NCERT

Young's Double-Slit Experiment demonstrates the interference phenomenon. It provides strong proof for the wave nature of light. This experiment shows how due to constructive and destructive interference, two coherent light sources create a pattern of bright and dark fringes.

The image is real when it is formed by the actual convergence of rays and it can be projected on a screen. For example, when the object is placed beyond focus, and the image is formed either by the convex lens or concave mirror.
The image is virtual, if it is formed by the apparent divergence of rays and it cannot be captured on a screen. For example, when the image is formed by the convex mirror or plane mirror.

Simple microscope: When the object is placed within the focal length, the simple microscope works on the principle of producing an erect, virtual and magnified image. To magnify small objects, the simple microscope uses a single convex lens.
Compound microscope: It uses two lenses including an eyepiece and objective. The objective lens creates a real, magnified, and inverted image which acts as the object for the eyepiece, which in turn magnifies it further.

In Ray Optics And Optical Instruments questions, when using lens and mirror formulas, sign convention helps in maintaining consistency. The sign convention determines whether quantities like image distance, object distance, and focal length are taken as positive or negative. The incorrect usage of these sign conventions mostly leads to wrong magnifications and image positions. For accurate results, it is essential to follow the Cartesian sign.

The EM wave speed in a vacuum is determined by the permeability and permittivity of free space. It is given by the formula:

$c=\frac{1}{\sqrt{?0_{}?0_{}}}$

The constant value is around $3*{10}^8\text{ m/s.}$

This speed forms the basis for many scientific and technological applications. It is a fundamental physical constant.

The electric field (E) and magnetic field (B) in an electromagnetic wave are always perpendicular to each other and to the direction of wave propagation. The EM waves are transverse in nature. When the EM waves are moving along the X-axis, then along the y-axis, the electric field may oscillate and the magnetic field along the z-axis.

Maxwell introduced the displacement current which ensures that when the conduction current is absent, the current continues in the circuits, such as in the capacitors. The displacement current plays a significant role in the derivation of electromagnetic waves. It shows that an electric field that is changing can produce a magnetic field. In free space, it enables wave propagation.

The capacitive reactance is equal to the inductive reactance at resonance:

$X_L=X_C??=\frac{1}{\sqrt{L?C}}$

In an AC circuit, the power factor cos? measures the efficiency of power usage. Between the current and voltage, it is the cosine of the phase angle. A power factor of 1 means all the supplied power is used effectively and a low power factor means more energy is wasted.

Average power:

$P=V_{\text{rms}}\cdot I_{\text{rms}}\cdot cos??$