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Quiz 11
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Q.1
A plano convex lens (µg=3.2) of radius of curvature R=10 cm is placed at a distance 'b' from a concave lens of focal length 20 cm. What should be the distance 'a' of a point object O from the plano-convex lance so that the position of final image is independent of 'b'?
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a)40 cm
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b) 60 cm
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c)30 cm
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d)20 cm
Explanation
Focal length of plano-convex lens is If object O is placed at a distance of 20 cm from the plano-convex lens rays become parallel and final image is formed at second focus of 20 cm from concave lens which is independent of bAnswer: (d)
Q.2
One of the refracting surface of a prism of angle 30° is silvered. A ray of light incident at an angle of 60° retraces its path. The refractive index of the material of the prism is
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a) √2
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b) √3
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c)3/2
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d)2
Explanation
From figure r2=0 r1=A=30 and i1=60° µ=sini1 / sinr1=√3Answer: (b)
Q.3
A ray of light falls on a transparent sphere with centre at C as shown in figure. The ray emerges from the sphere parallel to line AB. The refractive index of sphere is
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a) √2
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b) √3
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c)3/2
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d)1/2
Explanation
Figure deviation by sphere is δ=2 ( i-r) HEre δ=60° 60=2(i-r) i- r=30 r=60 - 30=30° Now µ=sini/ sinr µ=sin60/sin30=√3 Answer:(b)
Q.4
Two identical glass (µg=3/2) Reunion lenses of focal length f are kept in contact. The space between the two lenses is filled with water (µw=4/3). The focal length of the combination is
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a) f
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b) f/2
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c) 4f/3
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d) 3f/4
Explanation
Let R be the radius of curvature of each surface Then For ware lensNow by using formula for combination of lenses Answer: (d)
Q.5
Angle of minimum deviation is equal to the angle of prism A of an equilateral glass prism. The angle of incidence at which minimum deviation will be obtained is
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a)60°
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b) 30°
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c)45°
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d)sin-1 (2/3)
Explanation
A=δm=60°At minimum deviation i=[( A+δm) / 2]=60°Answer: (a)
Q.6
Refraction takes place at a concave spherical boundary separating glass air medium. For the image to be real, the object distance ( µg=3/2)
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a) should be greater than three times the radius of curvature of the refracting surface
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b) should be greater than two times the radius of curvature of the refracting surface
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c)should be greater than the radius of curvature of the refracting surface
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d)is independent of the radius of curvature of the refracting surface
Explanation
Object is in denser medium . Applying formula for refraction at curved sourceIn given problem for image to be real v should be positive which possible only if 1/ 2R > (3/2u)u > 3R Answer: (a)
Q.7
Mark the correct option
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a) If the incident rays are converging, we have real object
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b) If the final rays are converging, we have a real image
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c)The image of virtual object is called a virtual image
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d)If the image is virtual, the corresponding object is called a virtual object
Explanation
Answer:(b)
Q.8
A point source of light is placed in front of a plane mirror
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a) All the reflected rays meet at a point when produced backward
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b) Only the reflected rays close to the normal meet at a point when produced backward
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c) Only the reflected rays making a small angle with the mirror, meet at a point when produced backward
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d) Light of different colours make different images
Explanation
Answer: (a)
Q.9
On reflection from a plane surface, the following gets changed
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a)wavelength
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b) frequency
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c)speed
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d)amplitude
Explanation
Answer: (d)
Q.10
A plane mirror is approaching you at 10cm per second. You can see your image in it. At what speed will your image approach you
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a) 10 cm/s
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b) 5 cm/s
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c)20 cm/s
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d)15 cm/s
Explanation
Answer: (c)
Q.11
An object is approaching a plane mirror at 5 cms per second. A stationary observer sees the image. At what speed will the image approach the stationary observer
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a) 5 cms per second
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b) 20 cms per second
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c)10 cms per second
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d)15 cms per second
Explanation
Answer:(a)
Q.12
Which of the following letters do not suffers later inversion
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a) HGA
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b) HOX
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c) VET
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d) YUL
Explanation
Answer: (b)
Q.13
A small object is 10 cm in front of a plane mirror. A man stands 30 cm from the mirror behind the object and looks at the object's image. He should focus his eyes to see the image at a distance
