1994 JAMB Physics Past Questions & Answers - page 3

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21
The sound from a source travelled to the bottom of the sea and the echo was heard 4s later. If the speed of sound in sea water is 1500ms-1 the depth of the sea is
A 6000m
B 3000m
C 1500m
D 375m


Correct Option: B


Solution
t = 4s ; v = 150ms-1

depth = X ; but 2X = vt

X = \(\frac {vt}{2}\) = \(\frac {1500 \times 4}{2}\)

X = 3000m

22
The fundamental frequency of vibration of a sonometer wire may be halved by
A doubling the length of the wire
B doubling the mass of the wire
C reducing the tension by half
D reducing the absolute temperature by half


Correct Option: A


Solution
F \(\alpha\) \(\frac{1}{L}\)

23
A pipe of length 45cm is closed at the end. Calculate the fundamental frequency of the sound wave generated in the pipe if the velocity of sound in air is 360ms-1 [Neglect end correction]
A 5.5Hz
B 148.5Hz
C 200.0Hz
D 550.0Hz


Correct Option: C


Solution
\(\lambda\) = 4L = 4 X 45 = 180cm

F = \(\frac {V}{\lambda}\)

= \(\frac {360}{180 \times 10^{-2}}\)

= 200Hz

24
The note produced by a stretched string has a fundamental frequency of 400Hz. If the length of the string is doubled while the tension in the string is increased by a factor of 4, the frequency is
A 200Hz
B 400Hz
C 800Hz
D 1600Hz


Correct Option: B


Solution
F = \(\frac {1}{2L}\) \(\sqrt\frac{T}{m}\)

T2 = 4T1 ; L2 = 2L1

\(\frac{2F_1L_1}{2F_2L_2}\) = \(\sqrt\frac{T_1}{T_2}\)

\(\frac{400\times L_1}{F \times L_1}\) = \(\sqrt\frac{T_1}{4T_1}\)

F = 400Hz

25
To produce a parallel beam of light from a concave mirror, the distance at a which the lamp should be placed from the mirror is equal to
A the focal length
B two times the focal length
C the distance of the image
D two times the radius of curvature


Correct Option: A



26
Light of frequency 6.0 x 1014 Hz travelling in air is transmitted through glass or refractive index 1.5. Calculate the frequency of the light in the glass
A 4.0 x 1014Hz
B 6.0 x 1014Hz
C 7.5 x 1014Hz
D 9.0 x 1014Hz


Correct Option: A


Solution
1.5 = \(\frac{6 \times 10^{14}}{F}\)

F = 4 x 1014Hz

27
An object is placed in front of a converging lens of a focal length 20cm. The image is virtual and has a magnification of 2. What is the distance of the object from the lens?
A 5cm
B 10cm
C 30cm
D 40cm


Correct Option: B


Solution
V = -2u

F = \(\frac{uv}{u + v}\)

20 = \(\frac{uv - 2u}{u - 2u}\)

u = 10cm

28
An object is placed directly below a glass block of thickness 3.0cm. Calculate the lateral displacement of the object if the refractive index of the glass is 1.5.
A 1.0cm
B 1.5cm
C 2.0cm
D 2.5cm


Correct Option: A


Solution
1.5 = \(\frac{3}{\text {apparent displacement}}\)

Apparent depth = 2cm

displacement = 3 - 2

= 1cm

29
In a projection lantern of focal length f, the object distance u, is such that
A u > 2f > f
B u < f < 2f
C u = f < 2f
D f < u < 2f


Correct Option: D



30
If the Nigeria flag (green, white, green) is viewed in pure yellow light, which of the following set of colours would be observed on the flag?
A Green, yellow, green
B Red, yellow, red
C black, yellow, black
D Green, white, green


Correct Option: A



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