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Quiz 7
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Q.1
The distance between an object and a divergent lens is m times the focal length of the lens. The linear magnification produced by the lens will be equal to
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a) m
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b) 1/m
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c) (m+1)
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d) 1/(m+1)
Explanation
u = -mf substitute in lens formula and solve for v/u Answer:(d)
Q.2
A ray of light falls on the surface of spherical glass paper weight making an angle α with the normal and is refracted in the medium at an angle β. The angle of deviation of emergent ray from the direction of incident ray is
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a) ( α - β )
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b) 2( α - β )
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c) ( α - β )/2
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d) (β - α )
Explanation
from geometry of figure δ1 = α- β δ2 = α-β δ = δ1 +δ2 δ = 2( α - β ) Answer: (b)
Q.3
The minimum distance between an object and real image formed by a convex lens is
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a)1.5f
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b) 2f
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c)2.5f
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d)4f
Explanation
Answer: (d)
Q.4
A double convex air bubble in water would behave as
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a) divergent lens
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b) convergent lens
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c)concave mirror
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d)plane mirror
Explanation
Answer: (a)
Q.5
An object is placed at point distance x from the focus of convex lens and its image is formed at I as shown in figure. The distance x, x' satisfy the relation
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a) xx' / 2=f
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b) f2=xx'
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c)x+x'=2f
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d)x-x'=2f
Explanation
from Newton's formula Answer:(b)
Q.6
A convex lens and a concave lens having focal lengths 30cm and 20 cm are placed 10cms apart. The distance of a source of light at which it must be placed for the system to give parallel beam of rays, is
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a) 10 cms
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b) 20 cm
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c) 30 cm
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d) ∞
Explanation
For parallel rays image formed by the convex lens must be at the focal point of concave lens. i.e 30 cm form the convex or 20 cm from the concave thus object should be at infinity Answer: (d)
Q.7
The graph drawn with object distance along abscissa and image ( real) distance as ordinate for a convex lens is
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a)A straight line
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b) A circle
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c)A parabola
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d)A rectangular hyperbola
Explanation
Answer: (d)
Q.8
A convex spherical surface forms an inverted image of an object. The object is placed in air and image is formed in glass(µ=1.5). If the object and image distance from the spherical surface is 0.2m and 0.3m respectively the magnification is
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a) 1.5
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b) 0.67
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c)1
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d)0.45
Explanation
Answer: (c)
Q.9
A converging lens is to project image of a lamp 4 times the size of the lamp on a wall at a distance of 10m from lamp. The focal length of the lens is
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a) 1.6 m
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b) 2.67 m
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c)4.4 m
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d)-1.6 m
Explanation
v + u=10 mv/u=4 ∴ u=2 m and v=8m use lens formula to find f Answer:(a)
Q.10
A convex spherical surface separates glass and air. A ray incident in glass and parallel to principal axis is bent .... to the principal axis
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a) Towards
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b) Away
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c) At 45°
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d) At 10°
Explanation
Answer: (a)
Q.11
In a film projector a convex lens is usually placed between the bulb and the film. The purpose of this lens is to
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a)Make the image brighter
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b) Laterally invert the image
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c)Produce an image of the film on the screen
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d)Correct any distortions in the image
Explanation
Answer: (a)
Q.12
The focal length of convex lens of glass ( µ=1.5) is 2cm. the focal length of the lens when immersed in a liquid of refractive index=1.25 will be
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a) 5 cm
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b) 2.4 cm
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c)1 cm
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d)4 cm
Explanation
Use formula here n2 is refractive index of lens and n1 is refractive index of liquidAnswer: (a)
Q.13
Two thin lenses of focal length 20cm and -30cm are placed in contact with each other. The combination has focal length
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a) -10 cm
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b) 50 cm
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c)60 cm
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d)10 cm
Explanation
Answer:(c)
Q.14
A virtual image as big as the object is formed by a convex lens when the object is 10cm away from it. A real image twice as big as the object will be formed when it is placed at a distance from the lens
