Two mirrors at an angle 0° produce 5 images of a point. The number of images produced when θ is decreased to θ − 30° is
9
10
11
12
The radius of the light circle observed by a fish at a depth of 12 m is (refractive index of water = 4/3)
In Young's double slit experiment, the fringe width is β. If the entire arrangement is placed in a liquid of refractive index n, the fringe width becomes
nβ
A plano-convex lens (f = 20 cm) is silvered at plane surface. Now focal length will be
20 cm
40 cm
30 cm
10 cm
The light beams of intensities in the ratio of 9 : 1 are allowed to interfere. What will be the ratio of the intensities of maxima and minima ?
3 : 1
4 : 1
25 : 9
81 : 1
If x1 be the size of the magnified image and x2 the size of the diminished image in Lens Displacement Method, then the size of the object is
x1x2
x12x2
x1x22
In a common emitter configuration, a transistor has β = 50 and input resistance 1 kΩ. If the peak value of AC input is 0.01 V, then the peak value of collector current is
0.01 µA
0.25 µA
100 µA
500 µA
Half-life of a radioactive substance is 20 min. The time between 20% and 80% decay will be
20 min
30 min
40 min
25 min
C.
40 min
Given that the half-life of a radioactive substance is 2O min. So, t1/2 = 20 min.
For 20% decay, we have 80% of the substance left, hence
..... (i)
where N0 = initial undecayed substance and t20 is the time taken for 20% decay.
For 80% decay, we have 20% of the substance left, hence
..... (ii)
Dividing Eq.(i) and Eq.(ii), we get
4 = eλ(t80 - t20)
⇒ ln 4 = λ (t80 − t20)
(taking log on both sides)
⇒ 2 ln 2 = (t80 − t20)
⇒ t80 − t20 = 2 × t1/2
= 40 min
The energy released by the fission of one uranium atom is 200 MeV. The number of fissions per second required to produce 3.2 W of power is (Take, 1eV = 1.6 x 10-19 J)
107
1010
1015
1011