Three light sources A, B and C emit equal amount of radiant energy per unit time. The wavelengths emitted by the three source are 450 nm, 555 nm and 700 nm respectively. The brightness sensed by an eye for the sources are X_A, X_B and X_C respectively. Then,
(a) X_A > X_B, X_C> X_B
(b) X_A > X_B, X_B > X_C
(c) X_B > X_A, X_B> X_C
(d) X_B > X_A, X_C> X_B

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Solution

Correct option (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.
∴ 555 - 450 = 105
But 700 - 555 = 145
So, X_A's brightness will be greater than that of X_C_.

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