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Class 12 - Physics
Electromagnetic Waves Test 05

Question No. 1 to 5 are based on the given text. Read the text carefully and answer the questions:

Radio waves are produced by the accelerated motion of charges in conducting wires. Microwaves are produced by special
vacuum tubes. Infrared waves are produced by hot bodies and molecules also known as heat waves. UV rays are produced by
special lamps and very hot bodies like Sun.

1. Solar radiation is
a. transverse electromagnetic wave
b. longitudinal electromagnetic waves
c. both longitudinal and transverse electromagnetic waves
d. none of these
2. What is the cause of greenhouse effect?
a. Ultraviolet rays
b. X-rays
c. Infrared rays
d. Radiowaves
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3. Biological importance of ozone layer is
a. it stops ultraviolet rays
b. none of these.
c. it reflects radiowaves
d. It layer reduces greenhouse effect
4. Ozone is found in
a. troposphere
b. mesosphere
c. ionosphere
d. stratosphere
5. Earth's atmosphere is richest in
a. ultraviolet
b. infrared
c. X-rays

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d. microwaves
6. An observer is 1.8 m from an isotropic point source of light of power 250W. The rms value of the electric field at the
position of the observer is nearly:
a. 500 Vm-1
b. 50 Vm-1
c. 0.5 Vm-1
d. 5 Vm-1
7. Characteristic X-rays are produced due to
a. transfer of momentum in collisiori of electrons with target atoms
b. transition of electrons from higher to lower electronic orbits of an atom
c. heating of the target
d. transfer of energy in collision of electrons with atoms in the target
8. In ruby laser, the stimulated emission is due to transition from:
a. any higher state to ground state
b. metastable state to ground state
c. any higher state to lower state
d. metastable state to any lower state
9. Assertion: Soft and hard X-rays differ in frequency only.
Reason: The penetrating power of hard X-rays is more than the penetrating power of soft X-rays.
a. Assertion and reason both are correct statements and reason is correct explanation for assertion.
b. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
c. Assertion is correct statement but reason is wrong statement.
d. Assertion is wrong statement but reason is correct statement.
10. Assertion (A): In an electromagnetic wave, the magnitude of the magnetic field vector is much smaller than the
magnitude of the electric field vector.
Reason (R): The energy of electromagnetic waves is shared equally by the electric and magnetic fields.
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A.
c. A is true but R is false
d. A is false but R is true.
11. State True or False:
i. The electromagnetic waves have momentum.
a. True
b. False
ii. The velocity of light in vacuum can be changed by changing frequency.
a. True
b. False
12. Fill in the blanks:
a. The average value of magnetic field or electric field in plane electromagnetic wave is ________.
b. Microwaves are electromagnetic waves with frequency, in the range of ________.
13. Name the electromagnetic radiation which has the largest penetrating power.
14. Find the energy stored in a 90 cm length of a laser beam operating at 10 mW.
15. Identify the electromagnetic waves whose wavelengths lie in the range.
i. 10-11 m < λ < 10-14 m

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ii. 10-4 m < λ < 10-6 m

Write one use of each.
16. Why is the orientation of the portable radio with respect to the broadcasting station important?
17. Mrs. Rajlakshmi had a sudden fall and was thereafter unable to stand straight. She was in great pain. Her daughter Rita
took her to the doctor. The doctor took a photograph of Mrs. Rajlakshmi's bones and found that she had suffered a
fracture.
He advised her to rest and take the required treatment.
i. Name the electromagnetic radiation used to take the photograph of the bones.
ii. How is this radiation produced?
iii. Mention the range of the wavelength of this electromagnetic radiation.
iv. Write two values displayed by Rita.
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18. Electromagnetic waves with wavelength
i. λ is used in satellite communication.
1

ii. λ is used to kill germs in water purifies.

2

iii. λ is used to detect leakage of oil in underground pipelines.

3

iv. λ is used to improve visibility in runways during fog and mist conditions.
4

a. Identify and name the part of electromagnetic spectrum to which these radiations belong.
b. Arrange these wavelengths in ascending order of their magnitude.
c. Write one more application of each.

