Radio wave diffract around building although light waves do not. The reason is that radio waves [AMU 2000]

Travel with speed larger than c

Have much larger wavelength than light

Direction of the first secondary maximum in the Fraunhofer diffraction pattern at a single slit is given by (a is the width of the slit) [KCET 1999]

\[a\sin \theta =\frac{\lambda }{2}\]

\[a\cos \theta =\frac{3\lambda }{2}\]

\[a\sin \theta =\frac{3\lambda }{2}\]

If we observe the single slit Fraunhofer diffraction with wavelength l and slit width e, the width of the central maxima is 2q. On decreasing the slit width for the same l [UPSEAT 2004]

The spectrum will seems to shift to ultraviolet side

q increases or decreases depending on the intensity of light

When light is incident on a diffraction grating the zero order principal maximum will be [KCET 2004]

There is gravitational effect on the light

In a diffraction pattern by a wire, on increasing diameter of wire, fringe width [RPMT 1998]

There is gravitational effect on the light

Increasing or decreasing will depend on wavelength

Conditions of diffraction is [RPET 2001]

\[3\times {{10}^{6}}m/s\]moving towards the earth

Light is incident normally on a diffraction grating through which the first order diffraction is seen at 32o. The second order diffraction will be seen at

There is no second order diffraction in this case

The ratio of intensities of consecutive maxima in the diffraction pattern due to a single slit is

\[1:\frac{1}{{{\pi }^{2}}}:\frac{9}{{{\pi }^{2}}}\]

In the ideal double-slit experiment, when a glass-plate (refractive index 1.5) of thickness t is introduced in the path of one of the interfering beams (wavelength l), the intensity at the position where the central maximum occurred previously remains unchanged. The minimum thickness of the glass-plate is [IIT-JEE (Screening) 2002]

\[3.2\times {{10}^{-9}}kg-m/{{s}^{2}}\]

Assertion : Newton?s rings are formed in the reflected system. When the space between the lens and the glass plate is filled with a liquid of refractive index greater than that of glass, the central spot of the pattern is dark. Reason : The reflection is Newton?s ring cases will be from a denser to a rarer medium and the two interfering rays are reflected under similar conditions. [AIIMS 1998]

If both assertion and reason are true but reason is not the correct explanation of the assertion.

If both assertion and reason are true and the reason is the correct explanation of the assertion.

If both assertion and reason are true but reason is not the correct explanation of the assertion.

If the assertion and reason both are false.

If assertion is false but reason is true.

Radio waves and visible light in vacuum have [KCET 2000]

Same velocity but different wavelength

Biological importance of Ozone layer is [CBSE PMT 2001]

Ozone rays reduce greenhouse effect

Ozone layer controls \[{{O}_{2}}/{{H}_{2}}\] radio in atmosphere

If \[\overrightarrow{E}\] and \[\overrightarrow{B}\] are the electric and magnetic field vectors of E.M. waves then the direction of propagation of E.M. wave is along the direction of [CBSE PMT 1992, 2002; DCE 2002, 05]

The body moves with velocities greater than that of light

Which of the following statement is wrong [NCERT 1976]

Photographic plates are sensitive to ultraviolet rays

Infrared photon has more energy than the photon of visible light

Photographic plates are sensitive to ultraviolet rays

If assertion is true but reason is false.

Infrared rays are invisible but can cast shadows like visible light rays

Assuming that universe is expanding, if the spectrum of light coming from a star which is going away from earth is tested, then in the wavelength of light

If assertion is true but reason is false.

The spectrum will move to infrared region

If assertion is true but reason is false.

If the shift of wavelength of light emitted by a star is towards violet, then this shows that star is [RPET 1996; RPMT 1999]

If assertion is true but reason is false.

