Heat Transfer By Radiation: Convection And Conduction

Heat Transfer By Radiation

Edited By Vishal kumar | Updated on Jul 02, 2025 06:13 PM IST

Have you ever felt the warmth of the sun on your skin even though you were standing in the shade? This experience is a direct result of heat transfer by radiation. Unlike conduction and convection, radiation does not need any medium to transfer heat. It's what enables us to feel the heat from the sun despite the vast vacuum of space between us.

This Story also Contains

Heat Transfer by Radiation
Prevost Theory of Heat Exchange
Solved Example Based On Heat Transfer By Radiation
Summary

Heat Transfer By Radiation

In this article, we will cover the concept of Heat Transfer By Radiation. This concept falls under the chapter Properties of Solids and Liquids which is a crucial chapter in Class 11 physics. It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination (JEE Main), National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE and more. Over the last ten years of the JEE Main exam (from 2013 to 2023), a total of one question has been asked on this concept. And no direct question in NEET from this concept.

Heat Transfer by Radiation

The process of the transfer of heat from one place to another place without any requirement of the medium is called radiation. It means that the radiation does not need any material medium to propagate.

Characteristics of Radiation

The process of the transfer of heat from one place to another place without heating the medium is called radiation.

Characteristics of the radiation are given below:

The wavelength of thermal radiation ranges from 7.8×10−7 m to 4×10−7 m. The radiation heat transfer belongs to the infrared region of the electromagnetic spectrum. That is why thermal radiations are also called infrared radiations.
Everybody whose temperature is above zero Kelvin emits thermal radiation. Practically it is not possible to reach 0 Kelvin in a finite number of steps, so every material in this universe emits radiation.
The intensity of thermal radiation is inversely proportional to the square of the distance of the point of observation from the source (I∝1d2)
As it is an electromagnetic wave, it follows laws of reflection, refraction, interference, diffraction, and polarisation.

Radiation Pressure

When these thermal radiations fall on a surface they exert pressure on that surface, which is called Radiation pressure.

Radiation spectrum is obtained by quartz or rock salt prism because these materials do not have free electrons and interatomic vibrational frequency is greater than the radiation frequency, hence they do not absorb heat radiations.
Interaction of Radiation with Matter-

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When thermal radiations (Q) fall on a body, they are partly reflected, partly absorbed and partly transmitted as shown in the below figure.

So we can write

Q=Qa+Qt+Qr or QQ=QaQ+QtQ+Qrr or 1=a+r+t

Where

QaQ=a= Absorptance QrQ=r= Reflectance QtQ=t= Transmittance

If a = t = 0 and r = 1 then body is perfect reflector
If r = t = 0 and a = 1 then the body is a perfectly black body.
If, a = r = 0 and t = 1 the body is perfect transmitter
If t=0⇒r+a=1 or a=1−r
i.e. good reflectors are bad absorbers.

Prevost Theory of Heat Exchange

Everybody emits heat radiations at all finite temperatures (Except 0 K) as well as it absorbs radiations from the surroundings.

The amount of heat emitted/absorbed depends on the nature of the body, the temperature of the body and the cross-section of the body through which heat exchange is taking place.

The exchange of energy along various bodies takes place via radiation.

How the temperature of the body will vary will depend on the temperature of the surrounding

I. If surrounding temperature= body temperature

then Qemission =Qabsorbed

i.e the body will emit and absorb at the same rate

the temperature of the body remains constant (thermal equilibrium)

II. If surrounding temperature > body temperature

then Qemission <Qabsorbed

i.e. temperature of the body increases and it appears hotter.

III. If surrounding temperature < body temperature

then Qemission >Qabsorbed

i.e. temperature of the body decreases and consequently the body appears colder.

Solved Example Based On Heat Transfer By Radiation

Example 1: The intensity of radiation at a distance d from the source is

1) proportional to d

2) Inversely proportional to d

3) Inversely proportional to d²

4) proportional to d²

Solution:

The intensity of the radiation is given by :

Iα1d2

Hence, the answer is option (3).

Example 2: Q amount of heat falls on a surface. 20% heat is absorbed and 35% is reflected, then the transmittance is:

1) 0.35

2) 0.2

3) 0.45

4) 0.55

Solution:

When thermal radiations (Q) fall on a body, they are partly reflected, partly absorbed and partly transmitted as shown in the below figure.

So we can write

Q=Qa+Qt+Qr or QQ=QaQ+QtQ+Qrr or 1=a+r+t

Where

QaQ=a= Absorptance QrQ=r= Reflectance QtQ=t= Transmittance Qa=0.20QQr=0.35QQt=Q−Qa−Qr=0.45Qt=QtQ=0.45

Hence, the answer is option (3).

