Blackbody radiation-MCQs & answers-Jayam chemistry learners
Multiple choice questions and answers on blackbody and its radiation
The world around us possesses a lot of mysteries that need
thorough conceptualization to uncover and utilize for scientific inventions.
Scientists always quest to track those puzzles and invent new scientific
theories that simplify our life.
It was unbelievable that a physicist's speculation on the
ideal thermal emitter inaugurated a new era as quantum physics. It is nothing
but blackbody-a unique material with giant absorbing and emissive capabilities
of thermal radiation. This blog post discusses the multiple choice questions
and answers of blackbody and its radiation.
Table of contents:
Define blackbody &
its radiation
Multiple choice questions and answers on blackbody radiation
True or false
questions on blackbody & its radiation
Reasoning questions on
blackbody & its radiation
Video description of blackbody and its radiation
PPT notes of blackbody and its radiation
Difference between blackbody and gray body
Match the following
table on blackbody & its radiation
Our e-book:
Download an e-book on blackbody & its radiation from our store, "Jayam chemistry adda."
Define blackbody & its radiation:
A blackbody is an empty solid space with a rigid opaque
surface and a pinhole to emit thermal electromagnetic radiation that acts as a
reference to calculate the emissive powers of existent earthly objects.
Its opaque surface peculiarly sucks up all incident light
irrespective of their frequencies at a constant temperature. As a result, the
assessed value of the absorbing ability of a perfect blackbody is always one.
Yet an ideal blackbody is a hypothetical object with no practical existence.
So, a material whose absorbing capacity lies above 0.95 is called a partial
blackbody. Another common name for the partial blackbody is the grey body. Lamp
black, Platinum black, and Graphite are partial blackbodies for laboratory
purposes. This endless partial blackbodies list included all the universal
objects that emit thermal electromagnetic radiation, such as the sun, earth,
stars, the human body, planets, galaxies, and so on.
Even though the solar spectrum resembles blackbody curves at
longer radiation wavelengths, the sun is not a perfect blackbody because at
shorter radiation wavelengths solar spectrum deviates from blackbody curves
mostly. It confirms to us once again that blackbody is an imaginary presumed
material of the physicist Gustav Kirchhoff.
Next, the most crucial phrase of the blackbody topic is
blackbody radiation. Blackbody radiation is heat-assimilated electromagnetic
light radiation. Hotter objects emit this thermal electromagnetic radiation at
constant temperature conditions. Besides, they are uniform in all directions.
Hence the blackbody radiations are homogeneous and isotropic.
For example, the infrared radiation expelled from the human
body at room temperature is also blackbody radiation. The human body
temperature varies approximately between 30-35 degrees Celsius. And the room
temperature is also the same. It means a dynamic thermodynamic equilibrium
state establishes between the human body and its surrounding. Hence, following
Wien's displacement law, the emitted thermal radiation from the hot human body
lies in the infrared region.
(For a better view, click the image)
Similarly, examples of blackbody radiations include
sunlight, and electromagnetic radiations emitted from planets, stars, galaxies,
and even from earth.
A perfect blackbody expels thermal radiation only on
heating. When the blackbody is in a normal state (i.e., not heated), it absorbs
electromagnetic light. Hence, a cold object whose absorbing capabilities lie
above 0.95 emits blackbody radiation only when heated above 0 Kelvin. When they
are cool, they can absorb electromagnetic radiations that fall on their
surface, yet they cannot release thermal radiation. And they are known as grey
bodies.
For example, black spots on a polished silver vessel act as
blackbodies only on heating. And the smooth silver surface is a white body that
predominantly reflects the light radiation that falls on its surface. So, on
heating, it cannot act as an ideal radiation emitter like black spots.
A white body is opposite to a black body. It is a good
reflector of light. And it absorbs a small fraction of incident light that
falls on its surface. So, it is a poor emitter of electromagnetic radiation on
heating. Here we are not discussing the color of objects that affect their
emissive powers. Instead, the absorbing capabilities of materials at fixed
temperature conditions directly influence the object's emissive powers.
Obviously, the color of a substance symbolizes its absorbing power but all
black material are not partial blackbodies either. The black color surface
denotes that it absorbs all colored radiations of the visible region but not
the entire electromagnetic spectrum. Hence, it is not a perfect absorber.
Similarly, a white surface reflects all colored light radiations, and a white body
is a poor absorber of light radiations.