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a) 40 cm
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b) 30 cm
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c) 20 cm
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d) 15 cm
Explanation
Answer: (b)
Q.15
When a thin lens is put in contact with a thin concave lens of the same focal length, the resultant combination has a focal length equal to
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a)f/2
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b) 2f
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c)0
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d)∞
Explanation
for concave lend f is negativeResultant focal length F then 1/F=(1/f) - (1/f)=0 Thus F=∞Answer: (d)
Q.16
An qui-convex glass lens with radius of each face equal to 50cms is placed in air. If refractive index of glass with respect to air=3/2 then its focal length is
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a) 25 cm
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b) 50 cm
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c)100 cm
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d)75 cm
Explanation
Answer: (b)
Q.17
A thin lens has a focal length f and its aperture has diameter d. It forms an image of intensity I. Now the central part of the aperture upto diameter d/2 is blocked by an opaque paper. The focal length and the image intensity will change to
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a) f/2 and I/2
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b) f and I/4
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c)3f/4 and I/2
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d)f and 3I/4
Explanation
Answer:(d)
Q.18
An object is placed 1 metre in front of the curved surface of plano-convex lens whose plane surface is silvered. A real image is formed in front of the lens at a distance of 120 cms. Then the focal length of the lens is
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a) 100 cm
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b) 120 cm
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c) 109.1 cm
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d) 110.0 cms
Explanation
When plane surface of the plano-convex lens is silvered, the lens behaves as a concave mirror. The focal length of the lens< lens=2fm USe mirror formula to find fm then focal length of lens Answer: (c)
Q.19
A luminous object is lying in front of a screen at a distance D from it. What is the maximum focal length of a converging lens, which can be mounted between the object and the screen to cast the real image of the object on the screen
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a)3D
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b) D
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c)D/2
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d)D/4
Explanation
Answer: (d)
Q.20
A convex lens placed between an object and screen form a sharp image of the object on screen. A glass slab of refractive index µ is inserted between object and lens. In order to obtain the sharp image on the screen again the lens has to be displaced by a distance x. The thickness of glass is
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a) µx
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b) µ/x
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c)[x(µ - 1)] / µ
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d)µx / ( µ - 1)
Explanation
from the formula for lateral shift x=[t(µ - 1)]/ µ Answer: (d)
Q.21
For a spherical surface separating two media of refractive index µ1 and µ2, the focal length f is
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a) f=R/2
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b) f=R
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c)f=(µ1 - µ2 ) / µ2R
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d)µ2R/ (µ2 - µ1)
Explanation
Answer:(d)
Q.22
A bi-convex lens ( µ=1.5) of focal length 0.2 m acts as a divergent lens of power one dioptre when immersed in a liquid. the refractive index of the liquid is
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a) 1.33
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b) 1.67
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c) 1.25
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d) 1.2
Explanation
Answer: (b)
Q.23
An qui-convex lens is cut into two halves by a plane AB as shown in figure. The focal length of each half so obtained is
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a)f
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b) f/2
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c)2f
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d)3f/2
Explanation
Answer: (c)
Q.24
A double convex lens have radii of curvature as 2 cm and 30 cm and refractive index of the material of lens is 1.then the focal length of the lens is
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a) 24cm
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b) 10 cm
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c)15 cm
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d)120 cm
Explanation
Use lens maker's formula Answer: (a)
Q.25
The focal length of combination of two lenses separated by distance d
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a) Increases as their separation is increases
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b) Decreases as the separation is increased
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c) Is less than the focal length of either lens for all d
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d) Is less than the focal length of one of them is d is greater
Explanation
Answer: (a)
Q.26
A plano co-convex glass lens has a maximum thickness of 3cm. Refractive index of glass with respect to air=3/2 when seen normally through the curved surface the greatest thickness appears to be 5/2 cm. The radius of curvature of the spherical surface of the lens is
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a) 3 cm
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b) 7.5 cm
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c) 4.5 cm
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d) 5.0 cm
Explanation
