# 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**

^{-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**

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**

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**

T

_{2}= 4T

_{1}; L

_{2}= 2L

_{1}

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

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

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 10

^{14}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 10

^{14}Hz
B 6.0 x 10

^{14}Hz
C 7.5 x 10

^{14}Hz
D 9.0 x 10

^{14}Hz**Correct Option:** A

**Solution**

F = 4 x 10

^{14}Hz

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**

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**

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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