EM Spectrum — NEET Physics PYQ MCQs with Solutions

Solution: $\gamma$-rays have the shortest wavelength because they have higher frequency than UV-rays, X-rays and microwaves.

Solution: $E = h\nu = \dfrac{hc}{\lambda} \Rightarrow \lambda = \dfrac{hc}{E} = \dfrac{6.63\times10^{-34}\times3\times10^{8}}{15\times10^{3}\times1.6\times10^{-19}} \approx 0.828\times10^{-10}\,\text{m} = 0.828\,\text{\AA}$. This wavelength lies in the X-ray region.

Solution: It is an electromagnetic wave. The frequency of the microwave oven must match the resonant frequency of the water molecules so that the water molecules oscillate and large heat is developed.

Solution: The decreasing order of wavelength of various rays is Microwave $>$ Infrared $>$ Ultraviolet $>$ Gamma rays.

Solution: The velocity of electromagnetic waves in free space is given by $c = \dfrac{1}{\sqrt{\varepsilon_0\mu_0}}$.

Solution: Wavelength ranges: visible light $4000$–$7000\,\text{\AA}$, X-rays $10^{-3}$–$10^{2}\,\text{\AA}$, microwaves $10^{7}$–$10^{9}\,\text{\AA}$. So, $\lambda_x < \lambda_v < \lambda_m$, i.e. $\lambda_m > \lambda_v > \lambda_x$.

Solution: $\beta$-rays are fast moving electrons, so they are not electromagnetic waves. Heat rays, X-rays and $\gamma$-rays are all electromagnetic waves.

Solution: Wavelength ranges: X-rays $0.001$–$100\,\text{\AA}$, ultraviolet rays $10$–$4000\,\text{\AA}$, $\gamma$-rays $0.0001$–$1\,\text{\AA}$, cosmic rays upto $4\times10^{-3}\,\text{\AA}$. Thus, cosmic rays have the minimum wavelength.

Solution: A green house's glass transmits short-wavelength infrared radiation and visible light in, but absorbs the long-wavelength infrared radiation emitted back by the plants at night, trapping heat inside. This is called the green house effect.

Solution: Frequency ranges: $\gamma$-rays $3\times10^{18}$ to $3\times10^{21}\,\text{Hz}$, X-rays $3\times10^{16}$ to $3\times10^{18}\,\text{Hz}$, ultraviolet radiation $7.5\times10^{14}$ to $3\times10^{16}\,\text{Hz}$. Thus, the correct order is $a > b > c$.

Solution: The ozone layer extends from about 30 km to 50 km above the earth's surface and absorbs the major part of ultraviolet radiations coming from the sun. The range of ultraviolet radiation is $100$–$4000\,\text{\AA}$, so it blocks radiation of wavelength less than $3\times10^{-7}\,\text{m}$ (3000 \AA).

Solution: Space communication refers to sending, receiving and processing information through space, which occurs via ground (surface) wave propagation and sky wave propagation (among others), so a signal can reach another point as either a sky wave or a ground wave.

Solution: Due to their high penetrating power, X-rays are used for investigation of the structure of solids. The Laue spot method and rotating crystal method are used for this purpose: X-rays fall on the solid under investigation and their structure is recorded on a photographic plate.

Solution: To observe a particle, the wavelength used should be of the order of the particle's size: $\lambda \approx r = 3\times10^{-4}\,\text{cm} = 3\times10^{-6}\,\text{m}$. Frequency $\nu = \dfrac{c}{\lambda} = \dfrac{3\times10^{8}}{3\times10^{-6}} = 10^{14}\,\text{Hz}$.

Solution: Gamma rays have wavelength range $6\times10^{-14}\,\text{m}$ to $1\times10^{-10}\,\text{m}$, X-rays have wavelength range $1\times10^{-13}\,\text{m}$ to $3\times10^{-8}\,\text{m}$. Blue light and red light lie in the visible range, which extends from $4000\,\text{\AA}$ to $7800\,\text{\AA}$. Hence, red light has the longest wavelength.

Solution: Wavelength ranges: Gamma rays $6\times10^{-14}$ to $1\times10^{-10}\,\text{m}$, X-rays $1\times10^{-13}$ to $3\times10^{-8}\,\text{m}$, microwaves $10^{-3}$ to $0.3\,\text{m}$, radio waves greater than $0.1\,\text{m}$. So, radiowaves are the longest waves.