Example 3: If an amount of heat falls on the surface of a metal and 80% of the heat is reflected, then absorptance is:

1) 0.8

2) 0.2

3) 0.4

4) 0.6

Solution:

Absorptance is given by :

QaQ=a
where,
Q= thermal radiation Qa=Q−Qr=Q−0.8Q=0.2Qa=QaQ=0.2

Hence, the answer is option (2).

Example 4: Which of the following is incorrect for perfectly reflecting surface

1) a = 0

2) t = 0

3) r = 1

4) a = 1

Solution:

The radiant energy per unit area per unit time-

Qincident =Qabsorbed +Qreflected +Qtransmitted Qi=Qa+Qr+Qt1=QaQi+QrQi+QtQia=QaQir= absorptive power t=QrQi= reflective power ⇒a+r+t=1
For a perfectly reflecting body: reflective power r will be 1.

Since the body reflects all the radiation falling on the body.

⇒a=t=0

For a perfectly reflecting surface,

a=0,t=0&r=1

Hence, the answer is option (4).

Example 5: Which of the following relation between absorbance, transmittance and reflectance is correct?

1) a+r+t=Q
2) a+r+t=0.8
3) a+r+t=1
4) a+r+t=100

Solution:

When Thermal Radiation(Q) falls in a body -

Q=Qa+Qt+QrQQ=QaQ+QtQ+Qrr

1 = a + r + t

- wherein

Hence, the answer is option (3).

Summary

Heat transfer by radiation is a flow of energy via electromagnetic waves, predominantly in the infrared spectrum. All objects radiate according to their temperature. The hotter, the more energy they radiate. Since this is a non-medium-dependent process, it can take place through a vacuum. That's why we feel the heat from the sun. Some of the key concepts of this chapter are blackbody radiation, emissivity, and the Stefan-Boltzmann law which quantifies just how much power an object radiates. These include the greenhouse effect thermal insulation, and periodic energy-efficient building designs. The understanding of radiation helps in tapping the potential of solar energy and improved thermal imaging techniques for better climate modelling.
FAQ's
How is heat transferred by radiation?
Heat gets transferred by radiation as a process in which energy flows by directly transferring it through electromagnetic waves, even through a vacuum.
What is a blackbody in relation to radiation?
A blackbody is an idealised object which would, crossing the boundary from outside inward, perfectly absorb all incident radiation and, going outward, perfectly emanate it. It is used for comparison with thermal radiation.
Why are colour objects hotter in the sun than light-colour objects?
Dark colours absorb more radiation and hence transfer more energy to heat, while light colours reflect more radiation and absorb less.
Can radiation take place in a vacuum?
Yes, radiation can occur in a vacuum because it doesn't require any medium; it transfers energy through electromagnetic waves.
What is emissivity?
Emissivity is the relative ability of an object to give off radiation in comparison with a perfect blackbody. It ranges from 0 to 1.

Frequently Asked Questions (FAQs)

1. What is heat transfer by radiation?

Heat transfer by radiation is the process of energy transfer through electromagnetic waves, without the need for a physical medium. Unlike conduction and convection, radiation can occur in a vacuum and doesn't require direct contact between objects.

2. Why does a thermos flask have a silvered inner surface?

A thermos flask has a silvered inner surface to minimize heat transfer by radiation. The silvered surface has a very low emissivity, which means it reflects most of the thermal radiation back into the contents of the flask, helping to maintain the temperature of the liquid inside for longer periods.

3. What is the Stefan-Boltzmann law, and how does it relate to thermal radiation?

The Stefan-Boltzmann law describes the relationship between an object's temperature and the amount of thermal radiation it emits. It states that the total energy radiated per unit surface area of a black body is directly proportional to the fourth power of its absolute temperature. This means that as an object's temperature increases, the amount of radiation it emits increases dramatically.

4. How does the emissivity of a material affect its ability to radiate heat?

Emissivity is a measure of a material's ability to emit thermal radiation compared to a perfect black body. Materials with high emissivity (close to 1) are efficient at radiating heat, while those with low emissivity (close to 0) are poor radiators. This property affects how quickly an object can cool down through radiation.

5. How does the greenhouse effect relate to heat transfer by radiation?

The greenhouse effect is a process where certain gases in Earth's atmosphere trap heat by absorbing and re-emitting infrared radiation. Solar radiation passes through the atmosphere and warms the Earth's surface. The surface then emits infrared radiation, which is partially absorbed by greenhouse gases and radiated back towards the surface, leading to additional warming.