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Class 12 - Physics
Electromagnetic Waves Test 05

Solution

1. (a) transverse electromagnetic wave

2. (c) Infrared rays
Explanation: Greenhouse effect is due to infrared rays.
3. (a) it stops ultraviolet rays
Explanation: Ozone layer absorbs the harmful ultraviolet radiations coming from the sun.
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4. (d) stratosphere
Explanation: Ozone layer lies in stratosphere.
5. (b) infrared
Explanation: The atmosphere of earth is richest in infrared radiation.
6. (b) 50 Vm-1
Explanation: I = P

4πr
2

Also I = cε 0 E2
Hence, P

2
= cε0 E
2

4πr
−−−−−
or E = √ P

2
4πr c ε0
−−−−−−−−−−−−−−−−−−−−−
or E = √ 2
250

8 −12
4(3.14)(1.8) (3 × 10 )(8.85 × 19 )

= 48.1 Vm-1 = 50 Vm-1

7. (b) transition of electrons from higher to lower electronic orbits of an atom
Explanation: Characteristic X-rays are produced due to transfer of electrons from higher to lower electronic orbits of an
atom.
8. (b) metastable state to ground state
Explanation: Ruby laser is a three-level laser. In a ruby laser, the stimulated emission is due to the transition from a
metastable state to a ground state.
9. (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
10. (d) A is false but R is true.
11. State True or False:
i. (a) True
ii. (b) False
12. Fill in the blanks:
a. Zero
b. Giga hertz
13. γ -rays have the largest penetrating power.
14. Time taken by laser beam to move through a distance 90 cm is,
90 90cm −9
t = = = 3 × 10 s
c 10
3×10 cm/s

The energy contained in 90 cm length of laser beam is

U = Pt = (10 mW) × (3 × 10-9 s) = (10 × 10-3 Js-1) × (3 × 10-9 s) = 30 × 10-12 J

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15. i. Wavelength range of 10-11 m < λ < 10-14m shows the presence of both X-rays and Gamma rays as Gamma-rays have
wavelength ranges from 10-14 m - 10-11 m while the X-rays wavelength ranges from 10-12m - 10-8m.
Uses: (any one)
a. Gamma-radiations are used in medical treatment and for checking flaws in metal castings and for detection by
photographic plates or radiation detectors.
b. X-rays are used in medicine and dentistry and may be detected using photographic film.
ii. The wavelength range of 104m < λ < 10-6m shows the presence of Infrared, Visible and Microwave.
Uses : (any one)
a. Infrared radiation is useful for haze photography and is used by Earth resource satellites to detect healthy crops.
It can be used to study human and animal body heat patterns.
b. Visible light affects a photographic film, stimulates the retina in the eye and causes photosynthesis in plants.
c. Microwaves are used in radar, telemetry and electron spin resonance studies and in microwave ovens.
16. Transmitted carrier wave signals are plane-polarized and if the intensity of the signal is poor then the receiving antenna of
radio must be parallel to the component of either electric or magnetic field. Because energy is only due to amplitudes of
electric and magnetic components in EM wave, the magnitude of amplitude is in particular direction perpendicular to each
other and perpendicular to wave propagation.
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17. a. X-rays
b. By using X-rays tubes (Or: By bombarding a metal target with high energy electrons)
c. The wavelength range of X-rays is from (10 nm to 10 pm)
d. Alertness, empathy; concern for her mother, knowledgeable.
18. a.
i. In satellite communications, microwave is widely used. Hence λ , is the wavelength of the microwave.
1

ii. In water purifier, ultraviolet rays are used to kill germs. So, λ is the wavelength of UV rays.
2

iii. X-rays are used to detect leakage of oil in underground pipelines. So λ is the wavelength of X-rays
3

iv. Infrared rays are used to improve visibility on runways during fog and mist conditions. So, it is the wavelength of
infrared waves.
b. Wavelength of X-rays < wavelength of UV < wavelength of infrared < wavelength of microwave.
= λ3 < λ2 < λ4 < λ1

c. Radiation Uses

y-rays Gives informations on nuclear structure, medical treatment etc.

Medical diagnosis(Bone fracture identification (bone scanning)) and treatment study of crystal
X-rays
structure, industrial radiograph.

Preserve food, sterilizing the surgical instruments, detecting the invisible writings, fingerprints,
UV-rays
LASIK eye surgery etc.

Visible light To see objects

in physical therapy (To treat, muscular strain), for taking photography during the fog, haze,
Infrared rays
optical communication etc.

Micro wave and

In radar and telecommunication.
radio wave

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