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a)25 cm
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b) 35 cm
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c)45 cm
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d)40 cm
Explanation
Answer: (d)
Q.14
If you want to see your full image, then minimum size of the mirror
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a) Should be of your height
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b) Should be half of your height
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c)Should be twice of your height
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d)Depends upon distance from the mirror
Explanation
Answer: (b)
Q.15
In a concave mirror an object is placed at a distance x1 from the focus and the image is formed at a distance x2 from the focus. Then the focal length of the mirror is
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a)x1x2
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b)√(x1x2)
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c)(x1+x2) /2
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d)√(x1/x2)
Explanation
Answer:(b)
Q.16
If an object is placed unsymmetrical between two plane mirror, inclined at an angle of 72°, then the total number of images formed is
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a) 5
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b) 4
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c) 2
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d) infinity
Explanation
number images=360/θ Answer: (a)
Q.17
If the critical angle for total internal reflection from a medium to vacuum is 30° the velocity o light in the medium is
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a)3×108 m/s
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b) 1.5×108 m/s
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c)6×108 m/s
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d)√3×108 m/s
Explanation
use sinc=1/µand µ=c/vAnswer: (b)
Q.18
Light travels through a glass of thickness t and refractive index µ. If c is the velocity of light in vacuum, the time taken y light to travel through the plate is
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a) t/µc
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b) µtc
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c)µt/c
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d)tc/µ
Explanation
Answer: (c)
Q.19
If I1 and I2 be the size of the images respectively for two positions of the lens in the displacement method, then the size of the object is given by
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a)I1 I2
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b) √(I1 I2 )
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c) √(I1 / I2 )
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d)√(I2 / I1 )
Explanation
Answer:(b)
Q.20
Two thin lenses of focal length f1 and f2 are in contact and coaxial. Its power is same as the power of single lens given by
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a) (f1 + f2) /(f1 × f2)
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b) √(f1 / f2)
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c) √(f2 / f1)
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d) (f1 + f2) /2
Explanation
Answer: (a)
Q.21
When a light ray travels into a single medium then refractive index is
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a)directly proportional to wavelength of light
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b) inversely proportional to wavelength of light
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c)inversely proportional to square of wavelength of light
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d)directly proportional to the square of wavelength of light
Explanation
Answer: (c)
Q.22
The equation for two waves obtained by two light sources are as given by y1=A1sin3ωt and y2=A2cos(3ωt + π/6). What will be the value of phase difference at time t
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a) 3π/2
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b) 2π/3
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c)π
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d)π/2
Explanation
use cosθ=sin(π/2+θ) π/2 + π/6=2π/3Answer: (b)
Q.23
The path difference between two waves y1=A1sinωt and y2cos(ωt + φ) will be
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a) (λ/2π) φ
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b) (λ/2π) (φ+ π/2)
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c)(λ/2π) (φ- π/2)
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d)(λ/2π) (φ)
Explanation
Phase difference=π/2 + φ path difference is λ for phase difference of 2π∴ option 'b' is correct Answer:(b)
Q.24
Two coherent waves are represented by y1=a1cosωt and y2=a2sinωt. The resultant intensity due to interference will be
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a) a1 + a2
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b) a1 - a2
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c) a12 + a22
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d) a12 - a22
Explanation
phase difference between both the waves is π/2 Answer: (c)
Q.25
For a double slit experiment the necessary equation representing the coherent sources are y1=10sinωt and y2=10sin(ωt - π/6) then the value of intensity of bright fringes will be
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a)20
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b) 400
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c)100
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d)200
Explanation
Answer: (b)
Q.26
The phenomenon of diffraction can be observed, when the obstacle is
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a) Quit large as compared to the wavelength of light used
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b) Quit small as compared to the wavelength of light used
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c)Of the same order as the wavelength of light used
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d)Bear no relation with the wavelength of light used
Explanation
Answer: (c)
Q.27
Who first discovered the phenomenon of diffraction
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a) Fresnel
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b) Fraunhofer
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c)Arago
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d)Grimaldi
Explanation
Answer:(d)
Q.28
The correct relation between the size of the obstacle and the diffraction event is
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a) a/λ=1