6. How does the color of an object affect its ability to absorb or emit radiant heat?

The color of an object significantly affects its ability to absorb or emit radiant heat. Dark-colored objects generally absorb more radiation and emit more heat, while light-colored objects tend to reflect more radiation and emit less heat. This is why dark surfaces often feel hotter in sunlight compared to light surfaces.

7. How does the wavelength of thermal radiation change with temperature?

As the temperature of an object increases, the peak wavelength of its thermal radiation decreases. This relationship is described by Wien's displacement law. Hotter objects emit radiation with shorter wavelengths, while cooler objects emit radiation with longer wavelengths. This is why very hot objects can emit visible light, while cooler objects only emit infrared radiation.

8. What is a black body in the context of thermal radiation?

A black body is an idealized physical object that absorbs all electromagnetic radiation that falls on it, regardless of frequency or wavelength. It is also a perfect emitter of radiation. While no perfect black body exists in nature, some objects like a small hole in a hollow object can closely approximate black body behavior.

9. How does the distance between objects affect radiative heat transfer?

The intensity of radiative heat transfer decreases with the square of the distance between objects. This relationship is known as the inverse square law. As the distance between a heat source and an object doubles, the amount of radiant heat reaching the object decreases to one-fourth of its original value.

10. What is the role of radiation in maintaining Earth's energy balance?

Radiation plays a crucial role in maintaining Earth's energy balance. The Earth receives shortwave radiation from the Sun, absorbs some of it, and reflects the rest back to space. The absorbed energy warms the Earth, which then emits longwave infrared radiation. The balance between incoming solar radiation and outgoing terrestrial radiation determines the Earth's overall temperature.

11. What is the concept of a "heat island" in urban areas, and how does it relate to radiation?

A heat island is an urban area that is significantly warmer than its surrounding rural areas due to human activities. This phenomenon is closely related to radiation. Urban materials like concrete and asphalt absorb more solar radiation and have higher heat capacities, storing more heat during the day and releasing it slowly at night. Additionally, reduced vegetation in cities means less cooling through evapotranspiration.

12. What is the role of radiation in the formation of the ozone layer?

Radiation plays a crucial role in the formation and destruction of the ozone layer. High-energy ultraviolet radiation from the Sun breaks down oxygen molecules in the stratosphere, allowing the formation of ozone. Simultaneously, ozone absorbs ultraviolet radiation, protecting life on Earth. The balance between ozone formation and destruction is maintained through complex photochemical reactions driven by solar radiation.

13. What is the role of radiation in the process of photosynthesis?

Radiation, specifically in the form of visible light, is crucial for photosynthesis. Plants absorb light energy, primarily in the red and blue parts of the spectrum, using pigments like chlorophyll. This energy is then used to drive the chemical reactions that convert carbon dioxide and water into glucose and oxygen. The efficiency of photosynthesis depends on the intensity and spectral quality of the available light.

14. What is the role of radiation pressure in stellar evolution?

Radiation pressure plays a crucial role in stellar evolution, especially for massive stars. It results from the momentum carried by photons emitted from the star's core. In massive stars, radiation pressure provides significant support against gravitational collapse, balancing the star's enormous weight. As stars evolve and their core temperatures change, the balance between radiation pressure and gravity shifts, influencing the star's structure and fate.

15. What is the difference between heat transfer by radiation and by conduction?

Heat transfer by radiation occurs through electromagnetic waves and doesn't require a medium, while conduction involves the transfer of thermal energy through direct contact between particles. Radiation can occur in a vacuum, but conduction requires a physical medium. Additionally, radiation transfers heat much faster than conduction, especially over long distances.

16. How do infrared cameras detect heat?

Infrared cameras detect heat by sensing the infrared radiation emitted by objects. All objects above absolute zero temperature emit some level of infrared radiation. The camera's sensors detect this radiation and convert it into an electrical signal, which is then processed to create a visible image. Warmer objects appear brighter or in different colors on the camera's display.

17. What is thermal equilibrium in the context of radiative heat transfer?

Thermal equilibrium in radiative heat transfer occurs when two objects exchange the same amount of thermal radiation with each other, resulting in no net heat transfer. At equilibrium, the rate at which an object absorbs radiation equals the rate at which it emits radiation. This concept is important in understanding how objects interact thermally with their environment.

18. How does the presence of an atmosphere affect radiative heat transfer on a planet?

An atmosphere significantly affects radiative heat transfer on a planet. It can absorb, reflect, and re-emit radiation, altering the planet's energy balance. Greenhouse gases in the atmosphere absorb and re-emit infrared radiation, warming the planet's surface. The atmosphere also scatters and reflects some incoming solar radiation, reducing the amount that reaches the surface directly.