When the wavelength of light coming from a distant star is measured it is found shifted towards red. Then the conclusion is [JIPMER 1999]

The star is approaching the observer

The star recedes away from earth

96 + Mcqs in Wave Optics in Physics Page 1 McqOptions

1.	Radio wave diffract around building although light waves do not. The reason is that radio waves [AMU 2000]
A.	Travel with speed larger than c
B.	Have much larger wavelength than light
C.	\[\sqrt{\frac{mv}{k}}\]
D.	Are not electromagnetic waves
Answer» C. \[\sqrt{\frac{mv}{k}}\]

Discussion

2.	A heavenly body is receding from earth such that the fractional change in l is 1, then its velocity is [DCE 2000]
A.	C
B.	\[\frac{3C}{5}\]
C.	Ionosphere
D.	\[\frac{2C}{5}\]
Answer» B. \[\frac{3C}{5}\]

Discussion

3.	The condition for observing Fraunhofer diffraction from a single slit is that the light wavefront incident on the slit should be [MP PMT 1987]
A.	Spherical
B.	Cylindrical
C.	UV rays
D.	Elliptical
Answer» D. Elliptical

Discussion

4.	A parallel beam of monochromatic light of wavelength\[5000\,\,{AA}\]is incident normally on a single narrow slit of width 0.001 mm. The light is focused by a convex lens on a screen placed on the focal plane. The first minimum will be formed for the angle of diffraction equal to [CBSE PMT 1993]
A.	\[0{}^\circ \]
B.	\[15{}^\circ \]
C.	X-rays
D.	\[60{}^\circ \]
Answer» D. \[60{}^\circ \]

Discussion

5.	A parallel monochromatic beam of light is incident normally on a narrow slit. A diffraction pattern is formed on a screen placed perpendicular to the direction of incident beam. At the first maximum of the diffraction pattern the phase difference between the rays coming from the edges of the slit is [IIT?JEE 1995, 98]
A.	0
B.	\[\frac{\pi }{2}\]
C.	Mesosphere
D.	\[2\pi \]
Answer» E.

Discussion

6.	Direction of the first secondary maximum in the Fraunhofer diffraction pattern at a single slit is given by (a is the width of the slit) [KCET 1999]
A.	\[a\sin \theta =\frac{\lambda }{2}\]
B.	\[a\cos \theta =\frac{3\lambda }{2}\]
C.	X-rays
D.	\[a\sin \theta =\frac{3\lambda }{2}\]
Answer» E.

Discussion

7.	If we observe the single slit Fraunhofer diffraction with wavelength l and slit width e, the width of the central maxima is 2q. On decreasing the slit width for the same l [UPSEAT 2004]
A.	q increases
B.	q remains unchanged
C.	The spectrum will seems to shift to ultraviolet side
D.	q increases or decreases depending on the intensity of light
Answer» B. q remains unchanged

Discussion

8.	A beam of light of wavelength 600 nm from a distant source falls on a single slit 1 mm wide and the resulting diffraction pattern is observed on a screen 2 m away. The distance between the first dark fringes on either side of the central bright fringe is [IIT-JEE 1994; KCET 2004]
A.	1.2 mm
B.	1.2 cm
C.	\[0.2\,\,{AA}\]
D.	2.4 mm
Answer» E.

Discussion

9.	When light is incident on a diffraction grating the zero order principal maximum will be [KCET 2004]
A.	One of the component colours
B.	Absent
C.	There is gravitational effect on the light
D.	White
Answer» E.

Discussion

10.	What will be the angular width of central maxima in Fraunhoffer diffraction when light of wavelength \[6000\,{AA}\] is used and slit width is \[12\times {{10}^{-5}}cm.\] [RPMT 2004]
A.	2 rad
B.	3 rad
C.	\[\frac{C}{5}\]
D.	8 rad
Answer» D. 8 rad

Discussion

11.	In a diffraction pattern by a wire, on increasing diameter of wire, fringe width [RPMT 1998]
A.	Decreases
B.	Increases
C.	There is gravitational effect on the light
D.	Increasing or decreasing will depend on wavelength
Answer» B. Increases

Discussion

12.	The phenomenon of diffraction of light was discovered by [KCET 2000]
A.	Hygens
B.	Newton
C.	Blue
D.	Grimaldi
Answer» E.

Discussion

13.	Light of wavelength \[589.3\,nm\] is incident normally on the slit of width \[0.1\,mm.\] What will be the angular width of the central diffraction maximum at a distance of \[1\,m\] from the slit [BHU (Med.) 1999]
A.	\[0.68{}^\circ \]
B.	\[1.02{}^\circ \]
C.	\[3\times {{10}^{6}}Hz\]
D.	None of these
Answer» B. \[1.02{}^\circ \]

Discussion

14.	Conditions of diffraction is [RPET 2001]
A.	\[\frac{a}{\lambda }=1\]
B.	\[\frac{a}{\lambda }>>1\]
C.	\[3\times {{10}^{6}}m/s\]moving towards the earth
D.	None of these
Answer» B. \[\frac{a}{\lambda }>>1\]

Discussion

15.	Light is incident normally on a diffraction grating through which the first order diffraction is seen at 32o. The second order diffraction will be seen at
A.	48o
B.	64o
C.	1 mm
D.	There is no second order diffraction in this case
Answer» E.

