Sign Convention

Topic Content:

Sign convention is used to distinguish between image positions when they are in front of a curved mirror or behind it i.e. when real or virtual images are formed.

There are two types of sign conventions namely:

1. New Cartesian (NC)
2. Real-is-Positive (RIP)

The New Cartesian sign convention is easier to use because it is similar to using an x-y coordinate axis.

Examples 4.7.1:

An object is placed 10 cm in front of a concave mirror whose radius of curvature is 12 cm. Calculate the position, nature and magnification of the image produced.

Solution

 

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The same result is obtained irrespective of the convention used.

The image is:

• inverted
• real
• magnified

Magnification = -1.5. The negative sign tells us that the magnification is real and inverted, and 1.5 tells us that the image is 1.5 times larger than the object.

Examples 4.7.2:

An object, 6 cm in size, is placed at 30 cm in front of a concave mirror, of focal length 15 cm. At what distance from the mirror should a screen be placed in order to obtain a sharp image? Find the nature and size