6.4 Refraction, Reflection, and Absorption

Refraction, Reflection, and Absorption: A Comprehensive Guide

Light is a fascinating phenomenon, and understanding how it interacts with different materials allows us to explain many everyday and scientific occurrences. When light encounters a medium, it may undergo one of three processes: reflection, refraction, or absorption. This guide will break down these processes and provide insights into their applications and underlying principles.

What Happens When Light Meets a Medium?

When light transitions between two mediums, three primary processes occur:

Reflection

Reflection is when light bounces off a surface. The most recognizable example is a mirror, where the reflected light forms a clear image of the source. Reflection can be categorized into two types:

• Specular Reflection: Occurs on smooth surfaces where the light rays reflect uniformly, preserving the orientation of the rays.
• Diffuse Reflection: Happens on rough surfaces where the light rays scatter in various directions, diffusing the light.

Absorption

Absorption occurs when light enters a medium and is transformed into another form of energy, typically heat. For example, a black surface absorbs most light, converting it into heat energy.

Transmission

Transmission refers to the passage of light through a medium without being absorbed. Transparent materials, like glass, allow light to pass through, albeit with some refraction depending on the medium’s properties.

The Science of Reflection and Refraction

The Law of Reflection

The law of reflection states that the angle of incidence ($\theta_i$) is equal to the angle of reflection ($\theta_r$). This principle is observed in both specular and diffuse reflections.

Key Terms:

• Incident Ray: The incoming ray of light striking a surface.
• Reflected Ray: The ray bouncing off the surface.
• Normal Line: A line perpendicular to the surface at the point of incidence, dividing the angles equally.

Understanding Refraction

Refraction occurs when light travels from one medium to another, changing its speed and direction. This “bending” of light depends on the medium’s refractive index ($n$).

• Refracted Ray: The portion of the light that enters the new medium and bends.
• Angle of Refraction ( 
• $\theta_r$

   

• ): The angle between the refracted ray and the normal.

Key Equation: Snell’s Law

$n_1 \sin \theta_1 = n_2 \sin \theta_2$

​
Where:

• 
  $n_1$

   

  ​: Refractive index of the first medium.

• 
  $n_2$

   

  ​: Refractive index of the second medium.

• 
  $\theta_1$

   

  ​: Angle of incidence.

• 
  $\theta_2$

   

  ​: Angle of refraction.

Effects of Refraction:

• Toward the Normal: Light slows down as it moves to a denser medium (
  $n_2 > n_1$

   

  ​).

• Away from the Normal: Light speeds up as it moves to a less dense medium (
  $n_2 < n_1$

   

  ).

Index of Refraction and Wave Speed

The speed of light in a medium (

$v$

) is always slower than in a vacuum (

$c$

), where

$c = 3.00 \times 10^8 \, \text{m/s}$

. The ratio of these speeds defines the medium’s refractive index:

$n = \frac{c}{v}$

​

Properties of the Refractive Index:

1. It is always greater than or equal to 1.
2. A higher index indicates a slower speed of light in the medium.

Total Internal Reflection

When light travels from a denser medium to a less dense one, it may undergo total internal reflection if the angle of incidence exceeds the critical angle (

$\theta_c$

​). At this angle, the refracted ray lies along the boundary.

Critical Angle Formula:

$\sin \theta_c = \frac{n_2}{n_1}$

​​
Where

$n_1 > n_2$

​.

If

$\theta_1 > \theta_c$

, total internal reflection occurs. This principle is widely used in optical fibers, lasers, and microscopes.

Real-World Applications

1. Optical Instruments

Mirrors, lenses, and prisms utilize reflection and refraction principles to focus or redirect light.

2. Fiber Optics

Telecommunications heavily rely on total internal reflection to transmit light signals efficiently over long distances.

3. Photography

Filters and lenses adjust light transmission, reflection, and absorption to achieve desired effects.

4. Solar Panels

Selective coatings optimize light absorption to maximize energy conversion.

Practice Problems

1. The critical angle of a material is the angle of incidence for which the angle of refraction is:

A) 0°
B) 30°
C) 45°
D) 90°
E) 180°

2. A beam of light passes from medium 1 to medium 2 to medium 3. What is true about the respective indices of refraction ( $n_1, n_2, n_3$

​)?

D)

$n_2 > n_1 > n_3$

​

3. A wave moves from one medium to a second medium with a different index of refraction. Which of the following wave properties NEVER changes?

A) Frequency