# 1994 - JAMB Physics Past Questions and Answers - page 3

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 isA

6000m

B

3000m

C

1500m

D

375m

**correct option:**b

t = 4s ; v = 150ms

depth = X ; but 2X = vt

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

X = 3000m

Users' Answers & Comments^{-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

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

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

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

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

= 200Hz

Users' Answers & CommentsF = \(\frac {V}{\lambda}\)

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

= 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

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

T

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

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

F = 400Hz

Users' Answers & CommentsT

_{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 glassA

4.0 x 10

^{14}HzB

6.0 x 10

^{14}HzC

7.5 x 10

^{14}HzD

9.0 x 10

^{14}Hz27

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

V = -2u

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

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

u = 10cm

Users' Answers & CommentsF = \(\frac{uv}{u + v}\)

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

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

Apparent depth = 2cm

displacement = 3 - 2

= 1cm

Users' Answers & CommentsApparent 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