The quantity of heat energy supplied to the blackbody
enclosure directly influences the blackbody emissions radiant energies. When
heating a blackbody chamber to a high temperature gives high-frequency
radiations and vice versa. Hence, the released blackbody radiation is affected
only by the enclosure's temperature. Max Planck pointed out this directly
proportional relationship between blackbody radiation frequencies and the
magnitude of heat absorbed by the blackbody enclosure and developed a
mathematical formula by inserting a fixed physical quantity named Planck's
constant. And it is the blackbody radiation formula where E = hν. The blackbody radiation
formula suggests that at higher enclosure temperatures, thermal electromagnetic
radiation possesses higher frequencies. Evidently, this trend continues until
the emissions attain a peak state, after which the temperature rise decreases
the radiation frequency.
Multiple choice questions and answers on blackbody radiation:
1. What is a blackbody?
A. A celestial object
B. A chemical substance
C. A rigid solid enclosure
Answer:
A rigid solid enclosure
Explanation:
A hollow solidified enclosure having an opaque surface with
a pinhole is an assumption of a perfect blackbody by the physicist Gustav
Kirchhoff.
2. Which of the following is not an example of a partial
blackbody?
A. Heating filament of a toaster
B. Thin gold foil
C. Platinum black
Answer:
Thin gold foil
Explanation:
A perfect blackbody is unreal. All earthly objects with
absorbing abilities greater than 0.95 are imperfect blackbodies called grey
bodies. Thin gold foil absorbing ability is much less than the said value.
Hence, it is not an example of a partial blackbody.
3. A blackbody emits_________
A. Heat-assimilated electromagnetic radiations
B. Thermal radiations
C. Light radiations
Answer:
Heat-assimilated electromagnetic radiations
Explanation:
The thermal electromagnetic radiation emitted from a
blackbody is known as blackbody radiation. A blackbody emits the absorbed
electromagnetic light only on heating. Hence, it is a heat-combined
electromagnetic radiation.
4. Which of the following substance has the highest
absorbing power?
A. Polished Silver
B. Diamond
C. Lamp black
Answer:
Lamp black
Explanation:
Lamp black is a partial blackbody with 0.98 absorbing power.
It implies lamp black can intake 98% of the incident that falls on its surface.
The diamond and polished silver are not partial blackbodies whose absorbing
capacities lie below 0.95.
5. What is the emissivity of a pure copper filament?
A. less than one
B. greater than one
C. equal to one
Answer:
Less than one
Explanation:
When an object and a perfect blackbody are at a fixed
temperature state, their radiant emittances ratio is called emissivity. Anyhow
the emissivity of an ideal blackbody is always one. But all other earthly
materials' emissivity lies below one.
6. Which of the following is an application of blackbody
radiation?
A. cellular communications
B. satellite communications
C. Burglar alarm
Answer:
Burglar alarm
Explanation:
A burglar alarm is a security device with a loud alarm to
alert about the opening of doors and windows. It consists of blackbody
radiation detectors and sensors that connect with the control panel systems.
7. Blackbody radiations comprise _____________
A. Protons
B. Photons
C. Positrons
Answers:
Photons
Explanation:
Following Planck's law, blackbody radiation comprises
discrete radiant energy particles called photons.
8. The following are examples of hot bodies identical in
every respect in thermal equilibrium conditions. Which one among them cools
last?
A. A solid cube
B. A solid sphere
C. A solid cone
Answer:
A solid sphere
Explanation:
The rate of thermal emissions of a blackbody varies directly
with its surface area. Objects with a large surface area have high emissive
power. The solid sphere has a low surface area than the other hot bodies.
Hence, it cools last.
9. What is the color of blackbody radiation?
A. Blue
B. Red
C. It depends upon the enclosure temperature
Answer:
It depends upon the enclosure temperature
Explanation:
The color of blackbody radiation depends only on the
enclosure's temperature. The blackbody radiation exhibit colors when its
wavelength lies between 350-750 nm, and the enclosure's temperature ranges
between 3800-8300 Kelvin approximately. Initially, at 3800 K, the radiation
appears in dull red. At higher temperatures, the radiation color follows the
reverse VIBGYOR pattern. Finally, the radiation becomes violet-colored at 8257
Kelvin. At above 12000 K, the radiation becomes white. Then it enters the
ultraviolet region part of the spectrum with further temperature rise.
10. Which of the following surfaces are good reflectors of
blackbody radiations?
