Q67 / 90 · KVPY 2019 (SB/SX) · Physics

A certain stellar body has radius 50Rs and temperature 2Ts and is at a distance of 2×1010 AU from the earth. Here, AU refers to the earth-sun distance and Rs and Ts refer to the sun's radius and temperature, respectively. Take, both star and sun to be ideal black bodies. The ratio of the power received on earth from the stellar body as compared to that received from the sun is close to

  • A
    4×10-20
  • B
    2×10-6
  • C
    10-8
  • Correct answer
    D
    10-16

Solution

Intensity of radiation of sun on earth or solar constant, 

S1=P4πR02=4πRs2.σ.Ts44πR02

=σRs2R02.Ts4

where, Rs= radius of sun, 

Ro= radius of orbit

σ= Stefan-Boltzmann constant and Ts= temperature of sun.

Figure

Now, intensity of radiation received from steller body on earth's surface, 

S2=σ50Rs22×1010R02.2Ts4

⇒S2=2500×164×1020×σ.Rs2R02.Ts4

⇒S2=10-16S1   (from Eq. (i))

⇒S2S1=10-16

Hence, the correct answer is option: 'D'.

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