Discussion

16.	The ratio of intensities of consecutive maxima in the diffraction pattern due to a single slit is
A.	1 : 4 : 9
B.	1 : 2 : 3
C.	Spectrum of the colours
D.	\[1:\frac{1}{{{\pi }^{2}}}:\frac{9}{{{\pi }^{2}}}\]
Answer» D. \[1:\frac{1}{{{\pi }^{2}}}:\frac{9}{{{\pi }^{2}}}\]

Discussion

17.	In a single slit diffraction experiment first minimum for red light (660 nm) coincides with first maximum of some other wavelength l'. The value of l' is
A.	\[4400\,\,{AA}\]
B.	\[6600\,\,{AA}\]
C.	1 rad
D.	\[3500\,\,{AA}\]
Answer» B. \[6600\,\,{AA}\]

Discussion

18.	Angular width of central maxima in the Fraunhoffer diffraction pattern of a slit is measured. The slit is illuminated by light of wavelength \[6000\,{AA}\]. When the slit is illuminated by light of another wavelength, the angular width decreases by 30%. The wavelength of this light will be
A.	\[6000\,\,{AA}\]
B.	\[4200\,\,{AA}\]
C.	Remains unchanged
D.	\[1800\,\,{AA}\]
Answer» C. Remains unchanged

Discussion

19.	A flake of glass (refractive index 1.5) is placed over one of the openings of a double slit apparatus. The interference pattern displaces itself through seven successive maxima towards the side where the flake is placed. if wavelength of the diffracted light is \[\lambda =600nm\], then the thickness of the flake is
A.	2100 nm
B.	4200 nm
C.	\[\frac{a}{\lambda }<<1\]
D.	None of these
Answer» D. None of these

Discussion

20.	In the ideal double-slit experiment, when a glass-plate (refractive index 1.5) of thickness t is introduced in the path of one of the interfering beams (wavelength l), the intensity at the position where the central maximum occurred previously remains unchanged. The minimum thickness of the glass-plate is [IIT-JEE (Screening) 2002]
A.	2l
B.	\[\frac{2\lambda }{3}\]
C.	\[3.2\times {{10}^{-9}}kg-m/{{s}^{2}}\]
D.	l
Answer» B. \[\frac{2\lambda }{3}\]

Discussion

21.	Assertion : Newton?s rings are formed in the reflected system. When the space between the lens and the glass plate is filled with a liquid of refractive index greater than that of glass, the central spot of the pattern is dark. Reason : The reflection is Newton?s ring cases will be from a denser to a rarer medium and the two interfering rays are reflected under similar conditions. [AIIMS 1998]
A.	If both assertion and reason are true and the reason is the correct explanation of the assertion.
B.	If both assertion and reason are true but reason is not the correct explanation of the assertion.
C.	136°
D.	If the assertion and reason both are false.
E.	If assertion is false but reason is true.
Answer» B. If both assertion and reason are true but reason is not the correct explanation of the assertion.

Discussion

22.	Which one of the following have minimum wavelength [Pb PET 2001]
A.	Ultraviolet rays
B.	Cosmic rays
C.	\[3\,m/s\]
D.	\[\gamma -\]rays
Answer» C. \[3\,m/s\]

Discussion

23.	Which of the following electromagnetic waves have minimum frequency [Pb PET 2000]
A.	Microwaves
B.	Audible waves
C.	\[\frac{1}{5}\]
D.	Radiowaves
Answer» C. \[\frac{1}{5}\]

Discussion

24.	The region of the atmosphere above troposphere is known as [BCECE 2004]
A.	Lithosphere
B.	Uppersphere
C.	0.23
D.	Stratosphere
Answer» E.

