# Heat Transfer Questions and Answers – Reflectivity

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This set of Heat Transfer Multiple Choice Questions & Answers (MCQs) focuses on “Reflectivity”.

1. Reflectivity is defined as
a) Fraction of total energy transmitted by the body
b) Fraction of energy reflected by the body
c) Fraction of total energy absorbed by the body
d) Fraction of total energy absorbed and radiated by the body

Explanation: 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.

2. 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
a) 0.181
b) 0.171
c) 0.161
d) 0.151

Explanation: Reflectivity = fraction of total energy reflected by the body = 60/350 = 0.171.

3. 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
a) 0.8 W
b) 0.3 W
c) 0.9 W
d) 0.10 W

Explanation: 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.
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4. The value of reflectivity depends upon
(i) Body temperature
(iii) Nature of surface of body
Identify the correct statement
a) i and ii
b) ii and iii
c) ii only
d) i and iii

Explanation: It should be wavelength of incident radiation. It is the fraction of total energy reflected by the body.

5. When a surface absorbs a certain fixed percentage of striking radiation the surface is called
a) Grey body
b) Black body
c) White body
d) Opaque body

Explanation: It is absorbing a certain fixed amount of radiations.
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6. Which one is true regarding gases?
a) Reflectivity is unity
b) Absorptivity is zero
c) Reflectivity is zero
d) Transmissivity is zero

Explanation: Gases are known to reflect very little of the radiation incident on their surface.

7. Thin glass plate is an example of
a) Transparent body
b) Opaque body
c) Black body
d) White body

Explanation: For a transparent body, transmissivity is zero.

8. 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
a) 518.20 W/m2
b) 418.20 W/m2
c) 318.20 W/m2
d) 218.20 W/m2

Explanation: Rate of reflection = p Q 0 = 218.20 W/m2.

9. 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 is the energy that is absorbed by the body
b) B is the energy that is reflected by the body
c) C is the energy that is absorbed by the body
d) D is the energy that is transmitted by the body

Explanation: A = B + C + D.

10. Which one is true for an opaque body?
a) Transmissivity is zero
b) Reflectivity is zero
c) Absorptivity is zero
d) Reflectivity is unity

Explanation: For the opaque body, sum of absorptivity and reflectivity is unity.

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