# Image size Formula

The image size formula or magnification equation, relates the ratio of the image distance and object distance to the ratio of the image height and object height. The magnification equation is:

M= (image height)/(object height) = - (image distance)/(object distance)

The equation is:

M= hi / ho = - I/o

Where:

o: Object distance

I: Formed image distance

hi: image height

ho: object height

Image size Formula Questions:

1) A lens formed an image at 30 cm, the object of 2 cm height is placed in front at 1 m. What is the size of the image?

Answer:

From the image size formula:

hi/ho = - I/o

hi = -I/o * ho = - (30 cm / 100 cm) * 2 cm

hi = -0.15 cm

The minus sign indicates that the virtual image is inverted.

2) A lens forms an image at 25 cm from it, where the real object is placed at 1 m. The image size is inverted and of 4 cm, what is the height of the real object?

Answer:

From the image size formula, we find:

ho = - o/I * hi

ho = - 100 cm / 25 cm * (- 4 cm)

ho = 16 cm

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