Moving Screen & Objects!
A thin biconvex lens of focal length f = 2 5 cm forms a real image of an object on a screen. The object is separated by 5 cm from the lens. The screen is drawn closer to the lens by a distance of 1 8 cm . Through what distance must the object be shifted so that the image is again formed one the screen?
Give your answers in mm .
The answer is 0.5.
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1 solution
Sir, I have two questions.
1st Question :- In Bangladesh we are taught the equation, (1/u)+(1/v)=(1/f). We put positive sign for REAL Image & negative sign for Unreal Image. Is this wrong? Because, this isn't matching with your equation. Here, in this case, BOTH the Image & Object distance is REAL. So, we can simply write (1/5)+(1/v)=(1/25). But, if we do This, we get that the image comes out to be UNREAL. Where am I Wrong?
2nd Question :- If we keep an object at LESS Distance than the Focal Length of a Biconvex Lens, the Image should be UNREAL. But, here, even if we have put the object LESS Distance (5 cm) than the Focal Length (25 cm), how can the image be REAL? Maybe, I am wrong. But, please Reply & kindly help me to make my concept clear. I want to know the actual correct knowledge about this Formula & it's application. If possible, kindly provide me some Link or any other source where it is described correctly and more precisely.
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you are getting confused somewhere between ( v 1 − u 1 = f 1 ) (Len's Formula) with the Mirror Formula ( v 1 + u 1 = f 1 )
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Thanks a lot for pointing it out and sharing the idea. Thanks, Nishant.