Discussion

25.	Which scientist experimentally proved the existence of electromagnetic waves [AFMC 2004]
A.	Sir J.C. Bose
B.	Maxwell
C.	0.81 kg-m/s
D.	Hertz
Answer» D. Hertz

Discussion

26.	The wavelength 21 cm emitted by atomic hydrogen in interstellar space belongs to
A.	Radio waves
B.	Infrared waves
C.	\[m=M\]
D.	g-rays
Answer» B. Infrared waves

Discussion

27.	A TV tower has a height of 100 m. The average population density around the tower is 1000 per km2. The radius of the earth is \[6.4\times {{10}^{6}}\]m. the population covered by the tower is
A.	\[2\times {{10}^{6}}\]
B.	\[3\times {{10}^{6}}\]
C.	7.0 m/s
D.	\[6\times {{10}^{6}}\]
Answer» D. \[6\times {{10}^{6}}\]

Discussion

28.	Approximate height of ozone layer above the ground is [CBSE PMT 1991]
A.	60 to 70 km
B.	59 km to 80 km
C.	mgR
D.	100 km to 200 km
Answer» B. 59 km to 80 km

Discussion

29.	Which of the following rays has the maximum frequency [CBSE PMT 1994]
A.	Gamma rays
B.	Blue light
C.	\[MgL/9\]
D.	Ultraviolet rays
Answer» B. Blue light

Discussion

30.	An electromagnetic wave travels along z-axis. Which of the following pairs of space and time varying fields would generate such a wave [CBSE PMT 1994]
A.	\[{{E}_{x}},\,{{B}_{y}}\]
B.	\[{{E}_{y}},\,{{B}_{x}}\]
C.	\[1/\sqrt{m}\]
D.	\[{{E}_{y}},\,{{B}_{z}}\]
Answer» B. \[{{E}_{y}},\,{{B}_{x}}\]

Discussion

31.	In which one of the following regions of the electromagnetic spectrum will the vibrational motion of molecules give rise to absorption [SCRA 1994]
A.	Ultraviolet
B.	Microwaves
C.	40 m/s
D.	Radio waves
Answer» C. 40 m/s

Discussion

32.	Radio waves and visible light in vacuum have [KCET 2000]
A.	Same velocity but different wavelength
B.	Continuous emission spectrum
C.	v in backward direction
D.	Line emission spectrum
Answer» B. Continuous emission spectrum

Discussion

33.	The frequencies of X-rays, g-rays and ultraviolet rays are respectively a, b and c. Then [CBSE PMT 2000]
A.	a < b, b > c
B.	a > b, b > c
C.	6
D.	a < b, b < c
Answer» B. a > b, b > c

Discussion

34.	Biological importance of Ozone layer is [CBSE PMT 2001]
A.	It stops ultraviolet rays
B.	Ozone rays reduce greenhouse effect
C.	225%
D.	Ozone layer controls \[{{O}_{2}}/{{H}_{2}}\] radio in atmosphere
Answer» B. Ozone rays reduce greenhouse effect

Discussion

35.	If \[\overrightarrow{E}\] and \[\overrightarrow{B}\] are the electric and magnetic field vectors of E.M. waves then the direction of propagation of E.M. wave is along the direction of [CBSE PMT 1992, 2002; DCE 2002, 05]
A.	\[\overrightarrow{E}\]
B.	\[\overrightarrow{B}\]
C.	The body moves with velocities greater than that of light
D.	None of these
Answer» D. None of these

Discussion

36.	Electromagnetic waves are transverse in nature is evident by [AIEEE 2002]
A.	Polarisation
B.	Interference
C.	10%
D.	Diffraction
Answer» B. Interference

Discussion

37.	Which of the following waves have the maximum wavelength [AFMC 2002]
A.	X-rays
B.	I.R. rays
C.	1000 J
D.	Radio waves
Answer» E.

Discussion

38.	Which of the following shows greenhouse effect [CBSE PMT 2002]
A.	Ultraviolet rays
B.	Infrared rays
C.	108
D.	None of these
Answer» C. 108

Discussion

39.	Electromagnetic radiation of highest frequency is [Kerala PMT 2002]
A.	Infrared radiations
B.	Visible radiation
C.	4 : 1
D.	g-rays
Answer» E.

