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Question

An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. The magnification is:

This question was previously asked in
RRB ALP 2018 CBT 2 Fitter Question Paper (21-Jan-2019) (Shift 3)
The correct answer is
+0.6

To find the magnification of the object placed in front of a convex mirror, we can use the formula for magnification \( m \) in the context of mirrors:

\(m = \frac{h'}{h} = \frac{-v}{u}\)

where:

  • \( h' \) is the height of the image.
  • \( h \) is the height of the object.
  • \( v \) is the image distance.
  • \( u \) is the object distance (given as -10 cm since the object is on the left side of the mirror, according to the sign convention for mirrors).

For a convex mirror, the focal length \( f \) is positive. Given: \( f = 15 \, \text{cm} \) and \( u = -10 \, \text{cm} \).

Using the mirror formula,

\(\frac{1}{f} = \frac{1}{v} + \frac{1}{u}\)

Substitute the values:

\(\frac{1}{15} = \frac{1}{v} + \frac{1}{-10}\)

Rearrange to solve for \( \frac{1}{v} \):

\(\frac{1}{v} = \frac{1}{15} + \frac{1}{10}\)

Calculate the right-hand side:

\(\frac{1}{v} = \frac{2}{30} + \frac{3}{30} = \frac{5}{30} = \frac{1}{6}\)

Therefore,

\(v = 6 \, \text{cm}\)

Now calculate the magnification \( m \):

\(m = \frac{-v}{u} = \frac{-6}{-10} = +0.6\)

Hence, the magnification of the object by the convex mirror is +0.6.

Let's validate the options given in the question:

  • +0.3 - Incorrect. This does not match the computed magnification.
  • +0.6 - Correct. This matches the computed magnification.
  • -0.3 - Incorrect. The sign is wrong and does not match the computed magnification.
  • -0.6 - Incorrect. The sign is wrong and does not match the computed magnification.
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  5. The radius of curvature of a spherical mirror is 20 cm. Its focal length is:
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Important Questions from Mirrors and Images

  1. What type of mirror is used in the headlights of vehicles?

  2. A concave mirror forms a real and inverted image of a distant object at a distance of $15 \text{ cm}$ from the mirror.
    If an object is placed $20 \text{ cm}$ in front of this mirror, what will be the nature and magnification of the image formed?
  3. The number of images observable between two parallel mirror is

  4. An object is placed at a distance of \(\frac{f}{2}\) from a convex lens. The image will be

  5. Which of the following pair is correct?

    I. Mirror formula : (1/v) – (1/u) = (1/f)

    II. Lens formula : (1/v) + (1/u) = (1/f)

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