Question

an object 4cm high is placed at a distance of 10cm from a convex lens of focal length 20cm.find the position,size and nature of the image
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Answer 1

Given,

• height (h) = 4 cm
• object distance (u) = -10 cm
• focal length (f) = 20 cm.

According to lens formula,  1/v - 1/u = 1/f

1/v = 1/f + 1/u

⇒ 1/v = 1/20 - 1/10

⇒ 1/v = -1/20

⇒ v = -20

m = v/u

                   

⇒ m = -20/-10

⇒ m= 2

m= h'/h  

Here,

• h = height of object
• h'= height of image

⇒ 2 = h'/4

⇒ h' = 4×2

⇒ h'= 8 cm

Therefore, the image is real and inverted.