A. Black coarse surfaces
B. Polished surfaces
C. Rough surfaces
Answer:
Polished surfaces
Explanation:
Polished surfaces are even that makes them better reflectors
of light. They reflect light parallel to the incident light ray.
11. Intensity of blackbody radiation is maximum
at___________________
A. Longer radiation wavelengths
B. Shorter radiation wavelengths
C. λmax
Answer:
λmax
Explanation:
The blackbody curve depicts that energy distribution is not
uniform over broad wavelength ranges. As a result, the blackbody radiation
intensity rises with emissive power at shorter wavelengths. It continues until
it reaches the peak wavelength position λmax. For a fixed
temperature T, the radiation intensity is maximum at λmax.
12. The blackbody curve is the variation of
__________________ and ____________
A. Emissive power and temperature
B. Emissive power and wavelength
C. Temperature and wavelength
Answer:
Emissive power and wavelength
Explanation:
A blackbody curve is a graph between blackbody radiations'
emissive power and their wavelength at fixed temperature conditions. The
blackbody radiation graph is a hill-shaped curve depicting non-uniform spectral
energy densities of blackbody radiation emissions. Moreover, the crest position
of the blackbody curve shifts towards shorter wavelengths at higher
temperatures by Wien's displacement law.
13. Blackbody spectrum is a _____________________spectrum
A. Continuous radiation emission
B. Discontinuous radiation absorption
C. Line spectra of emitted blackbody radiations
Answer:
The continuous radiation emission spectrum
Explanation:
The blackbody spectrum is a patterned arrangement of
blackbody radiation emissions in the increasing order of frequencies or in the
decreasing order of their wavelengths. Besides, it is a continuous emission
spectrum due to the absence of distinct boundary separation between the
spectral bands. In the blackbody spectrum, one spectral band merges into the
other without partition. Like the electromagnetic spectrum, it is a
never-ending blackbody radiation emission spectrum.
14. By Lambert's law, the distribution of blackbody
radiation in various directions is ____________________
A. identical
B. irregular
C. intermittent
Answer:
Identical
Explanation:
Blackbody radiation emissions are homogeneous in all
directions. Hence, they are isotropic.
15. The blackbody emissions obey __________law
A. Quantum law
B. Energy conservation law
C. Biot-savart law
Answer:
Quantum law
Explanation:
Planck's quantum law was the first theory that invented the
particle character of energy. And it successfully explained the energy
distributions of blackbody curves for shorter and longer radiation wavelengths,
unlike Rayleigh-Jeans law and Wien displacement law. It elucidated the
interaction of electromagnetic radiations with the matter. Further, it calculated
the magnitude of blackbody emission energies from the radiation frequencies.
Answer whether the following statement is true or false:
1. White stars act as blackbodies
A. True
B. False
Answer:
False
Explanation:
A perfect blackbody is a hypothetical inexistent object.
White stars are partial blackbodies as they can emit hot thermal
electromagnetic radiations.
2. Blackbody radiations are hot
A. True
B. False
Answer:
True
Explanation:
Blackbody releases radiations only on heating. Consequently,
the blackbody radiations are heat-assimilated light radiations expelled from
the cavity of the blackbody chamber. The spectral energy densities of blackbody
emissions depend on the blackbody enclosure's temperature. Moreover, the
temperature is the sole factor influencing blackbody emissions.
3. Max Planck invented the term blackbody
A. True
B. False
Answer:
False
Explanation:
German physicist Gustav Kirchhoff in 1860, invented an ideal
absorber and emitter of electromagnetic radiations, irrespective of their wavelengths
in thermal equilibrium conditions, known as a blackbody.
4. The surface of a blackbody is opaque
A. True
B. False
Answer:
True
Explanation:
The surface of the blackbody was assumed opaque by Kirchhoff
to avoid reflection and transmission of incident light that falls on its
surface. It makes the blackbody a complete absorber and emitter of thermal
electromagnetic radiations in fixed temperature conditions, unlike other
earthly existent materials.
5. Blackbody radiations are visible only when their wavelength
lies below 300 nm
A. True
B. False
Answer:
False
Explanation:
Below 300 nm, the blackbody emissions lie in the ultraviolet
part of the blackbody spectrum that a human eye cannot detect. Hence they are
invisible.
Reasoning questions on blackbody & its radiation:
Read the assertion and explanation statements carefully. And
Pick the correct option from the following. The options are common to all
questions.