Discussion

40.	Ozone is found in [DPMT 2002]
A.	Stratosphere
B.	Ionosphere
C.	4 times
D.	Troposphere
Answer» B. Ionosphere

Discussion

41.	Which of the following are not electromagnetic waves [AIEEE 2002; CBSE PMT 2003]
A.	Cosmic rays
B.	Gamma rays
C.	Be quadrupled
D.	X-rays
Answer» D. X-rays

Discussion

42.	The maximum distance upto which TV transmission from a TV tower of height h can be received is proportional to [AIIMS 2003]
A.	\[{{h}^{1/2}}\]
B.	h
C.	Heat energy
D.	\[{{h}^{2}}\]
Answer» B. h

Discussion

43.	Pick out the longest wavelength from the following types of radiations [CBSE PMT 1990]
A.	Blue light
B.	g-rays
C.	1 : 2
D.	Red light
Answer» E.

Discussion

44.	Which of the following statement is wrong [NCERT 1976]
A.	Infrared photon has more energy than the photon of visible light
B.	Photographic plates are sensitive to ultraviolet rays
C.	If assertion is true but reason is false.
D.	Infrared rays are invisible but can cast shadows like visible light rays
Answer» B. Photographic plates are sensitive to ultraviolet rays

Discussion

45.	Assuming that universe is expanding, if the spectrum of light coming from a star which is going away from earth is tested, then in the wavelength of light
A.	There will be no change
B.	The spectrum will move to infrared region
C.	If assertion is true but reason is false.
D.	None of the above
Answer» C. If assertion is true but reason is false.

Discussion

46.	If the shift of wavelength of light emitted by a star is towards violet, then this shows that star is [RPET 1996; RPMT 1999]
A.	Stationary
B.	Moving towards earth
C.	If assertion is true but reason is false.
D.	Information is incomplete
Answer» C. If assertion is true but reason is false.

Discussion

47.	A star producing light of wavelength\[6000\,\,{AA}\]moves away from the earth with a speed of 5 km/sec. Due to Doppler effect the shift in wavelength will be \[(c=3\times {{10}^{8}}m/\sec )\] [MP PMT 1990]
A.	\[0.1\,\,{AA}\]
B.	\[0.05\,\,{AA}\]
C.	X-rays
D.	\[1\,\,{AA}\]
Answer» B. \[0.05\,\,{AA}\]

Discussion

48.	When the wavelength of light coming from a distant star is measured it is found shifted towards red. Then the conclusion is [JIPMER 1999]
A.	The star is approaching the observer
B.	The star recedes away from earth
C.	Marconi
D.	The star remains stationary
Answer» C. Marconi

Discussion

49.	If a star is moving towards the earth, then the lines are shifted towards [AIIMS 1997]
A.	Red
B.	Infrared
C.	\[4\times {{10}^{6}}\]
D.	Green
Answer» D. Green

Discussion

50.	A rocket is going away from the earth at a speed 0.2c, where c = speed of light. It emits a signal of frequency \[4\times {{10}^{7}}Hz\]. What will be the frequency observed by an observer on the earth [RPMT 1996]
A.	\[4\times {{10}^{6}}Hz\]
B.	\[3.2\times {{10}^{7}}Hz\]
C.	70 km to 100 km
D.	\[5\times {{10}^{7}}Hz\]
Answer» C. 70 km to 100 km

Discussion

Explore topic-wise MCQs in Physics.

Radio wave diffract around building although light waves do not. The reason is that radio waves [AMU 2000]

A heavenly body is receding from earth such that the fractional change in l is 1, then its velocity is [DCE 2000]

The condition for observing Fraunhofer diffraction from a single slit is that the light wavefront incident on the slit should be [MP PMT 1987]

A parallel beam of monochromatic light of wavelength\[5000\,\,{AA}\]is incident normally on a single narrow slit of width 0.001 mm. The light is focused by a convex lens on a screen placed on the focal plane. The first minimum will be formed for the angle of diffraction equal to [CBSE PMT 1993]

Direction of the first secondary maximum in the Fraunhofer diffraction pattern at a single slit is given by (a is the width of the slit) [KCET 1999]

If we observe the single slit Fraunhofer diffraction with wavelength l and slit width e, the width of the central maxima is 2q. On decreasing the slit width for the same l [UPSEAT 2004]

A beam of light of wavelength 600 nm from a distant source falls on a single slit 1 mm wide and the resulting diffraction pattern is observed on a screen 2 m away. The distance between the first dark fringes on either side of the central bright fringe is [IIT-JEE 1994; KCET 2004]

