A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is
(a) -30 cm
(b) -20 cm
(c) -40cm
(d) -60 cm
Answers
Answer:
- 20 cm
Explanation:
Answer: (2) - 20 cm
The focal length of the concave mirror can be written as
1/f = 1/v + 1/u
Where
f = focal length of the mirror
v = distance of the image from the pole
u = Distance of the object from the pole
Given: The image is formed in front of the mirror, so the image is real and is at a distance of 30 cm from the pole.
∴ v = -30cm
Magnifying factor (m) = m/u = -y/x
y = Image height
x = Object’s height
m = -5/10
Since m = v/u
-5/10 = v/u
-5/10 = (-30)/u
∴ u = 60cm
Since object distance is always negative, so u = -60cm
Apply mirror formula as
(1/ v) + (1/u) = 1/f
1 /(- 60) + 1/(- 30) = 1/f
-1/60 – 1/30 = 1/f
-90/1800 = 1/f
1 /(- 20) = 1/f or
f = – 20 cm
The focal length lies in front of the mirror. Therefore, the sign is negative
Answer:
b. option is right and brainliest please