A. Both assertion and explanation are correct
B. Assertion is true. And the explanation is false
C. Assertion is false. And the explanation is correct.
D. Both assertion and explanation are false.
Question-1:
Assertion: An ideal blackbody absorbing power is one.
Explanation: It has zero transmitting and reflecting powers.
Answer:
The assertion is true. And the explanation is false.
Clarification:
The absorbing power of a perfect blackbody is one as it
absorbs all incident electromagnetic radiations of all wavelengths that fall on
its opaque surface in fixed temperature conditions. Absorbing capacity is the
ratio between the magnitude of heat absorbed to the total amount of heat
incident on its surface. In the case of an ideal blackbody, the absorbed heat
amount and the aggregate of incident light are equal. Hence, its absorbing
power is one. Although its opaque surface inhibits reflection and transmission
through the blackbody, it indirectly influences the absorbing ability of a
blackbody. Unless the blackbody surface is a good absorber, these reasons
cannot control its absorbing capacities.
Question-2:
Assertion: The blackbody spectrum is a continuous emission
spectrum
Explanation: The blackbody emissions are uniform and
homogeneous
Answer:
The assertion is true. And the explanation is false.
Clarification:
The blackbody spectrum is a continuous arrangement of
emitted blackbody radiations based on their wavelengths or frequencies. The
spectral bands denote the specific wavelengths of released blackbody radiations
that consolidate with its neighbor band to give an uninterrupted spectral bands
array. The emission spectrum of a blackbody is continuous due to the merging of
spectral bands but not by the homogeneous and isotropic character of blackbody
radiations.
Question-3:
Assertion:
Blackbodies emit thermal electromagnetic radiation only on
heating.
Explanation:
It helps to attain the thermal equilibrium conditions in the
enclosure.
Answer:
Both the assertion and explanation statements are correct.
Clarification:
At the thermal equilibrium state, no heat change occurs.
Nevertheless, the heat absorption and emission proceed with an equal energy sum
to maintain the thermal equilibrium. Moreover, it is a dynamic state that
changes intermittently in periodic installments.
Heating a blackbody enclosure disturbs its thermal
equilibrium state by enhancing heat energy levels. So, to achieve the equilibrium
state once again, the blackbody expels its excess energy as thermal
electromagnetic radiation. Hence, the release of blackbody radiations after
heating the blackbody to maintain the thermal equilibrium condition with the
surrounding is a justified explanation.
Question-4:
Assertion:
The blackbody surface is opaque. It makes the blackbody
surface crucial in controlling thermal electromagnetic emissions.
Explanation:
The proportion of radiation emissions varies inversely with
the surface area.
Both the assertion and explanation statements are false.
Clarification:
Regarding the assertion, the opaque surface of a blackbody
restricts reflection and transmission through it, but it cannot control its
radiation emissions. Hence, the crucial role of opaque blackbody surface in
blackbody radiation emission is false.
And coming to the explanation portion, the proportion of
radiation emissions varies directly with the surface area. Large surface area
objects have high absorbing and emissive powers. It is due to the enhancement
in the rate of energy transmissions per unit volume.
Question-5:
Assertion:
A blackbody is a hollow solid enclosure with an
opaque-coated surface. So it is physically existent solid matter.
Explanation:
A perfect blackbody
is an imaginary theoretical assumption of the physicist Gustav Kirchhoff.
Answer:
The assertion is false, and the explanation is correct.
Clarification:
A blackbody is a hollow, solidified, closed enclosure with a
little hole to emit blackbody radiation. The discussed description of the
physical state of the blackbody hints that it is an existent earthly object.
But it is untrue due to its exceptional absorbing and emissive capacities of
radiations of all wavelengths without any bounds. It is impractical for the
existent universal objects discovered so far.
Question-6:
Assertion:
A blackbody can emit white-colored thermal electromagnetic
radiation when heated to high temperatures.
Explanation:
It is possible only when the blackbody surface is coated
white.
Answer:
The assertion is true. The explanation is false.
Clarification:
The blackbody enclosure's temperature controls the color of
released thermal electromagnetic emissions. White-colored blackbody radiation
is released when the enclosure temperature is nearly 12000 Kelvin. Hence, the
assertion statement is correct.
A blackbody is not necessarily a black-colored object. All
black materials are not blackbodies, either. The material's absorbing and
emitting power decides the blackbody from the black-colored substances.