U=-3 cm and v=-5/2 cm Object is in denser medium therefore Answer: (d)
Q.27
The greatest thickness of plano -convex glass lens appears to be 2cm when observed normally through the plane face and when observation is taken through the curved face the greatest thickness appears to be 20/9 cms. If real thickness is 3cm then the refractive index of the glass is
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a)1.35
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b) 1.50
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c)1.11
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d)1.2
Explanation
µ=Real depth / apparent depthAnswer: (b)
Q.28
An equiconvex lens is made from glass of refractive index 1.If the radius of each surface is changed from 5cm to 6 cm then the power
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a) Remains unchanged
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b) Increases by 3.33 dioptre approx
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c)Decreases by 3.33 dioptre approx
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d)Decreases by 5.5 dioptre approx
Explanation
Use lens makers formula power=1/fAnswer: (c)
Q.29
A glass concave lens is placed in a liquid in which it behaves like a convergent lens. If the refractive indices of glass and liquid be aµg and a µ l respectively then
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a) aµg=a µ l
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b) aµg < a µ l
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c)aµg > a µ l
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d)aµg ≥ 3a µ l
Explanation
Answer:(b)
Q.30
To plano convex lenses of radius of curvature of the curved surface as R and µ=1.5 will have equivalent focal length R if they are placed
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a) In contact with each other
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b) At a distance R from each other
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c) At a distance 3R from each other
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d) None is correct
Explanation
Answer: (a)
Q.31
An object is placed at a distance of 20cm from a convex lens of focal length 10cm. The image is formed on the other side of the lens at a distance
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a)20 cm
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b) 10 cm
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c)40 cm
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d)30 cm
Explanation
Answer: (a)
Q.32
A convergent beam of light converges to a point 20cm behind a concave lens on the principal axis. An inverted image of the same size is formed. Then the foal length of the lens is
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a) 20 cm
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b) 10 cm
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c)40 cm
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d)30 cm
Explanation
Answer: (b)
Q.33
An object is placed at a distance of 9 cm from the first principal focus of convex lens produces a real image at a distance of 25 cms from its principal focus. Then the focal length of the lens is
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a) 9 cm
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b) 25 cm
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c)15 cm
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d)17 cms
Explanation
USe Newton's formula x1x2=f2 here x1=9 and here x2=25 Answer:(c)
Q.34
How large will an image be formed of an artificial satellite 5m in diameter, passing at an altitude of 500 km if it is photographed by a camera with a lens of focal length 10cm
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a) 10-2 m
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b) 10-4 m
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c) 10-6 m
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d) 5×10-6 m
Explanation
Object distance is very large thus image will be formed at focal length f ∴ v=f I/O=f/u I=O(f/u) I=5(0.1/500,000)=10-6 Answer: (c)
Q.35
A lens is made of a material of refractive index µIt is immersed in a medium of refractive index µIf r1 and R2 are the radii of curvature of the lens surface, the focal length f is
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a)
0%
b)
0%
c)
0%
d)
Explanation
Answer: (b)
Q.36
A lens form a virtual, diminished image of an object placed at 2m from it. The size of image is half of the object. Which one of the following statement is correct regarding the nature and focal length of the lens
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a) concave, |f|=1 m
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b) convex, |f|=1 m
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c)concave, |f|=2 m
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d)convex, |f|=2 m
Explanation
Answer: (c)
Q.37
The curved face of a plano-convex lens has a radius of curvature of 250 mm. The refractive index of lens material is 1.The power of the lens is
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a) 0.2D
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b) -0.2D
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c)+2D
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d)-2D
Explanation
Answer:(c)
Q.38
A mark 'A" is placed on the surface of glass sphere is viewed through glass from a point directly opposite. Radius of sphere=10cm. Refractive index of glass is 1.Which of the following statement is correct
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a) the image formed is real and at a distance of 0.4 m from eye
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b) the image formed is virtual and at a distance of 0.2 m from eye
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c) the image formed is virtual and at a distance 0.3 m from the centre of the sphere
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d) the image formed is real and at a distance of 0.2m from point A
Explanation
u=-2R=-20cm µ1=1.5, µ=1 From formula for refraction at covered surface Image I is at 40 cm from point O and 30 cm from the centre of sphere Answer: (c)
Q.39
A convex lens of focal length 2m has an object placed at a distance of 0.5m from it. The image formed is
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a)Real, at a distance of 2/3m from the lens
