Question

Which of the following statements is not correct? [AFMC 1999]
A. Infra-red photons have more energy than photons of visible light
B. Infra-red rays are invisible but can cast shadows like visible light
C. Photographic plates are sensitive to ultraviolet rays
D. Photographic plates are sensitive to infra-red rays.

Answer 1

The photon is the qualitative unit of energy of the light wave. It is proportional to the frequency of the light wave. So, to compare the energy of different electromagnetic waves, we need to compare the frequencies of the given waves.

Formula used:
$E = \dfrac{{hc}}{\lambda }$
where E is the energy of the light (electromagnetic wave), h is the Plank’s constant, c is the speed of light and $\lambda$ is the wavelength of the light.

Complete step by step solution:
Light spectrum is the combination of electromagnetic waves of many different wavelengths of energy produced by the light source. As the frequency of the electromagnetic wave is the most characteristic property so the spectrum is characterised based on the range of the frequencies.

Electromagnetic rays	Range of frequency
$\gamma - rays$	${10^{20}} - {10^{24}}Hz$
X-rays	${10^{17}} - {10^{20}}Hz$
Ultraviolet rays	${10^{15}} - {10^{17}}Hz$
Visible spectrum	${10^{14.5}} - {10^{15}}Hz$
Infra-red	${10^{11}} - {10^{14.5}}Hz$
Microwave	${10^9} - {10^{11}}Hz$
FM radio wave	${10^6} - {10^9}Hz$
AM radio wave	${10^5} - {10^6}Hz$
Long radio wave	${10^0} - {10^5}Hz$

As the energy of the photon is directly proportional to the frequency of the wave so the energy of the visible light is more than that of infrared photons.
$E \propto \nu$
${\nu _{visible}} > {\nu _{IR}}$
So, the energy of the photons of visible light is more than those of infrared ray’s photon. Hence, the given statement that infrared photons have more energy than photons of visible light is incorrect. Infra-red rays are invisible rays which the human eye can’t detect but when an opaque object comes in the path of the infra-red rays then the absence of electromagnetic rays produces the shadow. Hence, the given statement is correct.

The silver salt used in the photographic plate is a light-sensitive plate. So it is sensitive to the ultraviolet rays which are a part of the rays coming out of the sun. Hence, the statement is correct. The silver salt used in the photographic plate is a light-sensitive plate. So it is sensitive to ultraviolet rays and infrared rays as well. Hence, the statement is correct.

Therefore, the correct option is A.

Note: While referring to the experimental values of the frequencies of different waves from electromagnetic spectra, we should be careful about the frequencies because there is a range of frequencies in which a particular wave fits into.