2×30 -1 / 60 = 1 / f

➡ -3 / 60 = 1 / f

.°. f = -20 cm

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