19. How does the albedo of a surface affect its interaction with radiant heat?

Albedo is a measure of a surface's reflectivity to solar radiation. Surfaces with high albedo (like snow or ice) reflect more incoming solar radiation, absorbing less heat. Surfaces with low albedo (like dark soil or asphalt) absorb more radiation and heat up more quickly. Changes in albedo can significantly affect local and global climate patterns.

20. What is the relationship between an object's temperature and the intensity of radiation it emits?

The relationship between an object's temperature and the intensity of radiation it emits is described by the Stefan-Boltzmann law. The law states that the total radiant heat energy emitted from a surface is proportional to the fourth power of its absolute temperature. This means that a small increase in temperature results in a large increase in radiant energy emission.

21. How do radiative cooling techniques work in building design?

Radiative cooling techniques in building design utilize the principle of thermal radiation to cool buildings passively. These techniques often involve using materials with high emissivity on roofs or exterior surfaces. These materials emit infrared radiation to the cold sky at night, effectively cooling the building. Some advanced materials can even achieve cooling during the day by emitting radiation in wavelengths that can pass through the atmosphere.

22. What is the role of radiation in the formation of frost on surfaces?

Radiation plays a crucial role in frost formation. On clear nights, surfaces can lose heat rapidly through radiation to the cold sky. If the surface temperature drops below the dew point and freezing point of water, frost can form. This is why frost is more likely to form on clear nights when there are no clouds to reflect the radiation back to the surface.

23. How does the concept of view factor affect radiative heat transfer between surfaces?

The view factor, also known as the configuration factor or shape factor, is a geometric quantity used in radiative heat transfer calculations. It represents the fraction of radiation leaving one surface that is intercepted by another surface. The view factor depends on the size, shape, and relative orientation of the surfaces involved. It's crucial in determining the amount of radiative heat exchange between objects, especially in complex geometries.

24. What is the difference between specular and diffuse reflection of thermal radiation?

Specular reflection occurs when radiation is reflected at a single angle equal to the angle of incidence, like a mirror. Diffuse reflection, on the other hand, scatters the incoming radiation in many directions. Most real surfaces exhibit a combination of both types of reflection. The nature of reflection affects how thermal radiation is distributed in an environment and can impact heat transfer calculations.

25. How do selective surfaces work in solar thermal applications?

Selective surfaces are designed to have high absorptivity for solar radiation (visible and near-infrared) but low emissivity for longer wavelength infrared radiation. In solar thermal applications, this property allows the surface to efficiently absorb solar energy while minimizing heat loss through radiation. These surfaces are often used in solar collectors to maximize energy capture and retention.

26. What is the concept of radiative forcing in climate science?

Radiative forcing is a measure of the difference between incoming solar radiation absorbed by the Earth and energy radiated back to space. It's expressed in watts per square meter. Positive radiative forcing tends to warm the Earth's surface, while negative forcing tends to cool it. This concept is crucial in understanding climate change, as greenhouse gases increase positive radiative forcing.

27. How does the presence of clouds affect radiative heat transfer in the atmosphere?

Clouds have a complex effect on radiative heat transfer in the atmosphere. During the day, they reflect some incoming solar radiation back to space, cooling the Earth's surface. At night, clouds act as a blanket, absorbing and re-emitting infrared radiation from the Earth's surface back downwards, reducing cooling. The net effect depends on the type, altitude, and thickness of the clouds, as well as the time of day.

28. How does the concept of optical depth relate to radiative heat transfer in the atmosphere?

Optical depth is a measure of the transparency of a medium to radiation. In the atmosphere, it quantifies how much radiation is absorbed or scattered as it travels through the air. A higher optical depth means more absorption or scattering. This concept is important in understanding how different layers of the atmosphere interact with both incoming solar radiation and outgoing terrestrial radiation.

29. What is the difference between longwave and shortwave radiation in atmospheric science?

In atmospheric science, shortwave radiation refers to the energy from the Sun, which is primarily in the visible and near-infrared parts of the spectrum. Longwave radiation, also called thermal or infrared radiation, is emitted by the Earth and its atmosphere. The atmosphere is largely transparent to shortwave radiation but absorbs much of the longwave radiation, contributing to the greenhouse effect.

30. How do aerosols in the atmosphere affect radiative heat transfer?

Aerosols, tiny particles suspended in the air, can significantly affect radiative heat transfer. They can scatter and absorb both incoming solar radiation and outgoing terrestrial radiation. Some aerosols, like sulfates, tend to have a cooling effect by reflecting sunlight back to space. Others, like black carbon, can absorb solar radiation and warm the atmosphere. The net effect depends on the type, size, and distribution of aerosols.