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b) a/λ=0
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c) a/λ=∞
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d) a/λ=150
Explanation
Answer: (a)
Q.29
The fringe width in single slit diffraction pattern is proportional to
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a)a/λ
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b) λ/a
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c)aλ
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d)λ
Explanation
Answer: (b)
Q.30
The ratio of intensities of consecutive maxima in the diffraction slit is
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a) 1:4:9
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b) 1:2:3
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c)1 : 4/9π2 : 4/25π2
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d)1: 4/π2 : 9/π2 :
Explanation
Answer: (c)
Q.31
A slit of width 12×10-7 m is illuminated by light of wavelength 6000Å. The angular width of the central maximum is approximately
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a) 30°
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b) 60°
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c)90°
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d)0°
Explanation
Answer:(b)
Q.32
An unpolarized beam of light is incident on a group of four polarizing sheets which are arranged in such a way that characteristic direction of each polarizing sheet makes an angle of 30° with that of the preceding sheet. The % age of incident light transmitted by first polarizer will be
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a) 100%
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b) 50%
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c) 25%
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d) 12.5%
Explanation
Answer: (b)
Q.33
Unpolarised light of intensity 32 Wm-2 passes through three polarizes such that the transmission axis of the last polarizer is crossed with that of the first. The intensity of final emerging light is 3W-The intensity of light transmitted by first polarizer will be
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a)32 Wm-2
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b) 16Wm-2
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c)8Wm-2
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d)4Wm-2
Explanation
Answer: (b)
Q.34
A beam of unpolarized light of intensity Io is passed first through another tourmaline crystal B oriented so that its principle plane is parallel to that of A. The intensity of final emergent light is I. The value of I is
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a) Io /2
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b) Io / 4
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c)Io / 8
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d)none of these
Explanation
Answer: (a)
Q.35
Two Nicols prism are oriented with their principal planes making an angle of 60°. Then the percentage of incident unpolarized light which passes through the system is
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a) 100
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b) 50
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c)12.5
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d)37.5
Explanation
Answer:(c)
Q.36
Out of the the following, of which colour, sensitivity of human eye is highest
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a) green
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b) red
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c) violet
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d) white
Explanation
Answer: (a)
Q.37
In Young's experiment if transparent thin sheet are placed in front of two thin slits such that the central bright fringe remains at the same position. Thickness and refractive index of both sheets are t1 and t2 and µ1 and µ2 respectively. In this case..
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a)t1 / t2=µ1 / µ2
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b) t2 / t1=µ1 / µ2
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c)t1 / t2=(µ2-1) / (µ1-1)
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d)t2 / t1=(µ2-1) / (µ1-1)
Explanation
Use formula shift=(µ - 1) tAnswer: (c)
Q.38
The ratio of resolving power of telescope, when light of wavelengths 4000Å and 5000Å are used, is
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a) 16 : 25
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b) 5:4
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c)4:5
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d)9:1
Explanation
Answer: (b)
Q.39
The diameter of the lens of telescope is 1.22 m. The wavelength of light is 5000Å. The resolving power of the telescope is ...m-1
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a) 2×105
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b) 2×106
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c)2×102
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d)2×104
Explanation
resolving power of telescope=D/1.22λ Answer:(b)
Q.40
Unpolarized light falls on two polarizers placed one on top the other. What must be the angle between the characteristic directions of the polarizer if the intensity of the transmitted light is one third of the incident beam
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a) 54.7°
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b) 35.3°
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c) 0°
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d) 60°
Explanation
When unpolarized light passes through first plate emergent ray intensity is half of the original For second plate I=Iocos2θ I/3=(I/2)cos2θ find θ Answer: (b)
Q.41
For a normal eye, the cornea of eye provides a converging power of 40 D and the least converging power of the eye lens behind the cornea is 20 D. Using this information, the distance between the retina and the cornea - eye lens can be estimated to be
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a) 5 cm
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b) 2.5 cm
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c) 1.67 cm
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d) 1.5 cm
Explanation
Total power 40+20 =60 D Image is formed at retina thus focal length = distance between retina and cornea Answer:(c)
Q.42
In Young’s double slit experiment, the slits are 2 mm apart and are illuminated by photons of two wavelengths λ1 = 12000 Å and λ2 = 10000 Å. At what minimum distance from the common central bright fringe on the screen 2 m from the slit will a bright fringe from one interference pattern coincide with a bright fringe from the other?
