When we move the lens back and forth, we find that there exist two positions (P1 and P2 as shown in Fig. 1b) where clear image can be formed on the screen. The separation between the two positions is measured to be 20 cm. Is the unknown lens convex or concave? What is the focal length? Hint: The solution of a quadratic equation ax +bx +c=0_is -b+yb? - 4ac 2a

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(a) As shown in Fig. la, the object and the screen is separated by 1 = 100cm. When we put a
convex lens in the middle point between the object and the screen, a clear image is formed
on the screen. What is the focal length of the lens? You may use either ABCD matrix or other
methods.
100cm
Fig. la
(b) Now we replace the lens in (a) with an unknown lens (focal length f), as shown in Fig. Ib.
Assume the distance between the object and the lens is S cm, what is the ABCD matrix from
the object to the screen? What can we know about this matrix if a clear image is formed on
the screen?
100cm
20cm
P2
Fig. 1b
(c) When we move the lens back and forth, we find that there exist two positions (P1 and P2 as
shown in Fig. 1b) where clear image can be formed on the screen. The separation between
the two positions is measured to be 20 cm. Is the unknown lens convex or concave? What is
the focal length? Hint: The solution of a quadratic equation ax + bx+c=0_is
ーb+V6 -4ac
2a
Transcribed Image Text:(a) As shown in Fig. la, the object and the screen is separated by 1 = 100cm. When we put a convex lens in the middle point between the object and the screen, a clear image is formed on the screen. What is the focal length of the lens? You may use either ABCD matrix or other methods. 100cm Fig. la (b) Now we replace the lens in (a) with an unknown lens (focal length f), as shown in Fig. Ib. Assume the distance between the object and the lens is S cm, what is the ABCD matrix from the object to the screen? What can we know about this matrix if a clear image is formed on the screen? 100cm 20cm P2 Fig. 1b (c) When we move the lens back and forth, we find that there exist two positions (P1 and P2 as shown in Fig. 1b) where clear image can be formed on the screen. The separation between the two positions is measured to be 20 cm. Is the unknown lens convex or concave? What is the focal length? Hint: The solution of a quadratic equation ax + bx+c=0_is ーb+V6 -4ac 2a
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