MCQ
Three light sources $A, B$ and $C$ emit equal amount of radiant energy per unit time. The wavelengths emitted by the three sources are $450 \mathrm{~nm}, 555 \mathrm{~nm}$ and $700 nm$ respectively. The brightness sensed by an eye for thesources are $X_A X_B$ and $X_C$ respectively. Then,
  • A
    $X_A>X_B, X_C>X_B$
  • B
    $\mathrm{X}_{\mathrm{A}}>\mathrm{X}_{\mathrm{B}}, \mathrm{X}_{\mathrm{B}}>\mathrm{X}_{\mathrm{C}}$
  • $X_B>X_A, X_B>X_C$
  • D
    $X_B>X_A, X_C>X_B$

Answer

Correct option: C.
$X_B>X_A, X_B>X_C$
Wavelength of light $B$ is $555\  nm$ . It has the highest luminosity; hence, $X_B$ will be highest.
Again, $450\  nm$ is nearer to $555\  nm$ than $700\  nm $.
$\therefore 555-450=105$
But $700-555=145$
So, $X_A$ 's brightness will be greater than that of $X_C$.

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