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a) 8 mm
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b) 6 mm
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c) 4 mm
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d) 3 mm
Explanation
Let n1 and n2 be the bright fringe coincide at position X on screen i.e 5th fringe of λ1 coincide with 6th fringe of λ2 Answer:(b)
Q.43
A parallel beam of fast moving electrons is incident normally on a narrow slit. A fluorescent screen is placed at a large distance from the slit. If the speed of the electrons is increased, which of the following statements is correct?…[NEET 2013]
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a) Diffraction pattern is not observed on the screen in the case of electrons
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b) The angular width of the central maximum of the diffraction pattern will increase
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c) The angular width of the central maximum will decrease
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d) The angular width of the central maximum will be unaffected
Explanation
From De Broglie hypothesis, increase is velocity of electron is decrease in wave length as λ= h/mv From the width of central maxima Thus decrease in wave length decreases the angular width Answer:(c)
Q.44
The angle of a prism is 'A'. One of its refracting surfaces is silvered. Light rays falling at an angle of incidence 2A On the first surface returns back through the same path after suffering reflection at the silvered surface. The reaction index µ,of the prism is
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a) ½cosA
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b) tanA
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c) 2sinA
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d) 2cosA
Explanation
It is given that light reflect back on same path thus angle of incidence on reflected surface is zero. Or angle r2=0 Now r1 + r2 = A r1 = A µ=2cosA Answer:(d)
Q.45
A beam of light of λ = 600 nm from a distant source falls on a single slit 1 mm wide and the resulting diffraction pattern is observed on a screen 2m away. The distance between first dark fringes on either side of the central bright fringe is
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a) 2.4 cm
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b) 2.4mm
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c) 1.2 cm
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d) 1.2mm
Explanation
Distance between first dark fringes = width of central bright fringe (2x) 2x=2.400×10-3 m = 2.4 mm Answer:(b)
Q.46
Diameter of a plano - convex lens is 6 cm and thickness at the centre is 3 mm. If speed of light in material of lens is 2 × 108 m/s, the focal length of the lens is :
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a) 15 cm
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b) 20 cm
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c) 30 cm
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d) 10 cm
Explanation
In problem radius of curvature not given can be calculated as follows AA’ is diameter of lens, r is the radius of curvature OA is radius, OC = r-0.3 From triangle OAC r2 = (r-0.3)2 + 32 r2 = r2 – 0.6r + 0.09 + 9 0.6r = 9.09 r= 9.09/0.6 =15.15 given velocity of light is medium is 2 × 108 m/s, so refractive index = 1.5 Now using lens formula f= 30.3 approx = 30cm Answer:(c)
Q.47
In the Young's double-silt experiment, the intensity of light at a point on the screen where the path difference is λ is K, (λ being the wavelength of light used). The intensity at a point where the path difference is λ/4,will be …
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a) K/2
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b) zero
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c) K
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d) K/4
Explanation
In double –slit experiment intensity is same of bright fringes If path difference is nλ bright fringes. For dark fringes Given path difference is λ/4 thus we have to use formula Here if path difference λ the δ = 2π I1 = I2 = I I’= 4I = K Now if path difference λ/4 then δ =2π/4=π/2 I"=2I If 4I = K then 2I = K/2 Answer:(a)
Q.48
If the focal length of objective lens is increased then magnifying power of
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a) microscope and telescope both will decrease
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b) microscope will decrease but that of telescope will increase
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c) microscope will increase but that of telescope decrease
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d) microscope and telescope both will increase
Explanation
Microscope The total magnification, when the image is formed at infinity, is Thus, the total magnification, when the image is formed at near point, is Thus increasing the focal length of objective lens magnification of microscope decreases. Telescope Thus increasing the focal length of objective lens magnification of telescope increases Answer:(b)
Q.49
In a double slit experiment, the two slits are 1 mm apart and the screen is placed 1 m away. A monochromatic light of wavelength 500 nm is used. What will be the width of each slit for obtaining ten maxima of double slit within the central maxima of single slit pattern?
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a) 0.02 mm
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b) 0.2 mm
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c) 0.1 mm
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d) 0.5 mm
Explanation
width of central maxima Position of 10 maxima Given width of central maximum = Position of 10th bright fringe Answer:(b)
Q.50
The refracting angle of a prism is A, and refractive index of the material of the prism is cot (A/2). The angle of minimum deviation is :
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a) 180° + 2A
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b) 180° - 3A
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c) 180° - 2A
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d) 90° – A
Explanation
Also From (i) and (ii) Dm =π-2A Answer:(c)
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