The focal distance of a plane mirror is

Question

The focal distance of a plane mirror is

1. A. +1 cm
2. B. − 1 cm
3. C. 2 cm
4. D. Infinity (Correct answer)

Correct Answer

Option D — Infinity

Detailed Solution & Explanation

The correct answer is Infinity.

Key Points

• The focal length of a plane mirror is infinity, because a plane mirror can be regarded as a spherical mirror of infinite radius of curvature.
• The relationship between focal length and radius of curvature is:
• f = R / 2
• For a plane mirror the surface is flat, so R = infinity, and therefore f = infinity/2 = infinity.
• Consequently its power (P = 1/f) is zero, since a plane mirror neither converges nor diverges a beam of light.
• The image formed by a plane mirror is always virtual, erect, of the same size as the object, laterally inverted, and formed as far behind the mirror as the object is in front of it.

Exam Tip

• contrast the three mirror types — a concave mirror converges light and has a positive focal length by the Cartesian convention, a convex mirror diverges light, and a plane mirror does neither.
• Convex mirrors are used as rear-view mirrors because they give a wider field of view with a diminished, erect image.

प्रश्न (हिन्दी में)

समतल दर्पण की फोकस दूरी होती है:

1. A. +1 सेमी
2. B. − 1 सेमी
3. C. 2 सेमी
4. D. अनन्त