2 marksShort AnswerSection A

Define and show on a diagram, the following terms relating to a concave mirror : (i) Aperture (ii) Radius of curvature

Q: Define and show on a diagram, the following terms relating to a concave mirror : (i) Aperture (ii) Radius of curvature

### (i) Aperture The diameter of the circular outline of the reflecting surface of a spherical mirror is called its **aperture**. In ray diagrams, it is represented by the linear distance $MN$ between the extreme edges of the mirror. ### (ii) Radius of Curvature The radius of the sphere of which the reflecting surface of a spherical mirror forms a part is called its **radius of curvature**. It is represented by the letter $R$. The distance $PC$ between the pole ($P$) and the centre of curvature ($C$) is equal to the radius of curvature ($R = PC$). ### Diagram

Light — Reflection and Refraction

Spherical Mirrors

Official Answer

(i) Aperture

The diameter of the circular outline of the reflecting surface of a spherical mirror is called its aperture. In ray diagrams, it is represented by the linear distance $MN$ between the extreme edges of the mirror.

(ii) Radius of Curvature

The radius of the sphere of which the reflecting surface of a spherical mirror forms a part is called its radius of curvature. It is represented by the letter $R$ . The distance $PC$ between the pole ( $P$ ) and the centre of curvature ( $C$ ) is equal to the radius of curvature ( $R = PC$ ).

Diagram

Aperturediameter of reflecting surfaceRadius of curvatureradius of spherePCMN

Marking Scheme

10.5 marks
20.5 marks
31 mark

Hint

Recall that the aperture is the width/diameter of the mirror's reflecting face, while the radius of curvature is the radius of the complete sphere from which the mirror was cut.

Quick Oral Answer

The aperture represents the light-gathering area of the mirror. A mirror with a larger aperture captures more light rays, resulting in a brighter and sharper image.

Analysis & Explanation

The reflecting surface of a spherical mirror is a section of a large sphere. The center of this sphere is the Centre of Curvature ( $C$ ), and its radius is the Radius of Curvature ( $R$ ). The physical size or boundary of the mirror that intercepts light is its circular aperture. For Class 10, we only study spherical mirrors of small aperture, meaning the aperture $MN$ is much smaller than the radius of curvature $R$ , which mathematically ensures that the principal focus $F$ lies exactly midway between $P$ and $C$ ( $R = 2f$ ).

Common Mistakes

1Confusing the aperture with the curved length of the mirror instead of the straight-line diameter $MN$ .
2Drawing the centre of curvature inside the reflecting side of a convex mirror (it lies behind it) or behind a concave mirror (it lies in front of it).

Interesting Facts

The larger the aperture of a mirror or telescope, the more light it can collect, which is why astronomical telescopes use mirrors with massive apertures to view faint, distant stars.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2011?

This question carries 2 marks in the CBSE Class 10 Science 2011 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 "Spherical Mirrors" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2011 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2011 paper.