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b) Virtual, at a distance of 2/3 from the lens
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c)Real, at a distance of 0.4m from the lens
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d)virtual, at a distance of 0.4 m from the lens
Explanation
Answer: (b)
Q.40
A convex lens form a real image of an object placed at a distance u from the lens, at a distance v from it. The focal length f of the lens is
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a) u > f > v
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b) v > f > u
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c)less than u as well as v
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d)more than u as well as v
Explanation
Answer: (c)
Q.41
A spherical convex surface separates object and image space of refractive index 1.0 and 1.If radius of curvature of the surface is 0.1m, its power is
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a) 2.48 D
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b) - 2.48 D
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c)3.3 D
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d)-3.3D
Explanation
P=µ2 - µ1 / (µ2R) P=(1.33-1) / (1.33×1)=2.48D Answer:(a)
Q.42
An object is placed in air at a distance of 1 m from the curved end of long glass rod ( µ=1.5). The image of an object placed in air is formed inside the glass rod The image formed is erect and half the size of the object. The curved end of the rod is
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a) Convex, of radius of curvature 0.5m
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b) Concave, of radius of curvature 0.5m
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c) convex, of radius of curvature 6m
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d) concave, of radius of curvature 6m
Explanation
Answer: (a)
Q.43
Two thin lenses of focal lengths f1 and f2 are separated by a horizontal distance d ( where d< f1, d< f2) and their centres are displaced by a vertical separation shown Δ as shown in figure. Taking the origin of the coordinates,C, at the centre of the first lens, the x and y co-ordinates of the focal point of the lens system, for a parallel beam of rays coming from the left, are given by
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a)
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b)
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c)
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d)
Explanation
First lens forms image at f1 which acts as object for lens II For lens II , u=f1 - d For lens formula Thus image is formed at distance v+d from origin x-coordinate is For Y coordinateFor II lens m=I/O=v/u O=-Δ (negative sign taken as the image is formed by the fist lens is Δ distance below the principal axis of lens II) Now y=I +Δ Answer: (c)
Q.44
A convex lens of focal length 1.0m and a concave lens of focal length 0.25m are 0.75 m apart. Parallel beam of light is incident on the convex lens. The beam emerging after refraction from both lenses is
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a) Parallel to principal axis
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b) Convergent
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c)Divergent
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d)None of the above
Explanation
Answer: (a)
Q.45
An object of liner dimensions L is placed along the axis of a thin lens placed in air. the size of the image is
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a) Lv/u
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b) Lv2/ u2
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c)Lu2/ v2
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d)L ( f/ f-v)2
Explanation
Form lens formula Answer:(b)
Q.46
The linear magnification of an image is m. The magnification for area will be
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a) m
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b) m2
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c) m1/2
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d) m4
Explanation
Answer: (b)
Q.47
A convex lens is placed over a plane mirror as shown in figure. When a pin is placed as shown the image formed coincides with the pin itself. The focal length of lens is
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a)0.5 m
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b) 0.25 m
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c)1.0 m
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d)2.0 m
Explanation
Answer: (a)
Q.48
A convex lens of foal length 40 cm is held coaxially above a concave mirror of focal length 180cm. A luminous point object placed d cm above the lens on its axis gives rise to an image coincident with itself. Then d is equal to
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a)15 cm
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b) 18 cm
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c)40 cm
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d)38 cm
Explanation
Answer: (a)
Q.49
The distance between the convex lens and plane mirror is 10cm. the parallel rays of light are incident on the convex lens. After reflection from the plane mirror the final image is formed at the optical centre of the lens. The focal length of the lens is
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a)10 cm
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b) 20 cm
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c)40 cm
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d)5 cm
Explanation
Answer:(b)
Q.50
A convex lens produces an image of a real object on a screen with magnification of 1/2 . When the lens is moved 30cm to wards the object, the magnification of the image on the screen isThe focal length of the lens is
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a)30 cm
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b) 60 cm
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c)20 cm
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d)15 cm
Explanation
This is the question of displacement method.x=v-u=30 cm v2 / u2=I1 / I2 v2 / u2=m1 / m2 v2 / u2=1 /2×2=1/4v=2u ∴ u=30 cm and v=60 cm by using lens formula we get f=20 cmAnswer: (c)
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