Complete Physics Notes on Basic Concept of Light  for BPSC and Other Competitive Exams in 2025

Complete Physics Notes on Basic Concept of Light  for BPSC and Other Competitive Exams in 2025

Physics-2

Basic Concept of Light

Light is a form of energy that enables us to see. It travels in straight lines and exhibits both wave-like and particle-like properties.

1. Reflection of Light

• Concept: Reflection is the bouncing back of light when it strikes a surface.
• Types of Reflection:
  • Regular/Specular Reflection: Occurs when light strikes a very smooth surface (like a mirror), resulting in a clear, sharp image. Parallel rays of light remain parallel after reflection.
  • Irregular/Diffuse Reflection: Occurs when light strikes a rough or uneven surface (like a wall or paper), scattering the light in various directions. This is why we can see non-luminous objects from all angles.
• Laws of Reflection:

• Angle of Incidence = Angle of Reflection (∠i=∠r): The angle between the incident ray and the normal (a line perpendicular to the surface at the point of incidence) is equal to the angle between the reflected ray and the normal.
• Incident ray, reflected ray, and the normal all lie in the same plane.
• Mirrors:
  • Plane Mirror: Forms a virtual, erect, and laterally inverted image of the same size as the object, located as far behind the mirror as the object is in front.
  • Spherical Mirrors:
    • Concave Mirror (Converging Mirror): Reflects light inward to a focal point. Used in shaving mirrors, headlights, solar furnaces, and by dentists. Can form both real and virtual images, depending on object position.
    • Convex Mirror (Diverging Mirror): Reflects light outward. Always forms virtual, erect, and diminished images. Used as rearview mirrors in vehicles (due to a wider field of view) and as security mirrors in shops.

2. Refraction of Light

• Concept: Refraction is the bending of light as it passes from one transparent medium to another (e.g., from air to water or glass), due to a change in its speed.
• Laws of Refraction (Snell’s Law):
  1. Incident ray, refracted ray, and the normal all lie in the same plane.
  2. Snell’s Law: The ratio of the sine of the angle of incidence (sini) to the sine of the angle of refraction (sinr) is constant for a given pair of media and a given color of light. This constant is called the refractive index (n).
     • n=sinrsini​=speed of light in medium, speed of light in vacuum​
     • Higher refractive index means light bends more towards the normal and travels slower in that medium.
• Effects of Refraction:

• A stick appearing bent in water.
• The apparent depth of a swimming pool appearing shallower than its real depth.
• Twinkling of stars (due to atmospheric refraction).
• Formation of rainbows (due to dispersion and refraction in water droplets).
• Mirage (due to varying refractive index of air layers).
• Total Internal Reflection (TIR):

• Concept: When light travels from a denser medium to a rarer medium, and the angle of incidence in the denser medium exceeds a certain value called the critical angle, the light is reflected back into the denser medium instead of being refracted.
• Conditions for TIR:

• Light must travel from a denser medium to a rarer medium.
• The angle of incidence must be greater than the critical angle.
• Applications: Optical fibers (used in telecommunications and endoscopy), sparkling of diamonds, periscopes, binoculars, right-angled prisms.

3. Optics (Lenses and Vision Defects)

• Lenses: Transparent materials that refract light to form images.
  • Convex Lens (Converging Lens): Thicker in the middle, thinner at the edges. Converges parallel rays of light to a focal point. Used in magnifying glasses, cameras, telescopes, and to correct hypermetropia (far sightedness). Can form both real and virtual images.
  • Concave Lens (Diverging Lens): Thinner in the middle, thicker at the edges. Diverges parallel rays of light. Always forms virtual, erect, and diminished images. Used in peepholes, some telescopes, and to correct myopia (near sightedness).
• Power of a Lens (P): The ability of a lens to converge or diverge light rays.
  • Formula: P=f1​ (where f is the focal length in meters).
  • Unit: Dioptre (D).
  • Convex lens: Positive power.
  • Concave lens: Negative power.
• Dispersion of Light: The phenomenon of splitting white light into its constituent colors (VIBGYOR – Violet, Indigo, Blue, Green, Yellow, Orange, Red) when it passes through a prism or water droplets. This occurs because different colors of light travel at slightly different speeds in a medium, thus having different refractive indices.
• Scattering of Light: The phenomenon where light rays deviate from their straight path when they strike an obstacle (like dust particles or gas molecules).
  • Rayleigh Scattering: Explains why the sky appears blue (blue light scatters more) and why the sun appears red at sunrise/sunset (red light scatters least and travels further).
• Human Eye and Vision Defects:
  • Myopia (near-sightedness): Distant objects appear blurry. Light converges in front of the retina. Corrected using a concave lens.
  • Hypermetropia (Farsightedness): Near objects appear blurry. Light converges behind the retina. Corrected using a convex lens.
  • Presbyopia: Age-related vision defect where the eye loses its ability to focus on near objects. Corrected using bifocal lenses (or progressive lenses).
  • Astigmatism: Blurred vision at all distances due to an uneven curvature of the cornea or lens. Corrected using cylindrical lenses.