If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Course: Class 10 Physics (India)>Unit 1

Lesson 5: Sign convention

# Sign convention for mirrors (& lenses)

Let's explore the sign conventions (rules to assign +ve and -ve values) we use for mirrors and lenses. These rules are called Cartesian sign conventions. Created by Mahesh Shenoy.

## Want to join the conversation?

• was the Cartesian Sign Convention developed for making calculations convenient?
• yes cartesian is developed 4 making calculation convenient.
• what is incident direction?
I am total noob :D
(1 vote)
• In Physics, incident means the "incoming" (striking, falling) light onto the surface; hence if You please direction.
• it's said that the object is placed on the right side thus is positive. But, according to NCERT Pg 68, the first convention states that the object is always placed to the left of the mirror and that the incident direction is from the left-hand side i.e negative. Doesn't that make this claim incorrect?
• So, in concave and convex mirrors, a couple of figures have an opposite sign?
(1 vote)
• + and - signs are applied relying on the incident direction. The incident direction is the side of which is opposite the object or image you are trying to show. The positive side is always the one opposite the object or image you put in front of the mirror.
• I couldn't understand the sign convention.
(1 vote)
• Sign convention is a set of rules to set signs for image distance, object distance, focal length for mathematical analysis of image formation.

According to it:
- Object is always placed to the left of mirror
- All distances are measured from the pole of the mirror.
- Distances measured in the direction of the incident ray are positive and the distances measured in the - direction opposite to that of the incident rays are negative.
- Distances measured along y-axis above the principal axis are positive and that measured along y-axis below the principal axis are negative.
• If I use any other method instead of cartesian sign convention will I get the same answer
(1 vote)
• Depends on the method.
• if an object is placed between the focus and the pole of a concave mirror, the image formed will be virtual. will that not make the focal length positive? as the sign of the focal length represents the side where the image is formed?
(1 vote)
• No. Focal length is the distance from the pole to the focus. It will hence, always remain negative in the case of a concave mirror.
(1 vote)
• What if we put reflecting side at right hand side ??
(1 vote)
• how can images be formed in front or behind the mirror ?
(1 vote)
• Image formed in front of the mirror means that the image so formed is obtained by a screen in front of the mirror.It is a real image.
Similarly image behind the mirror means that the image is formed is virtual(such as the image formed in a plane mirror).
(1 vote)
• Something doesn't make sense here; in my Kaplan MCAT book it says that for concave (converging) mirrors the focal length is positive and for convex (diverging) mirrors the focal length is negative. The focal length would be negative in the video shown for a concave mirror while my book says it should be positive. This is highly confusing and I would like to know if Kaplan MCAT uses the cartesian system or another system or is the cartesian system universal?
(1 vote)