Physics MCQs for NEET — Practice Questions with Answers

The energy density of an electromagnetic wave is given by the formula \( u = \frac{1}{2} (\epsilon_0 E^2 + \frac{B^2}{\mu_0}) \). The unit of energy density is Joules per cubic meter (\( Jm^{-3} \)). This is because energy density is the amount of energy stored in a given system or region of space per unit volume.

The speed of light in a vacuum is constant and does not depend on the wavelength. Hence, the ratio of the velocities of light rays of any two wavelengths in a vacuum is always 1:1.

The human eye is sensitive to light in the wavelength range of approximately 400 nm (violet) to 700 nm (red). This range is known as the visible spectrum.

In an electromagnetic wave, the direction of the cross product $ \vec{E} \times \vec{B} $ is the direction of propagation of the wave. This follows from the right-hand rule in vector mathematics.

X-rays have wavelengths in the range of 0.01 to 10 nanometers, which corresponds to 0.1 to 100 angstroms. Therefore, the wavelength of X-rays is of the order of 1 angstrom.

For a plane electromagnetic wave traveling in free space, the relationship between the electric field (E) and magnetic field (B) is given by the equation \( B = \frac{E}{c} \), where \( c \) is the speed of light (\( 3 \times 10^8 \) m/s). Given \( E = 6.3 \) V/m, we find \( B = \frac{6.3}{3 \times 10^8} = 2.1 \times 10^{-8} \) T. Since the wave is traveling along the x-direction and \( \vec{E} \) is along the y-direction, \( \vec{B} \) must be along the z-direction (\( \hat{k} \)). Thus, \( \vec{B} = 2.1 \times 10^{-8} \hat{k} \) T.

The waves used in communication are typically radio waves. These waves have the appropriate wavelength and frequency to travel long distances and penetrate through obstacles, making them ideal for communication purposes. $ ext{Radio waves}$ are used in various communication technologies like television, radio broadcasting, and cell phones.