Reflectivity is defined as
It is total energy reflected by the body. Its value depends upon the nature of the surface of the body, its temperature and wavelength of incident radiations.
Of the radiant energy 350W/m2 incident upon a surface 250W/m2 is absorbed, 60W/m2 is reflected and the remainder is transmitted through the surface. Workout the value for reflectivity for the surface material
Reflectivity = fraction of total energy reflected by the body = 60/350 = 0.171.
A thin metal plate of 4 cm diameter is suspended in atmospheric air whose temperature is 290 K. This plate attains a temperature of 295 K when one of its face receives radiant energy from a heat source at the rate of 2 W. If heat transfer coefficient on both surfaces of the plate is stated to be 87.5 W/m 2 K, workout the energy lost by reflection
Heat loss by convection from both sides of the plates = 2 h A d t = 1.1 W. Energy lost by reflection = 2.0 – 1.1 = 0.9 W.
The value of reflectivity depends upon
(i) Body temperature
(ii) Wavelength of reflected radiation
(iii) Nature of surface of body
Identify the correct statement
It should be wavelength of incident radiation. It is the fraction of total energy reflected by the body.
When a surface absorbs a certain fixed percentage of striking radiation the surface is called
It is absorbing a certain fixed amount of radiations.
Which one is true regarding gases?
Gases are known to reflect very little of the radiation incident on their surface.
Thin glass plate is an example of
For transparent body, transmissivity is zero.
Radiant energy with an intensity of 800 W/m2 strikes a flat plate normally. The absorptivity is thrice the reflectivity and twice the transmissivity. Determine the rate of reflection
Rate of reflection = pQ0 = 218.20 W/m2.
The total radiant energy A impinging upon a body would be partially or totally absorbed, reflected from its surface or transmitted through it. Identify the correct statement
A = B + C + D.
Which one is true for opaque body?
For opaque body, sum of absorptivity and reflectivity is unity.