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Most wonderful Properties of Light

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🌈 1. Properties of Light

Light is a form of energy that travels as an electromagnetic wave but also behaves like particles (photons). Here are its key properties:

✔ Speed

• In vacuum: 3 × 10⁸ m/s
• Slows down in water, glass, etc.

✔ Reflection

• Bouncing of light off a surface.
• Law: Angle of incidence = angle of reflection
• Example: Mirrors.

✔ Refraction

• Bending of light when it passes from one medium to another.
• Caused by a change in light’s speed.
• Example: A straw looks bent in water.

✔ Diffraction

• Light spreads out when it passes through a narrow slit.
• More noticeable with small slits or obstacles.

✔ Interference

• Overlapping light waves produce bright and dark patterns.
• Seen in thin films like soap bubbles.

✔ Polarization

• Filters light to vibrate in one direction only.
• Used in sunglasses and LCD screens.

✔ Dispersion

• Splitting of white light into its color components (spectrum).
• Example: Rainbow, prism.

✔ Absorption & Transmission

• Materials may block, absorb, or allow light.
  • Transparent: light passes through (glass)
  • Translucent: partial transmission (frosted glass)
  • Opaque: blocks light (wood)

✔ Photoelectric Effect

• Light hits a metal → ejects electrons.
• Shows that light behaves as particles (photons).
• Einstein won the Nobel Prize for this!

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🔄 2. Can We Bend Light?

Yes, light can change direction under certain conditions — though not like bending a wire!

🌊 A. Refraction

• Most common method.
• Light bends when entering a medium with different density.
• Applications: Lenses, eyeglasses, magnifying glasses.

🌠 B. Gravitational Bending

• Predicted by Einstein.
• Massive objects bend spacetime, and light follows the curve.
• Known as gravitational lensing (see below).

🔍 C. Prisms and Lenses

• Prisms split and bend light (dispersion).
• Lenses focus or spread light.

🌅 D. Atmospheric Bending

• Light bends due to air layers of different temperatures.
• Examples: Mirages, flattened Sun at sunset.

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🌌 3. Gravitational Lensing

Gravitational lensing is a powerful effect of Einstein’s General Relativity.

🌀 What Happens?

When light from a distant galaxy passes near a massive object (like a black hole or galaxy cluster), the object’s gravity bends the light.

• Acts like a cosmic magnifying glass.
• Helps us observe distant galaxies or objects hidden behind other ones.

🧭 Types of Gravitational Lensing:

Type	Description	Use Case
Strong Lensing	Creates arcs, rings (Einstein rings), multiple images	Seen in Hubble images
Weak Lensing	Slight distortion in galaxy shapes	Used in dark matter mapping
Microlensing	Temporary brightening as a star passes in front of another	Detects exoplanets

🌠 Why It’s Important:

• Confirms Einstein’s theory.
• Maps dark matter (which doesn’t emit light).
• Reveals early and distant galaxies.

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🕳️ 4. What is Dark Matter?

Dark matter is an invisible form of matter that makes up about 27% of the universe.

🕵️‍♂️ We Can’t See It, But We Know It’s There!

Evidence for dark matter comes from:

🔄 Galaxy Rotation

• Galaxies rotate too fast to be held together by visible matter alone.
• Invisible “extra mass” is holding them in.

🔭 Gravitational Lensing

• Light bends more than expected → hidden mass is present.

🌌 Structure Formation

• The way galaxies formed after the Big Bang needs dark matter to explain it.

🤔 What Could It Be?

Dark matter is probably made of unknown particles like:

• WIMPs (Weakly Interacting Massive Particles)
• Axions
• Sterile Neutrinos

It is not:

• Normal atoms
• Black holes
• Ordinary gas
• Antimatter

📊 Composition of the Universe:

Component	Percentage
Dark Energy	68%
Dark Matter	27%
Normal Matter	5%

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🧠 Summary

Concept	What It Means
Light	Electromagnetic wave and particle
Bending of Light	Through refraction, lenses, gravity
Gravitational Lensing	Gravity bending light → space telescope effect
Dark Matter	Invisible matter that shapes the universe

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