Light – Reflection and Refraction — Part 2: Sign convention and the mirror formula: visual lesson, CBSE Class 10 Science

Lesson

Mirror formula and magnification

Part 2 — turning ray diagrams into numbers with a sign convention and one simple formula.

In this lesson you will

Apply the New Cartesian sign convention to spherical mirrors
Use the mirror formula to find image distance, object distance or focal length
Calculate magnification and interpret its sign and size

Diagram

The New Cartesian sign convention

The pole is the origin and the principal axis is the x-axis. The object is placed to the left, so incident light travels left to right. Distances measured in the direction of the incident light are positive; distances measured against it are negative. Heights above the axis are positive; heights below are negative.

Concept

What the signs tell you

Once the convention is fixed, the signs of the answers carry meaning.

Table of the signs of object distance, focal length, image distance and magnification for concave and convex mirrors
Quantity	Concave mirror	Convex mirror
Object distance u	Negative	Negative
Focal length f	Negative (F in front)	Positive (F behind)
Image distance v	Negative if real; positive if virtual	Always positive (virtual)
Magnification m	Negative if real and inverted; positive if virtual and erect	Positive, less than 1

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