Question

One half of a convex lens of focal length 10 cm is covered with a black paper. Can such a lens produce an image of a complete object placed at a distance of 30 cm from the lens? Draw a ray diagram to justify your answer.
A 4 cm tall object is placed perpendicular to its principal axis of a convex lens of focal length 20 cm. The distance of the object from the lens is 15 cm. Find the nature, position and the size of the image.

Answer 1

To observe a convex lens which can produce the complete image of the object whereas half of the lens is covered. Because light coming from the object can be refracted from the other half of the lens. Therefore, the intensity of light will be reduced.

Object size $h=+4cm$

Object distance $u=–20cm$

Focal length $f=–10cm$

Image distance $v=?$

Image size $h^{\prime }=?$

we know,

$\frac{1}{f}=\frac{1}{u}+\frac{1}{v}$

$\frac{1}{v}=\frac{1}{f}-\frac{1}{u}$

$\frac{1}{v}=\frac{-1}{20}$

$v=-20$

The screen should be placed at 20 cm from the mirror.The image is real.

Also magnification m=-1

So, the image is inverted and enlarged.