31. What is the concept of radiative cooling in space technology?

Radiative cooling in space technology refers to the process of dissipating heat from spacecraft or satellites by emitting thermal radiation into space. Since there's no atmosphere in space to conduct or convect heat away, radiation is the primary method of heat transfer. Space engineers design radiators and use special coatings to optimize this process, ensuring that spacecraft components don't overheat.

32. How does the phenomenon of radiative heat transfer contribute to the formation of dew?

Radiative heat transfer plays a crucial role in dew formation. On clear nights, surfaces like grass or car windshields can cool rapidly by radiating heat to the cold sky. If the surface temperature drops below the dew point of the surrounding air, water vapor in the air condenses on the surface, forming dew. This process is most effective when there are no clouds to reflect the radiation back to the surface.

33. What is the concept of radiative lifetime in atomic physics, and how does it relate to thermal radiation?

Radiative lifetime in atomic physics refers to the average time an atom or molecule remains in an excited state before emitting a photon and returning to a lower energy state. This concept is related to thermal radiation because the emission of thermal radiation involves transitions between energy states in atoms or molecules. The radiative lifetime affects the rate at which an object can emit thermal radiation.

34. How does the presence of greenhouse gases affect the Earth's radiative balance?

Greenhouse gases like carbon dioxide and methane affect the Earth's radiative balance by absorbing and re-emitting longwave radiation emitted by the Earth's surface. While they allow shortwave solar radiation to pass through relatively unimpeded, they trap some of the outgoing longwave radiation. This leads to a net warming effect, as more energy is retained in the Earth system than would be without these gases.

35. How does the concept of black body radiation apply to stars?

Stars, including our Sun, closely approximate black body radiators. The spectrum of radiation emitted by a star is similar to that of a black body at the star's surface temperature. This concept allows astronomers to estimate a star's temperature by analyzing its spectrum. Hotter stars emit more energy at shorter wavelengths, appearing bluer, while cooler stars emit more at longer wavelengths, appearing redder.

36. What is the difference between radiative and non-radiative energy transfer in molecules?

Radiative energy transfer in molecules involves the emission or absorption of a photon, resulting in a change in the molecule's energy state. Non-radiative energy transfer, on the other hand, involves energy exchange without the emission of light. This can occur through collisions with other molecules (collisional deactivation) or internal conversion processes. Both types of energy transfer are important in understanding molecular behavior and chemical reactions.

37. How does the concept of thermal radiation apply to the cosmic microwave background?

The cosmic microwave background (CMB) is thermal radiation left over from the early universe. It closely follows the spectrum of a black body with a temperature of about 2.7 Kelvin. The CMB represents the cooled remnant of the hot, dense state of the early universe, and its study provides crucial information about the universe's age, composition, and evolution.

38. How does the concept of radiative transfer apply to the study of planetary atmospheres?

Radiative transfer in planetary atmospheres describes how radiation is transmitted, absorbed, and emitted as it passes through the atmosphere. This concept is crucial for understanding planetary energy balance, atmospheric structure, and climate. It involves complex interactions between radiation and atmospheric components like gases, clouds, and aerosols. Models of radiative transfer are essential tools in planetary science and climate studies.

39. What is the relationship between an object's temperature and the peak wavelength of its thermal radiation?

The relationship between an object's temperature and the peak wavelength of its thermal radiation is described by Wien's displacement law. This law states that the wavelength at which an object emits the most intense radiation is inversely proportional to its absolute temperature. As an object's temperature increases, the peak wavelength of its emission shifts towards shorter wavelengths (higher frequencies).

40. How does radiative heat transfer contribute to the urban heat island effect?

Radiative heat transfer contributes significantly to the urban heat island effect. Urban materials like concrete and asphalt absorb more solar radiation during the day and have higher heat capacities, storing more heat. At night, these materials slowly release the stored heat through radiation. Additionally, the vertical surfaces of buildings can trap and re-radiate heat, creating "urban canyons" that retain warmth. The reduced vegetation in cities also means less cooling through evapotranspiration and shading.

41. What is the concept of radiative cooling in nighttime frost formation?

Radiative cooling plays a crucial role in nighttime frost formation. On clear nights, the Earth's surface radiates heat to the cold sky more rapidly than it receives heat from the atmosphere. This can cause the surface temperature to drop below the dew point and freezing point of water, leading to frost formation. The process is most effective when there are no clouds to reflect the radiation back to the surface, which is why frost is more common on clear nights.