We cover the convention which will help us solve numerical questions relating to Light in general. They are called the sign convention.

1. There is an imaginary line passing through the pole of a Mirror or optical centre of a Lens. This line is important as it acts as the principal axis. Normally, in the coordinate system, it’s the XX’ axis which is treated as the principal axis.

2. The purpose of sign convention is to assign +ve or -ve sign to object distance(u), image distance(v) and focal length(f) for Mirrors and Lens

3. Two important part to remember is: –

(i) Distances measured in the direction of light is taken as positive otherwise negative, and
(ii) Distance measured upward from the principal axis is taken as positive and distances measured downwards from the principal axis is taken as negative.

4. Actual use: – Assuming Light is coming from LEFT side

(i) Case I (object distance): Object is normally placed on the left side of Mirror or Lens. Light is coming from the left of the Mirror or Lens. As the distance of an object is measured from the pole (in Mirrors) or optical centre (in Lens), therefore, to measure the object distance we move from pole to the object. Object is on left side o the pole. Note that light is moving from left to right and we are measuring object distance from right to left. As we are measuring in opposite direction of light, the object distance is assigned -ve value. We write -u.

(ii) Case II (Image distance): Image is formed on the left side or right side of Mirror or Lens depending on where the object is placed. Assuming light is from left to right, if image is formed to the left of Mirror or Lens, it is written as -v. If image is formed to the right of Mirror or Lens is written as +v.

(iii) Case III (Focal Length): It’s is bit tricky. Just remember, if the focus lies to the left of Mirror or Lens, its -f and if the focus is to the right of Mirror or Lens, its +f. We assume that the light is from left to right.

(iv) Case IV (Height): If an object or image is above the principal axis they are written as +u or +v. If object or image are below principal axis, we write them as -u or -v respectively.

This is all you should know. But, still there can be confusion. A little practice will go a long way to clear out any confusion.

The most neglected part of the physics chapter Light: Reflection and Refraction are Sign Convention for solving numerical questions.  Download the complete NOTES in PDF——–> Complete Notes are here.