Question 22 of 29intermediate🔧 ApplyMCQ1 mark

A spherical mirror and a thin spherical lens have each a focal length of −15-15 cm. The mirror and the lens are likely to be

Exercise: EXERCISES | Q: 4 | (Chapter: Page 26)
For More Understanding

Explanation

Option (A) is correct. According to the sign convention used in the NCERT textbook, a concave mirror has a negative focal length, as seen in Example 9.2 where a concave mirror is given with 'Focal length, f = -15.0 cm'. Similarly, a concave lens (diverging lens) also has a negative focal length. Since both the mirror and the lens have a focal length of -15 cm, both are concave. A convex mirror would have a positive focal length, and a convex lens would also have a positive focal length.

Solution Steps

  1. Step 1: Identify that the focal length is negative (-15 cm) for both the mirror and the lens.

  2. Step 2: Recall from Example 9.2 that a concave mirror has negative focal length (f = -15.0 cm is explicitly stated for a concave mirror).

  3. Step 3: Apply the sign convention for lenses - concave lenses have negative focal lengths while convex lenses have positive focal lengths.

  4. Step 4: Conclude that since both have negative focal lengths, both the mirror and the lens are concave.

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