1. Direction of Incident Ray
In Class 10 CBSE problems, we assume that:
The incident ray always travels from left to right.
This direction is taken as the positive direction along the principal axis.
Therefore:
• Right side of Pole → Positive
• Left side of Pole → Negative
2. Reference Point
All distances are measured from the
Pole (P) of the mirror along the principal axis.
3. Horizontal Sign Rules (Along Principal Axis)
• Distance measured in the direction of incident light → Positive
• Distance measured opposite to incident light → Negative
Since light is assumed left → right:
• Right side → Positive
• Left side → Negative
4. Object Distance (u)
The object is placed in front of the mirror (left side).
\[
u = \text{Always Negative}
\]
5. Image Distance (v)
• Image formed in front of mirror → Negative
• Image formed behind mirror → Positive
Therefore:
Real image → v negative
Virtual image → v positive
6. Focal Length (f)
• Concave mirror → Focus in front → \( f \) negative
• Convex mirror → Focus behind → \( f \) positive
7. Radius of Curvature (R)
\[
f = \frac{R}{2}
\]
• Concave mirror → \( R \) negative
• Convex mirror → \( R \) positive
8. Vertical Sign Rules (Height)
• Height measured above principal axis → Positive
• Height measured below principal axis → Negative
So:
Erect image → Image height positive
Inverted image → Image height negative
9. Magnification Formula Connection
For mirrors:
\[
m = -\frac{v}{u}
\]
• If \( m \) is positive → Image is erect
• If \( m \) is negative → Image is inverted
10. Mirror Formula
\[
\frac{1}{f} = \frac{1}{v} + \frac{1}{u}
\]
Before substituting:
1. Identify mirror type
2. Assign sign of focal length
3. Object distance is always negative
4. Decide image nature
5. Assign correct sign to image distance
6. Substitute carefully
11. Special Case – Plane Mirror
• Object distance → Negative
• Image distance → Positive
• Focal length → Infinite
• Radius → Infinite
• Magnification → +1
Plane mirror always forms virtual, erect, same-size image.
12. Quick Summary Table
| Quantity |
Concave Mirror |
Convex Mirror |
Plane Mirror |
| u |
Negative |
Negative |
Negative |
| v (Real) |
Negative |
Not formed |
Not formed |
| v (Virtual) |
Positive |
Positive |
Positive |
| f |
Negative |
Positive |
Infinite |
| m (Real) |
Negative |
— |
— |
| m (Virtual) |
Positive |
Positive |
+1 |
13. Golden Memory Rules
Left side = Negative
Right side = Positive
Concave mirror → f negative
Convex mirror → f positive
Object distance → Always negative
Real image → v negative
Virtual image → v positive
Above principal axis → Positive
Below principal axis → Negative
