KEAM2007 Questions on Electromagnetic Waves

Questions on electromagnetic waves are simple at the 12^th class level and you can easily score full marks in this section. Here are the three questions which appeared in the Kerala Medical (Question No.1) and Engineering (Question Nos.2 & 3) Entrance 2007 question papers:

(1) An electromagnetic radiation has energy of 13.2 keV. Then the radiation belongs to the region of

(a) visible light (b) ultraviolet (c) infrared

(d) X-ray (e) microwave

If an electron accelerated by a potential difference of 13.2 kilo volt strikes a target, the maximum energy of the photon generated will be 13.2 kilo electron volt. You know that an accelerating voltage of 13.2 kV in an X-ray tube can produce X-rays. So, the correct option is (d).

If the energy given were in the electron volt (eV) range, you will be in confusion. To be fool proof, it will be better to calculate the wave length of the radiation, noting that the product of the wave length in angstrom (Ǻ) and the energy in electron volt (in the case of photons) is approximately 12400. The wave length of the photon in the present case is therefore given by

λ = (12400/13200) Ǻ = 0.94 Ǻ, which is in the X-ray region.

(2) The dielectric constant of air is 1.006. The speed of electromagnetic wave traveling in air is a×10⁸ ms^–1 where ‘a’ is about

(a) 3 (b) 3.88 (c) 2.5 (d) 3.2 (e) 2.8

This is an unusually simple question which the question setter might have deliberately included! You know that the speed of electromagnetic waves in air is almost equal to that in free space and hence the answer is 3 [Option (a)].

Generally, in any medium, the speed (v) of electromagnetic waves is given by

v = c/√(μ_rε_r) where c is the speed in free space (which is 3×10⁸ ms^–1), μ_r is the relative permeability and ε_r is the relative permittivity (dielectric constant) of the medium.

(3) a. The wave length of microwaves is greater than that of UV-rays

b. The wave length of IR rays is less than that of UV-rays

c. The wave length of microwaves is less than that of IR rays

d. Gamma ray has the shortest wave length in the electromagnetic spectrum

Of the above statements

(A) a and b are true (B) b and c are true

(C) c and d are true (D) a and c are true

(E) a and d are true

This too is a simple knowledge based question and the correct option is (E).