An object is placed 25cm in front of a convex mirror has its image formed 5cm behind the mirror. what is the focal length of the convex mirror
8.20cm
6.25cm
5.60cm
4.50cm
Explanation
Video Explanation
Post your Contribution
Discussions (16)
1. Object distance (u) is always negative if the object is in front of the mirror (which is almost always the case).
2. Image distance (v) is:
Positive if the image is formed behind the mirror (as in convex mirrors).
Negative if the image is formed in front of the mirror (as in concave mirrors with real images).
3. Focal length (f) is:
Negative for concave mirrors.
Positive for convex mirrors.
incorrect calculation, a convex mirror always produces a virtual image, as such, it should be -5cm not positive andthe answer will be 8.3
Another instance. The Ai is different from your explanation and I think the Ai is the correct one here because for a Convex mirror, virtual is negative unlike your explanation.
The calculation is correct. I will explain. In lens and mirror calculation, there are two methods or ways of solving. The first is ''Real is positive method'' and the second is ''New cartesian Method''. for new cartesian method, u is taken to be negative and v is taken to be positive so that the focal length of the convex mirror will be positive at the final answer. for the real is positive method, u is positive and v is taken to be negative, so the final answer will be negative focal length. either positive or negative answer, it doesn't matter. the question above was solved using new cartesian method. two answers of 6.25 and -6.25 cannot be in the options. Jamb examiners know what they're doing. Thanks.
The answer is -6.25
For a convex mirror, the image is virtual and formed behind the mirror, so it is negative. v=-5
The focal length is 6.25 cm. The negative sign confirms it is a convex (diverging) mirror. If your calculation for v or f in a convex mirror comes out positive, you've likely missed a sign!
