Q43. A Galilean telescope has a convex objective (fₒ = +80 cm) and concave eyepiece (fₑ = -5 cm). The final image is virtual, 25 cm in front of the eyepiece. Required tube length is:
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Q43. A Galilean telescope has a convex objective (fₒ = +80 cm) and concave eyepiece (fₑ = −5 cm). The final image is virtual, 25 cm in front of the eyepiece. Required tube length is:

G1 Physics · Accepted Answer

GIVEN: Galilean telescope fₒ = +80 cm, fₑ = −5 cm; final image virtual at 25 cm in front of eyepiece. Step 1: Distant-object intermediate image would be at x = fₒ = 80 cm to the right of objective. Step 2: For concave eyepiece, fₑ = −5 cm and desired final image vₑ = −25 cm. Step 3: Lens formula 1/fₑ = 1/vₑ − 1/uₑ ⇒ 1/uₑ = 1/vₑ − 1/fₑ. Step 4: 1/uₑ = (−1/25) − (−1/5) = −1/25 + 5/25 = 4/25 ⇒ uₑ = +25/4 = +6.25 cm. Step 5: Positive uₑ means a VIRTUAL OBJECT on the right of concave eyepiece, i.e.…