Date:
Experiment Number:
Aim: To find the value of v for different values of u in case of a
concave mirror and to find the focal length.
Apparatus Required: Optical Bench, Uprights, Needles,
Concave Mirror
Theory:
Mirror formula
1 1 1
= +
𝑓 𝑣 𝑢
Where,
f is focal length of mirror
u is object distance
v is image distance
𝑢𝑣
or, f =
𝑢+𝑣
Diagram:
Observations:
Range of optical bench = 0 cm to 150 cm
Least count of optical bench = 0.1 cm
Observation Table:
Serial
𝒖𝒗
Number Position of u= - |𝒃 − 𝒂| v = - |𝒄 − 𝒂| f=
𝒖+𝒗
Mirror Object Image
Upright Upright Upright cm cm cm
( a ) cm ( b ) cm ( c ) cm
1. 75 39 30 - |39 − 75|= - 36 - |30 − 75|= - 45 -20.00
2. 75 40 28 - |40 − 75|= - 35 - |28 − 75|= - 47 -20.06
3. 75 42 24 - |42 − 75|= - 33 - |24 − 75|= - 51 -20.03
Calculations:
(−36)(−45) (−35)(−47) (−33)(−51)
f1 = f2 = f3 =
−36−45 −35−47 −33−51
1620 1645 1683
f1 = f2 = f3 =
−81 −82 −84
f1 = -20.00 cm f2 = -20.06 cm f3 = -20.03 cm
f1 + f2 + f3
f=
3
(−20.00) +(−20.06) +(−20.03)
f=
3
−60.09
f=
3
f = -20.03 cm
Result: focal length of given concave mirror = -20.03 cm
Sources of errors:
1. Error may be due to parallax.
2. Errors may occur due to carelessness of experimenter.
3. The uprights on the optical bench may not be vertical.
Precautions:
1. Parallax should be removed from tip to tip.
2. The screws of the holder and uprights should be tight.
3. The uprights must hold needles exactly vertical.
