Question

Two convex lenses of focal length 20cm and 1 cm constitute a telescope

Answer 1

Telescope contains two convex lenses e.g., objective and eye lens .

For objective ,

f₀ = 20 cm , u₀ = -100 cm , v₀ = ?

use formula,

1/f₀ = 1/v₀ - 1/u₀

1/20cm = 1/v₀ - 1/-100cm

1/20 = 1/v₀ + 1/100

1/v₀ = -1/100 +1/20 = 4/100 = 1/25

v₀ = 25 cm

Now, magnification = -(v₀/u₀)[1 + D/fₐ]

Here,v₀ = 25cm , u₀ = 100 cm , D = 25cm , fₐ = focal length of eye lens = 1cm

so, magnification = -(25cm/100cm)[1 + 25/1]

= -26/4 = -6.5

Again, for eye lens

fₐ = 1cm , vₐ = -25cm , uₐ = ?

1/fₐ = 1/vₐ - 1/uₐ

1/1cm = 1/-25cm - 1/uₐ

1/1 + 1/25 = -1/uₐ

26/25 = -1/uₐ

1.04 = -1/uₐ

uₐ = -0.96 cm so, |uₐ| = 0.96 cm

Hence, length of tube = |uₐ|+ |v₀| = 0.96 + 25 = 25.96cm

Answer 2

Answer:

hyy mate

above answer is right