A convex lens of focal length 10 cm is placed at a distance of 12 cm from a wall. How far from the lens should an object be placed so as to form its real image on the wall?

Explanation

Here, the real image is formed on the wall which is at a distance of 12 cm from the convex lens. This means that the distance of image from the convex lens or image distance will be 12 cm. Since a real image is fomred on the right side of the lens, so this image distance will be positive.
Now, Image distance, `v=+- 12` cm (A real image)
Object distance, u=? (To be calculated)
Focal length, f=+10 cm (it is convex lens)
Putting these values in the lens formula:
`1/v-1/u=1/f`
We get: `1/12-1/u=1/10`
`1/12-1/10=1/u`
`(5-6)/(60)=1/u`
`-1/60=1/u`
So, Object distance, u=-60 cm
Thus, the obect should be place at a distance of 60 cm in front of the convex lens. The minus sign shows that the obect is on the left side of the lens.