# The focal length of a concave mirror is 2.0 cm…

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A. 2.7cm

B. 2.3cm

C.2.5cm

D. 2.0cm

A

## EXPLANATION…

If the focal length (f) is 2cm, and the object distance (u) is 8cm, then it follows that real image will be formed, this is because the object distance is more than the focal length. The reason for determining if a real image will be formed first is because virtual image formula has a negative v, while real image formula has a positive v. hence:

1/f = 1/u + 1/v

where v = image distance from the mirror, u = object distance from the mirror given as 8cm, and f = focal length which is given as 2cm.

Therefore, 1/v = 1/f – 1/u

1/v = 1/2 – 1/8

1/v = 4/8 – 1/8

1/v = (4 – 1)/8

1/v = 3/8

v = 8/3 = 2.667 = 2.7cm

### Now for the right answer to the above question:

1. Option A is correct.
2. Option B is incorrect.
3. C is incorrect.
4. D is not the correct answer.
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## KEY-POINTS…

You may please note these/this:

• Virtual image will only be formed when the object distance from the concave mirror is below the focal length, that is, the object is placed between the pole and the focus. In this case, v is negative.
• But if the object is placed anywhere more than the focal length, a real image will be formed, and v will be positive.
• This question is for concave mirror, but for convex mirror, all images formed by a convex mirror are virtual, so v is always negative for convex mirrors.
• Please beware of this sign convention.

Use the questions and answers session to raise further questions and deal more on this topic…

/ culled from 2017 JAMB-UTME physics past question 1 /

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