But a blackbody's surface appears black due to absorption of
all wavelength light radiations. When the blackbody surface is coated white,
the blackbody will reflect off all incident light radiations and become a poor
radiation absorber.
Question-7:
Assertion:
A hot blackbody glows in the dark.
Explanation:
The black body reflects light in the dark.
Answer:
The assertion is true. The explanation is false.
Clarification:
When heating a blackbody in thermal equilibrium conditions
to high temperature, then taken to a dark room, it glows due to the emission of
thermal electromagnetic radiation. Additionally, a perfect blackbody's
reflecting power is zero. Hence the explanation is wrong.
Video description of blackbody and its radiations:
Absorption of light radiations of suitable energies is a
characteristic of every chemical element as it depends upon various factors,
such as their composition, electron configuration, etc. But the presumption of
a hypothetical object with uneven absorbing capabilities of all radiation
wavelengths of the electromagnetic spectrum laid a new pathway to thermal
energy exchanges.
In the video, you observe that the blackbody accepts all
wavelength light radiations that fall on its surface coming from the light
source in thermal equilibrium conditions. The inserted radiation suffers a
collision with the walls of the rigid container. And the blackbody container
has a pinhole on one of its walls for radiation emission, known as a blackbody
hole. The radiation has very few chances of escaping through the blackbody hole
during its striking with the walls. But a large quantity of blackbody
radiations struggles inside with internal collisions when the blackbody is in a
cold state (not heated).
On heating, the blackbody must emit all detained radiations as heat-assimilated electromagnetic light to maintain the thermal equilibrium conditions of the enclosure. The blackbody emissions are affected only by the enclosure temperature.
(Click the image to view the video on blackbody)
Hence, the energy distribution of blackbody radiations
is non-uniform over a wide range of temperatures.
When the enclosure temperature is high, the magnitude of the
blackbody photon is also high. So, knowing the temperature of an emitted
blackbody radiation helps to measure the radiation wavelengths, aka their
energies.
PPT notes of blackbody and its radiation:
So far, we have discussed the definitions of blackbody and
its radiation. But the invention of the blackbody did not give birth to
Kirchhoff's law alone. Additionally, several theories, such as Wein
displacement law, Rayleigh-Jeans law, Planck quantum law, and Stefan-Boltzmann
law, rose to interpret the thermal emissions of earthly objects.
It expanded the applications of blackbody radiations in
lightning, heating and thermal imaging, and security fields. Electric heaters,
incandescent lights, night vision equipment, and burglar alarms are some
equipment developed using the theoretical concepts of blackbody radiations.
Here is a link to our PowerPoint presentation on blackbodies and their radiation
Our e-book:
Download a PowerPoint e-book on Kirchhoff's law from our store, "Jayam chemistry adda."
Difference between blackbody and graybody:
| Blackbody | Gray body |
|---|---|
| 1. It absorbs incident light of all wavelengths falling on its surface at a constant temperature | 1. Its surface absorptivity is independent of incident light wavelength and temperature conditions. And it absorbs some fraction of incident ligh that falls on its surface |
| 2. The absorptivity of a blackbody is one.And its reflecting and transmitting powers are zero | 2. Its absorptivity varies between 0 and 1. And it has definite reflective and transmitting powers depending upon the nature of the substance |
| 3. The energy density of radiations emitted by a blackbody is independent of its nature, size, and shape. Instead,it depends only on temperature conditions | 3. The energy density of emitted radiations by a gray body depends upon its nature,size, shape, and composition |
| 4. The blackbody's emissive power is a universal constant at each wavelength in the thermodynamic equilibrium state | 4. At particular wavelength and in thermal equilibrium conditions, the emissive power of a gray body is the product of its absorbing power and the blackbody's emissive power at that wavelength |
| 5. The blackbody radiation is isotropic in nature. The emissivity of a blackbody is one | 5. The gray body radiations are anisotropic in nature. The emissivity of the gray body varies between 0 and 1 |
Match the following on blackbody & its radiation:
| Column-A | Column-B |
|---|---|
| A. Diamond | 1. Example of a blackbody |
| B. James Clerk Maxwell | 2. Thermal electromagnetic light |
| C. Graphite | 3. Example of a gray body |
| D. Gustav Kirchhoff | 4. Light is electromagnetic |
| E. Blackbody radiation | 5. Ideal emitter of light |
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