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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