Optics Notes with PDF

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1. The Electromagnetic Spectrum & Light Ranges

• Solar Radiation Reaching Earth:
  • Wavelength range: Approximately 100 nm to 10^6 nm.
• Ultraviolet (UV) Radiation: Wavelengths below 400 nm.
  • UV-A: 320-400 nm (causes tanning).
  • UV-B: 280-320 nm (can lead to skin cancer).
  • UV-C: Below 280 nm (most dangerous, but largely absorbed by atmospheric oxygen).
• Visible Light:
  • Wavelength range: Approximately 400-700 nm (or 380-750 nm, 390-780 nm, 3900-7600 Å).
  • Human eyes can typically perceive light from about 380 nm to 750 nm.
  • The human eye is most sensitive to yellowish-green light (~555 nm or 530-570 nm).
    • Incorrect statement: “The human eye is most sensitive to red colour.”
• Infrared (IR) Radiation: Wavelengths above 700 nm.

2. Fundamental Properties of Light

• Nature of Light: Exhibits wave-particle duality (behaves like both a wave and a particle).
• Photon Energy:
  • Inversely proportional to its wavelength.
  • Shorter wavelength = Higher energy.
  • Among given options, 200-280 nm radiation has the highest energy per quantum.
• Speed (Velocity) of Light:
  • In a vacuum: 3 × 10^8 m/s (maximum possible speed for energy/information).
  • In different media: Light travels slower than in a vacuum. Speed is minimum in glass.
  • Speed in air depends on temperature, pressure, and humidity.
  • Sunlight takes about 8 to 8.5 minutes to travel from the Sun to Earth.
• Rectilinear Propagation: Light travels in a straight line because its wavelength is very small.

3. Behavior of Light: Reflection, Refraction, and Scattering

A. Refraction

• Definition: The bending of light when it passes from one medium to another.
• Changes during Refraction:
  • Changes: Velocity/Speed and Wavelength.
  • Remains Constant: Frequency.
• Direction of Bending:
  • Rarer to Denser Medium (e.g., air to glass): Light slows down and bends towards the normal.
  • Denser to Rarer Medium (e.g., glass to air): Light speeds up and bends away from the normal.
• Examples & Effects:
  • A stick dipped in water appears short and bent.
  • A coin in water appears closer to a person in the air; a coin in the air appears higher to a person underwater.
  • Advanced sunrise and delayed sunset occur due to atmospheric refraction.
  • The sun and moon appear slightly elliptical near the horizon due to atmospheric refraction.

B. Total Internal Reflection (TIR)

• Principle & Condition:
  • Occurs when light travels from a denser medium to a rarer medium.
  • The angle of incidence must be greater than the critical angle.
• Applications:
  • Optical Fibres: Transmit light signals with minimal loss (very low power consumption, no radio frequency interference).
  • Endoscopes: Medical instruments for viewing inside the body.
  • Diamond Brilliance: Due to high refractive index and multiple TIRs.
  • Mirage: An optical illusion caused by TIR in layers of air of different temperatures.
  • Traffic Signal Beads: Glow brightly due to TIR.

C. Scattering

• Rayleigh Scattering: Scattering of light by particles much smaller than its wavelength.
  • Shorter wavelengths (blue, violet) are scattered much more than longer wavelengths (red, orange).
• Atmospheric Phenomena:
  • Blue Sky: Due to the scattering of blue light by air molecules.
  • Red Sun at Sunrise/Sunset: Blue light is scattered away, leaving red/orange light to reach the observer.
  • View from Space: The sky appears black because there is no atmosphere to scatter light.

D. Dispersion & Rainbows

• Dispersion: The splitting of white light into its constituent colors (VIBGYOR) by a prism.
• Rainbow:
  • A natural spectrum caused by reflection, refraction, and dispersion in water droplets.
  • Color sequence: Red, Orange, Yellow, Green, Blue, Indigo, Violet (VIBGYOR). Central color is green.
  • Formed in the direction opposite to the Sun. Cannot be seen at 12 noon when the sun is overhead.

E. Interference

• Definition: The combination of waves leading to a new wave pattern.
• Example: The formation of glittering colors on a thin soap film or soap bubble.

