To find the focal length of a convex mirror using a convex lens.
An optical bench with four uprights (2 fixed upright in middle two outer uprights with lateral movement), convex lens, convex mirror, a lens holder, a mirror holder, 2 optical needles (one thin, one thick), a knitting needle, a half meter scale.
Focal length of a convex mirror ƒ = R/2
Where R is radius of curvature of the mirror.
Actual length of knitting needle, x = 15 cm.
Observed distance between image needle I and back of convex mirror, y = 15 cm
Index error = y - x = 15 – 15 = 0 cm No index correction
S.No. | Position of:/Object needle 0 (cm)/Lens Lcm/Mirror M cm/Image needle I (cm) | Radius of Curvature MI (cm) | |||
1 | 25 | 50 | 56 | 70.5 | 14.5 |
2 | 28.5 | 50 | 60 | 73.3 | 13.3 |
3 | 31.5 | 50 | 65 | 78.4 | 13.4 |
4 | 30.5 | 50 | 60 | 74 | 14 |
Mean R = 13.8
Mean corrected MI = R = 13.8 cm ƒ= R/2 = 6.9cm
The focal length of the given convex mirror = 6.9 cm
Get all latest content delivered to your email a few times a month.