Question

Q)write the correct alternative...

1) The focal length of convex mirror is______
while that concave mirror is______
1) Positive
2)Nagative
3) Positive and Negative
4)None of these​

Answer 1

Answer :

3

Explanation:

positive & negative

Answer 2

Solution:

The focal length of convex mirror is positive, while that of the concave mirror is negative.

We know that, if the image is formed on the left side of a mirror, it is negative.

We also know that, if the image is formed on the right side of a mirror, it is positive.

In convex mirror:

• In convex mirror, reflection of ray of light takes place in left side of the mirror.
• But, since it's a diverging mirror, the ray gets extended and it appears to meet at a particular point at right hand side.
• So, the image formation takes place in right side of the mirror.
• Therefore, the focal length of the convex mirror is always positive.

In concave mirror:

• In concave mirror, reflection of light rays takes place in left side of the mirror and the image is also formed at left side, since the mirror is converging.
• Therefore, the focal length of the concave mirror is always negative.

Hence, the answer is Option-C

Know more:

Know more about sign convention for concave mirror and convex mirror:

Concave mirror:

• Object distance(u) = Negative
• Image distance(v) = Negative
• Focal length(f) = Negative
• Magnification(m) = Negative

Convex mirror:

• Object distance(u) = Negative
• Image distance(v) = Positive
• Focal length(f) = Positive
• Magnification(m) = Positive

Note:

• Object distance is always negative for both mirror and lens, because the object is placed at the left side of the mirror/lens.
• If magnification is positive, the image formed is virtual and erect.
• If magnification is negative, the image formed is real and inverted.