Vandanawanted to see the stars of the night sky. She knows that she needs a telescope to
see those distant stars. She finds out that the telescopes, which are made of lenses, are
called refracting telescopes and the ones which are made of mirrors are called reflecting
telescopes. So she decided to make a refracting telescope. She bought two lenses, L1 and
L2. Out of which L1 was bigger and L2 was smaller. The larger lens gathers and bends the
light, while the smaller lens magnifies the image. Big, thick lenses are more powerful. So to
see far away,she needed a big powerful lens. Unfortunately,she realized that a big lens is
very heavy.
Heavy lenses are hard to make and difficult to hold in the right place. Also since the light is
passing through the lens, the surface of the lens has to be extremely smooth. Any flaws in
the lens will change the image. It would be like looking through a dirty window.
If the powers of the lenses L1 and L2 are in the ratio of 4:1,
what would be theratio of the focal length of L1 and L2?
a) 4:1
b) 1:4
c) 2:1
d) 1:1
Answers
Answer :
The ratio of the powers of the lenses L₁ and L₂ = 4 : 1
We have to find the ratio of their focal lengths.
\star⋆ Principle Focus (F) : It is the point of intersection of all reflected rays for which the incident rays strike the mirror (with small aperture) parallel to the principal axis. In convex lens it is real and in the concave lens it is virtual..
The distance from pole to focus is called focal length.
Ratio of powers = 4 : 1
Power of L₁ = 4x
Power of L₂ = 1x
We know that power is the reciprocal of focal length.
∴ Focal length of L₁ = 1/4x
∴ Focal length of L₂ = 1/x
Ratio of focal lengths ;
➝ f₁ : f₂ = 1 : 4
Hope this will help ...
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Answer: 1:4
Explanation:The ratio of the powers of the lenses L₁ and L₂ = 4 : 1
We have to find the ratio of their focal lengths.
\star⋆ Principle Focus (F) : It is the point of intersection of all reflected rays for which the incident rays strike the mirror (with small aperture) parallel to the principal axis. In convex lens it is real and in the concave lens it is virtual..
The distance from pole to focus is called focal length.
Ratio of powers = 4 : 1
Power of L₁ = 4x
Power of L₂ = 1x
We know that power is the reciprocal of focal length.
∴ Focal length of L₁ = 1/4x
∴ Focal length of L₂ = 1/x
so,
1:4