4. Light and Color

• Color of Light:
  • Determined by its wavelength.
  • Visible Spectrum Order (by increasing wavelength): Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).
• Color of Objects:
  • The color of an opaque object is due to the color of light it reflects; it absorbs other colors.
  • A white object reflects all light; a black object absorbs all light.
• Color Mixing:
  • Light (Additive): Primary colors are Red, Green, Blue (RGB). Mixing them equally produces white light.
  • Pigments (Subtractive): Primary colors are Cyan, Magenta, Yellow.
• Applications:
  • Danger/Traffic Signals (Red): Red light is used because it has the longest wavelength and is scattered the least, allowing it to travel farthest.
  • Summer Clothing: White/light-colored clothes are preferred as they reflect light and absorb less heat.
  • Color of Water Bodies: The sea appears blue due to reflection of the sky and scattering by water molecules.

5. The Human Eye and Vision

• Image Formation:
  • Light is focused by the lens to form a real, inverted, and smaller image on the retina (which acts like camera film).
  • The brain interprets these signals and corrects the inversion.
• Key Features:
  • Persistence of Vision: An image persists on the retina for about 1/16 of a second. This is the principle behind motion pictures.
  • Least Distance of Distinct Vision (Near Point): For a normal eye, this is 25 cm.
  • Stereopsis (Depth Perception): Having two eyes provides depth perception because each sees a slightly different image.
  • Ciliary Muscles: Control the curvature (and thus focal length) of the eye lens.
    • Relaxed: Lens thin, for seeing distant objects.
    • Contracted: Lens thick, for seeing nearby objects.
• Defects of Vision:
  • Hypermetropia (Long-sightedness): Can see far objects clearly, but not nearby ones. Image focuses behind the retina. Corrected with a convex lens.
  • Myopia (Short-sightedness): Can see near objects clearly, but not distant ones. Corrected with a concave lens.
• Eye Safety: Viewing the sun directly, especially during an eclipse, can cause retinal burns from ultraviolet light.
• Dark Adaptation: The delay in seeing in a dark room is due to the reformation of Rhodopsin in rod cells and pupil dilation.

6. Optical Instruments and Applications

A. Mirrors

• Plane Mirrors:
  • The minimum height to see a full-length image is half the person’s height.
  • The radius of curvature is infinity.
  • Rotating the mirror by an angle θ rotates the reflected ray by 2θ.
  • Parallel mirrors produce an infinite number of images.
• Convex Mirrors (Curved Outwards):
  • Always form a virtual, erect, and diminished image.
  • Provide a wider field of view. Used as rear-view mirrors in vehicles.
• Concave Mirrors (Curved Inwards):
  • Can form real or virtual images.
  • When the object is between the pole and the focus, the image is virtual, erect, and magnified.
  • Applications: Shaving/makeup mirrors, dentist mirrors, searchlights/headlights.

B. Lenses

• Lens Power: Measured in Dioptres (D). Power (D) = 1 / Focal Length (meters).
  • Convex (converging) lenses have positive (+) power.
  • Concave (diverging) lenses have negative (-) power.
  • A sunglass typically has 0 diopters of power.
• Lens Combinations: A convex and concave lens of the same focal length in contact act like a plane glass sheet.
• Air Bubble in Water: Behaves as a divergent (concave) lens.

C. Other Technologies

• Holography: A technique to create a true 3D image using interference of light.
• CD/DVD Players: Use a laser beam to read information.
• CRT Monitors: Use a raster scan method where an electron beam hits a phosphorous-coated screen.
• CD Surface: Rainbow colors are caused by reflection and diffraction of light from its layered structure.

7. Related Wave Phenomena

• Sound & Signal Transmission:
  • In TV broadcasts, Frequency Modulation (FM) is used for audio due to its wide frequency range and minimal noise.
  • Radio waves are produced by rapidly oscillating electrical currents.
• Doppler Effect:
  • The change in observed frequency/wavelength due to relative motion between source and observer.
  • Used by police radar to detect speeding vehicles (with radio waves).
  • A blue light source moving away from an observer would appear shifted towards longer wavelengths (e.g., yellow/red).

Know More About Optics:

• Optics – Old Year Questions
• Optics One Liner Questions & Answers