Question

12. A concave lens has a focal length of 10 cm. An object 2.5 cm high is placed 30 cm from the lens. Determine the position and size of the image. (3)​

Answer 1

Answer:

Size and the position of the image is 0.625 and -7.5 cm respectively.

Given-

Focal length of the concave lens is negative so f = -10 cm

Object distance from the lens is u = -30 cm

As, we know that

1/f = 1/v - 1/u where f is the focal length, v is the image distance from the lens and u is the object distance from the lens.

By putting the values in the above formula we get -

v = - 7.5 cm

And we know that magnification is equals to height of the image to the height of the object.

So, m = v/u = H1/H0

By putting the values we get -

H1/2.5 = -7.5/ -30

H1= 0.625

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Answer 2

Explanation:

• Size and the position of the image is 0.625 and -7.5 cm respectively.

Given-

Focal length of the concave lens is negative so f = -10 cm

Object distance from the lens is u = -30 cm

As, we know that

1/f = 1/v - 1/u where f is the focal length, v is the image distance from the lens and u is the object distance from the lens.

By putting the values in the above formula we get -

v = - 7.5 cm

And we know that magnification is equals to height of the image to the height of the object.

So, m = v/u = H1/H0

By putting the values we get -

H1/2.5 = -7.5/ -30

H1= 0.625