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title: Absorptivity, Reflectivity, and Transmissivity
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# Absorptivity, Reflectivity, and Transmissivity
Unit 3, Lecture 22

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## Radiation Heat Transfer

- Course: Heat and Mass Transfer (DI05019071)
- Unit 3: Radiation
- Lecture 22: Absorptivity, Reflectivity, and Transmissivity

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Welcome to Unit 3. We are shifting from conduction and convection to radiation, the only heat transfer mode that doesn't require a medium.
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## Lecture Agenda

- 1. Introduction to Thermal Radiation
- 2. Electromagnetic Spectrum
- 3. Interaction of Radiation with Matter
- 4. Definitions: Absorptivity, Reflectivity, Transmissivity
- 5. Conservation of Energy Relationship
- 6. Surface Characteristics

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Today's agenda covers the fundamentals of how radiation interacts with different materials.
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## What is Thermal Radiation?

- Radiation is energy emitted by matter in the form of electromagnetic waves (or photons).
- Does NOT require an intervening medium (can occur in a vacuum).
- All matter at a temperature above absolute zero (0 K) emits thermal radiation.
- Travels at the speed of light.

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Unlike conduction or convection, radiation works best in a vacuum. It's how solar energy reaches Earth.
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## The Electromagnetic Spectrum

- Thermal radiation spans a specific portion of the EM spectrum.
- Wavelength range: ~0.1 to 100 micrometers (μm).
- Includes a portion of ultraviolet (UV), all visible light, and infrared (IR).
- Heat transfer is mostly concerned with the infrared region.

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Thermal radiation is just one part of the EM spectrum, primarily in the infrared region, though visible light also carries significant thermal energy.
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## Interaction of Radiation with Matter

- When thermal radiation strikes a surface, three things can happen:
- 1. It is absorbed by the material (increases internal energy).
- 2. It is reflected away from the surface.
- 3. It is transmitted through the material.
- Total incident radiation is called Irradiation (G).

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Think of light hitting a window. Some bounces off, some goes through, and some heats the glass itself.
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## Absorptivity (α)

- Absorptivity (α): The fraction of incident radiation absorbed by the surface.
- Mathematical definition: α = G_abs / G
- Where G_abs is absorbed radiation and G is total incident irradiation.
- Range: 0 ≤ α ≤ 1
- Depends on the nature of the surface, temperature, and wavelength of incident radiation.

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Absorptivity tells us how well a material takes in radiative energy. Dark, matte surfaces typically have high absorptivity.
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## Reflectivity (ρ)

- Reflectivity (ρ): The fraction of incident radiation reflected by the surface.
- Mathematical definition: ρ = G_ref / G
- Where G_ref is reflected radiation.
- Range: 0 ≤ ρ ≤ 1
- Types of reflection: Specular (mirror-like) and Diffuse (scattered in all directions).

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Reflectivity is crucial for insulation and cooling. Mirrors have high reflectivity (specular), while rough white walls reflect diffusely.
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## Transmissivity (τ)

- Transmissivity (τ): The fraction of incident radiation transmitted through the material.
- Mathematical definition: τ = G_tr / G
- Where G_tr is transmitted radiation.
- Range: 0 ≤ τ ≤ 1
- Solid opaque materials generally have τ = 0.

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Transmissivity matters for glass, water, and gases. Most solid objects you interact with are opaque and don't transmit thermal radiation.
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## Conservation of Energy

- By the principle of conservation of energy:
- Total Incident Radiation = Absorbed + Reflected + Transmitted
- G = G_abs + G_ref + G_tr
- Dividing by G gives:
- 1 = α + ρ + τ

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This is the fundamental equation for surface radiation properties. The sum of these three fractions must always equal 1.
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## Special Cases: Opaque Surfaces

- For opaque materials (e.g., metals, wood, brick):
- No radiation is transmitted: τ = 0
- The conservation equation simplifies to:
- α + ρ = 1
- Therefore, a highly reflective opaque surface must have low absorptivity, and vice versa.

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In engineering, we deal mostly with opaque surfaces. If a metal reflects 90% of radiation, it can only absorb 10%.
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## Special Cases: Transparent Gases

- For highly transparent media (e.g., dry air, pure gases):
- Very little radiation is absorbed or reflected.
- α ≈ 0, ρ ≈ 0
- Transmissivity approaches 1: τ ≈ 1
- Note: Some gases (like CO2 and water vapor) absorb specific wavelengths.

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Standard air doesn't interact much with thermal radiation, but greenhouse gases like CO2 are notable exceptions because they absorb IR.
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## Summary

- Thermal radiation requires no medium and travels as EM waves.
- Irradiation (G) interacts via absorption, reflection, and transmission.
- Properties: α (absorptivity), ρ (reflectivity), τ (transmissivity).
- Conservation law: α + ρ + τ = 1.

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Remember the fundamental relationship α + ρ + τ = 1. This forms the basis for analyzing how any material handles radiation.
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## Next Lecture Preview

- Topic: Black, white, and grey bodies
- Key questions to ponder:
- - What makes a surface a 'perfect' emitter and absorber?
- - How do real surfaces differ from ideal theoretical surfaces?

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Next time, we will define theoretical ideal surfaces like the black body, which serves as a standard for comparing all real surfaces.
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