When light is incident on a diffraction grating the zero order principal maximum will be [KCET 2004]

What will be the angular width of central maxima in Fraunhoffer diffraction when light of wavelength \[6000\,{AA}\] is used and slit width is \[12\times {{10}^{-5}}cm.\] [RPMT 2004]

In a diffraction pattern by a wire, on increasing diameter of wire, fringe width [RPMT 1998]

The phenomenon of diffraction of light was discovered by [KCET 2000]

Light of wavelength \[589.3\,nm\] is incident normally on the slit of width \[0.1\,mm.\] What will be the angular width of the central diffraction maximum at a distance of \[1\,m\] from the slit [BHU (Med.) 1999]

Conditions of diffraction is [RPET 2001]

Light is incident normally on a diffraction grating through which the first order diffraction is seen at 32o. The second order diffraction will be seen at

The ratio of intensities of consecutive maxima in the diffraction pattern due to a single slit is

In a single slit diffraction experiment first minimum for red light (660 nm) coincides with first maximum of some other wavelength l'. The value of l' is

Angular width of central maxima in the Fraunhoffer diffraction pattern of a slit is measured. The slit is illuminated by light of wavelength \[6000\,{AA}\]. When the slit is illuminated by light of another wavelength, the angular width decreases by 30%. The wavelength of this light will be

Which one of the following have minimum wavelength [Pb PET 2001]

Which of the following electromagnetic waves have minimum frequency [Pb PET 2000]

The region of the atmosphere above troposphere is known as [BCECE 2004]

Which scientist experimentally proved the existence of electromagnetic waves [AFMC 2004]

The wavelength 21 cm emitted by atomic hydrogen in interstellar space belongs to

A TV tower has a height of 100 m. The average population density around the tower is 1000 per km2. The radius of the earth is \[6.4\times {{10}^{6}}\]m. the population covered by the tower is

Approximate height of ozone layer above the ground is [CBSE PMT 1991]

Which of the following rays has the maximum frequency [CBSE PMT 1994]

An electromagnetic wave travels along z-axis. Which of the following pairs of space and time varying fields would generate such a wave [CBSE PMT 1994]

In which one of the following regions of the electromagnetic spectrum will the vibrational motion of molecules give rise to absorption [SCRA 1994]

Radio waves and visible light in vacuum have [KCET 2000]

The frequencies of X-rays, g-rays and ultraviolet rays are respectively a, b and c. Then [CBSE PMT 2000]

Biological importance of Ozone layer is [CBSE PMT 2001]

If \[\overrightarrow{E}\] and \[\overrightarrow{B}\] are the electric and magnetic field vectors of E.M. waves then the direction of propagation of E.M. wave is along the direction of [CBSE PMT 1992, 2002; DCE 2002, 05]

Electromagnetic waves are transverse in nature is evident by [AIEEE 2002]

Which of the following waves have the maximum wavelength [AFMC 2002]

Which of the following shows greenhouse effect [CBSE PMT 2002]

Electromagnetic radiation of highest frequency is [Kerala PMT 2002]

Ozone is found in [DPMT 2002]

Which of the following are not electromagnetic waves [AIEEE 2002; CBSE PMT 2003]

The maximum distance upto which TV transmission from a TV tower of height h can be received is proportional to [AIIMS 2003]

Pick out the longest wavelength from the following types of radiations [CBSE PMT 1990]

Which of the following statement is wrong [NCERT 1976]

Assuming that universe is expanding, if the spectrum of light coming from a star which is going away from earth is tested, then in the wavelength of light

If the shift of wavelength of light emitted by a star is towards violet, then this shows that star is [RPET 1996; RPMT 1999]

A star producing light of wavelength\[6000\,\,{AA}\]moves away from the earth with a speed of 5 km/sec. Due to Doppler effect the shift in wavelength will be \[(c=3\times {{10}^{8}}m/\sec )\] [MP PMT 1990]

When the wavelength of light coming from a distant star is measured it is found shifted towards red. Then the conclusion is [JIPMER 1999]

If a star is moving towards the earth, then the lines are shifted towards [AIIMS 1997]

A rocket is going away from the earth at a speed 0.2c, where c = speed of light. It emits a signal of frequency \[4\times {{10}^{7}}Hz\]. What will be the frequency observed by an observer on the earth [RPMT 1996]