Below is a concise list of key definitions from Chapter 9: Physical Optics 

Below is a concise list of key definitions from Chapter 9: Physical Optics of the 11th Class Physics syllabus for the Federal Board of Intermediate and Secondary Education (FBISE), aligned with the National Book Foundation (NBF) curriculum. These definitions cover the essential concepts related to physical optics as typically presented in this chapter.

1. Physical Optics: The branch of optics that studies the wave nature of light, including phenomena like interference, diffraction, and polarization.
2. Wavefront: A surface joining all points of a wave that are in the same phase of oscillation (e.g., crests or troughs).
3. Ray: A line perpendicular to a wavefront, indicating the direction of wave propagation.
4. Huygens’ Principle: Every point on a wavefront acts as a source of secondary spherical wavelets, and the new wavefront is the tangent to these wavelets.
5. Interference: The superposition of two or more light waves, resulting in regions of increased or decreased intensity.
6. Constructive Interference: The superposition of light waves where crests align, producing a region of higher intensity (bright fringe).
7. Destructive Interference: The superposition of light waves where a crest aligns with a trough, producing a region of lower or zero intensity (dark fringe).
8. Coherent Sources: Light sources that emit waves with a constant phase difference and the same frequency.
9. Young’s Double-Slit Experiment: An experiment demonstrating the interference of light, producing alternating bright and dark fringes on a screen due to light passing through two slits.
10. Fringe Spacing (Δy): The distance between consecutive bright or dark fringes in an interference pattern, given by Δy = λL/d, where λ is wavelength, L is the distance to the screen, and d is slit separation.
11. Diffraction: The bending or spreading of light waves around the edges of an obstacle or through an aperture.
12. Diffraction Grating: An optical device with a large number of equally spaced slits that disperses light into its component wavelengths, producing a spectrum.
13. Grating Element (d): The distance between consecutive slits in a diffraction grating.
14. Diffraction Grating Equation: The condition for constructive interference in a diffraction grating, given by d sinθ = nλ, where d is the grating element, θ is the angle of diffraction, n is the order, and λ is the wavelength.
15. Polarization: The restriction of light wave oscillations to a single plane, resulting in polarized light.
16. Plane-Polarized Light: Light in which the electric field oscillates in a single plane perpendicular to the direction of propagation.
17. Polarizer: A device that filters light to produce plane-polarized light by allowing only waves oscillating in a specific direction to pass.
18. Malus’ Law: The intensity of polarized light passing through a polarizer is proportional to the square of the cosine of the angle between the polarizer’s axis and the light’s polarization direction, given by I = I₀ cos²θ.
19. Brewster’s Angle: The angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent medium with no reflection, given by tanθ_B = n₂/n₁, where n₁ and n₂ are refractive indices.
20. Optical Path Difference: The difference in the distance traveled by two light waves, expressed in terms of wavelength, affecting interference patterns.
21. Thin Film Interference: The interference of light waves reflected from the top and bottom surfaces of a thin film, producing colorful patterns (e.g., soap bubbles).

These definitions encapsulate the core concepts of physical optics from Chapter 9 of the FBISE 11th Class Physics curriculum. If you need further explanation or additional terms, please let me know!