Physics · Thermal Properties Of Matter · NEET
No. This is the key difference. Thermal radiation is carried by electromagnetic waves, so it travels through vacuum. That is why heat from the Sun reaches Earth across empty space. Conduction and convection both need a material medium and cannot work in vacuum.
Between the Sun and Earth there is mostly vacuum. Conduction passes energy from one particle to the next, so it needs particles in contact. In vacuum there are no such particles, so conduction stops. Radiation is electromagnetic waves that move on their own, so it crosses the empty space and warms us.
They are closely linked but not exactly the same. A body near room temperature radiates mostly in the infrared band, so we often call it heat waves. But thermal radiation actually has a continuous spectrum of wavelengths. A very hot body (like the Sun or a bulb filament) radiates visible light too, not just infrared.
Thermal radiation travels at the speed of light, about 3 x 10^8 m/s, because it is an electromagnetic wave. Conduction is much slower because energy has to pass step by step from one vibrating particle to the next. This is why you feel the Sun's warmth instantly but a metal spoon in hot tea heats up only gradually.
No. Every body above absolute zero (0 K) emits thermal radiation all the time. Hotter bodies radiate much more, and the peak shifts to shorter wavelengths. At room temperature bodies still radiate, but the wavelength is in the infrared, so we cannot see it with our eyes.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is heat energy that moves as electromagnetic waves, mainly infrared. It leaves a hot body and travels outward like light, without needing any material to carry it.
Radiation. Both conduction and convection need a material medium, but radiation travels as electromagnetic waves and can cross empty space.
Feeling the warmth of the Sun or of a nearby fire. In both cases heat reaches you without warm air or solid contact carrying it directly.
The flask walls are silvered and shiny. Shiny surfaces reflect radiation back, so the hot contents lose very little heat by radiation. The vacuum between the walls stops conduction and convection.
Yes. The total power radiated grows with the fourth power of absolute temperature (Stefan-Boltzmann law), and the peak wavelength shifts with temperature (Wien's law). These are studied next in black body radiation.
