Question

A concave mirror has a focal length of 20 cm. At what distance from the mirror
should a 4 cm tall object be placed so that it forms an image at a distance of 30 cm from the mirror ? Also calculate the size of the image formed.

Answer 1

Answer:

The object (u) should be placed 60cm away from the mirror. Size of Image formed is 2cm.

Explanation:

From the mirror formula, we know that-

1/f = 1/v + 1/u

So,

1/u = 1/f-1/v

Putting in the values

1/u = 1/-20 - 1/-30 (we took the values as minus because it is a concave mirror)

1/u = -1/60

u = -60cm

So we found out that the object should be kept 60cm away from the mirror.

Now to calculate the size of the image, we will use magnification formula.

Magnification formula-

m = -v/u = h'/h

Putting in the values

- (-30)/-60 = h'/4

-1/2 = h'/4

h' = -4/2

h' = -2cm

So, the size of the image is 2cm (as size of image can't be negative)

Also, the negative number means the image is inverted.

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