NCERT

For mirrors and lenses, the Cartesian sign convention is used. For lenses: For convex lenses, the focal length is positive and for concave lenses, it is negative. Distances to the right of the optical center are positive.
For mirrors: For concave mirrors, the focal length is positive and for convex, it is negative. The distances to the right of the optical center are positive. The sign convention allows for consistent calculations for formulas like mirror and lens formulas, and for ray diagrams.

Diffraction leads to the formation of patterns of varying intensity. When around obstacles, waves bend and spread through the narrow opening, it is called diffraction. The interference results in a new wave pattern and involves the superposition of two or more coherent waves. Both these phenomena produce patterns of light and dark regions; the interference results from the combination of multiple waves and the diffraction arises from a single wave interacting with an aperture or obstacle. When the size of the aperture or obstacle is comparable to the wavelength of the wave, diffraction patterns are typically observed.

A coherent light source in Young's Double Slit Experiment illuminates two closely spaced slits, and produces two overlapping light waves. The interference of these waves constructively or destructively based on their phase difference lead to a pattern of bright and dark fringes on a screen, When the path difference is an integral multiple of the wavelength, it is constructive interference (bright fringes) and when the path difference is an odd multiple of half the wavelength, it is destructive interference (dark fringes). Through observable interference patterns, Young's Double Slit Experiment, shows the wave nature of light.

The presesnt day theory for structure of Atom is developed through many discoveries and hypothesises. In class 12 Physics, Atom chapter includes development of the sturucture of atom, and theories in the path of development of present theory. Students can check the ordered points below;

1. Thomson's Model of the Atom
2. Rutherford's Nuclear Model
3. Bohr's Model of the Hydrogen Atom
4. De Broglie's Hypothesis
5. Energy Emission Spectrum

These throries have been used to introduced the current theory of structure of atom.

Students who are preparing for the class 12 board exams needs additional practice questions after completing the NCERT Exercises. We have provided practice questions with accurate and atep-by-step solutions for students to better prepare for the board exams. Students can check the below provided link to access our additional practice questions along with previous year questions.

Chapter 12 Atoms NCERT Solutions 

Electromagnetic waves do not require a medium to propagate because they generate self-sustaining oscillations of magnetic and electric fields. Hence, it can travel through the vacuum of space. A changing magnetic field generates an electric field and vice versa. This property allows the wave to carry energy across space. The phenomenon is applicable to sunlight and other forms of EM radiation that travel through space to reach Earth. They cover millions of kilometers while reaching Earth. The EM wave theory is fundamental to understanding phenomena like cosmic observations, satellite communication, and solar radiation.

The entire range of electromagnetic radiation, arranged to increase and decrease the wavelength is called the electromagnetic (EM) spectrum. In the following order, it includes - radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. All these regions have unique properties and applications. For example, X-rays used in medical imaging,  visible light in everyday vision, infrared in remote controls and thermal imaging and radio waves are used in communication. This is an important concept that helps in classifying and utilizing different EM waves.

The photoelectric effect when light is projected in a metal surface, it's surface ejects electron (which are in outermost shell or free electron). This happens because the light provides additional energy to the electron to detach itself from metal surface. Well to do this light must have a certain amount of energy which in other words can be said the right frequency of light is required for photoelectric effect. 

This frequency must be above a certain minimum value (called threshold frequency) fixed for every metal, regardless of its intensity. Photoelectric effect cannot be explained by the wave theory of light.