.An object is placed 50 cm in front of a convex lens of focal length 30 cm, if the object is
moved 30 cm away from the lens in 5 sec. Calculate the average velocity of the image
during this process.
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Answer:
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Answer:
20 cm
Explanation:
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12th
Physics
Ray Optics and Optical Instruments
Combination of Mirror and Lenses
An object is placed 30 cm a...
PHYSICS
An object is placed 30 cm away from a convex lens of focal length 10 cm and a sharp image is formed on a screen. Now a concave lens is placed in contact with the convex lens. The screen now has to be moved by 45 cm to get a sharp image again. The magnitude of focal length of the concave len is (in cm)
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ANSWER
An object is placed 30 cm in front of convex lens of focal length 10 cm :
Given : u
1
=−30 cm f
1
=10 cm
From lens formula,
v
1
1
−
u
1
1
=
f
1
1
∴
v
1
1
−
−30
1
=
10
1
⟹v=15 cm
Thus image is formed at a distance of 15 cm behind the lens.
Now a concave lens of focal length f
2
is placed in contact with convex lens. so the screen has to be shifted by 45 cm further away.
Now the new image distance v
2
=15+45=60 cm
Let the focal length of combination of lens be F.
Using lens formula,
v
2
1
−
u
1
1
=
F
1
∴
60
1
−
−30
1
=
F
1
⟹F=20 cm
Focal length of combination of lenses
F
1
=
f
1
1
+
f
2
1
∴
20
1
=
10
1
+
f
2
1
⟹f
2
=−20 cm