Question

A converging mirror forms a real image of height 4 centimetre, of an object of height 1 cm placed 20 CM away from the mirror calculate the image distance what is the focal length of the mirror?

Answer 1

# Converging mirror is actually concave mirror!

hi = 4cm

ho = 1cm

u=-20cm

(a) v=?

hi/ho=-v/u

where,

hi=height of the image

ho=height of the object

v=distance of the image from the mirror

u=distance of the object from the mirror

4/1=-v/-20

4 = v/20

v = 80cm

Let focal length of the mirror to be 'f'

1/f = 1/v + 1/u

1/f = 1/80 + 1/(-20)

1/f = 1/80 - 1/ 20

1/f = -3/80

f = -80/3 cm

Answer 2

ANSWERS :-


Here,


height of image, h2 = –4cm

height of object, h1 = +1cm

⚡⚡ object distance , u = – 20cm , minus sign for the object distance is measured from pole in a direction opposite to the direction of incidence of light.

we have to calculate the image distance , v = ?

As m = h2/h1 = –v/u

putting the values given above, we get

–4/1 = –v/–20

or,

v = –80cm

⚡⚡Negative sign of v indicates that the image is on the side of the object it must be real and inverted.

Now ,

we use mirror formula,to calculate focal length

1/f = 1/u + 1/v

putting u = –20 cm and v = –80cm ,

1/f = 1/-20 + 1/-80

or,

1/f = –4–1/80 = –5/80

so,

f = –80/5 = – 16cm

✴ negative sign of (f) indicates that mirror is concave.

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