Physics MCQs for NEET — Practice Questions with Answers

The radiation pressure exerted by an electromagnetic wave on a perfectly reflecting surface is given by $ P = rac{2I}{c} $, where $ I $ is the intensity of the wave and $ c $ is the speed of light. Given that $ I = 10 ext{ W/m}^2 $ and the area $ A = 20 ext{ cm}^2 = 20 imes 10^{-4} ext{ m}^2 $, the force $ F $ can be calculated as $ F = PA = rac{2IA}{c} $. Substituting the given values, we get $ F = rac{2 imes 10 imes 20 imes 10^{-4}}{3 imes 10^8} ext{ N} ightarrow F = 1.33 imes 10^{-10} ext{ N} $. Thus, the correct option is $ 1.33 imes 10^{-10} ext{ N} $.

The intensity $ I $ of light at a distance $ r $ from an isotropic point source is given by $ I = rac{P}{4 ext{π}r^2} $, where $ P $ is the power of the source. Given $ P = 40 ext{ W} $ and $ r = 2 ext{ m} $, we have $ I = rac{40}{4 ext{π}(2)^2} = rac{40}{16 ext{π}} = rac{10}{4 ext{π}} ext{ W/m}^2 $. The rms value of the electric field $ E_{ ext{rms}} $ is related to the intensity by $ I = rac{1}{2} c ext{ε}_0 E_{ ext{rms}}^2 $, where $ c $ is the speed of light and $ ext{ε}_0 $ is the permittivity of free space. Rearranging and solving for $ E_{ ext{rms}} $, we get $ E_{ ext{rms}} = igg(rac{2I}{c ext{ε}_0}igg)^{1/2} $. Substituting the values, $ E_{ ext{rms}} = igg(rac{2 imes rac{10}{4 ext{π}}}{3 imes 10^8 imes 8.854 imes 10^{-12}}igg)^{1/2} ightarrow E_{ ext{rms}} ightarrow 17.3 ext{ V/m} $. Thus, the correct option is $ 17.3 ext{ V/m} $.

Gamma rays are a form of electromagnetic radiation with high energy and short wavelength. They are used in medicine to destroy cancer cells because they can penetrate deep into tissues and cause ionization, which damages the DNA of cancer cells, leading to their destruction. Thus, the correct option is gamma rays.

The given equation $\oint \vec{E} \cdot d\vec{t} = -\frac{d\Phi_B}{dt}$ is Faraday's law of electromagnetic induction. This law states that a changing magnetic field within a closed loop induces an electromotive force (EMF) in the loop. The negative sign indicates the direction of the induced EMF, which opposes the change in magnetic flux, as per Lenz's law.

In an electromagnetic wave, both the electric field (\(\vec{E}\)) and the magnetic field (\(\vec{B}\)) oscillate perpendicular to each other and to the direction of wave propagation. This is a fundamental characteristic of electromagnetic waves.

The average total energy density (u) of an electromagnetic wave is given by the equation: \[ u = \frac{1}{2} \epsilon_0 E_{rms}^2 \] where \( \epsilon_0 \) is the permittivity of free space (\( 8.854 \times 10^{-12} \) F/m) and \( E_{rms} \) is the rms value of the electric field. Given \( E_{rms} = 720 \) N/C: \[ u = \frac{1}{2} \times 8.854 \times 10^{-12} \times (720)^2 \approx 4.58 \times 10^{-6} \text{ J/m}^3 \] Therefore, the correct option is \( 4.58 \times 10^{-6} \text{ J/m}^3 \).

The wavelength range of visible light lies approximately between 4000 \( A^\circ \) (400 nm) and 7000 \( A^\circ \) (700 nm). This range corresponds to the spectrum of light that is visible to the human eye. Therefore, the correct option is \( 4000 \text{ A}^\circ \text{ to } 7000 \text{ A}^\circ \).