To determine the approximate value of the focal length of a given concave mirror, you focus the image of a distant object formed by the mirror on a screen. The image obtained on the screen, as compared to the object is always:

Q: To determine the approximate value of the focal length of a given concave mirror, you focus the image of a distant object formed by the mirror on a screen. The image obtained on the screen, as compared to the object is always:

The correct option is **(b)**. When a distant object (at infinity) is focused using a concave mirror, its image is formed at the principal focus ($F$). According to the properties of image formation by a concave mirror, the image of an object at infinity is real, inverted, and highly diminished (point-sized). Since it is obtained on a screen, it must be a real image, which is always inverted.

Also asked in:2016

Light — Reflection and Refraction

Reflection of Light

Options

(a)Laterally inverted and diminished

(b)Inverted and diminished

(c)Erect and diminished

(d)Erect and highly diminished

Official Answer

The correct option is (b).

When a distant object (at infinity) is focused using a concave mirror, its image is formed at the principal focus ( $F$ ). According to the properties of image formation by a concave mirror, the image of an object at infinity is real, inverted, and highly diminished (point-sized). Since it is obtained on a screen, it must be a real image, which is always inverted.

concave mirrordistant objectfocusreal and inverteddiminished

Marking Scheme

11 mark

Hint

A distant object is at infinity. Think about where the parallel rays from infinity converge after reflecting off a concave mirror and what the nature of that image is.

Quick Oral Answer

The image of a distant object formed by a concave mirror is real, inverted, and highly diminished, located at its principal focus.

Analysis & Explanation

Why (b) is correct: A distant object is considered to be at infinity. The rays coming from it are parallel to the principal axis. After reflection from a concave mirror, these rays converge at the principal focus ( $F$ ), forming a real and inverted image. Thus, the image is inverted and diminished.
Why (a) is incorrect: Lateral inversion is a phenomenon where the left side of an object appears as the right side in a plane mirror. Real images formed by spherical mirrors are vertically inverted (upside down).
Why (c) and (d) are incorrect: Real images formed on a screen are always inverted, never erect.

Common Mistakes

1Confusing real images with erect images.
2Selecting 'laterally inverted' instead of vertically 'inverted'.

Interesting Facts

This experiment of focusing a distant tree or building on a screen is the standard practical method used to find the approximate focal length of a concave mirror in school laboratories.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2016?

This question carries 1 mark in the CBSE Class 10 Science 2016 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Light — Reflection and Refraction" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Reflection of Light" from CBSE Class 10 Science.

Has this question appeared in other CBSE Science papers?

Yes, a similar question appeared in the 2016 CBSE Class 10 Science paper.

What type of question is this in the CBSE Class 10 Science 2016 paper?

This is a MCQ question from Section B in the CBSE Class 10 Science 2016 paper.