Reflection and Refraction of Light

• Light and Sound Waves and Reflection & Refraction is covered

Light

• Visible light is a transverse wave and part of the Electromagnetic spectrum.
• Reflection and refraction can happen with light.
• In transverse waves, particles vibrate perpendicularly to energy transfer direction.

Sound

• Sound waves are longitudinal.
• Sound waves are illustrated as lines showing compressions (closer lines) and rarefactions (further lines).
• Reflection, which creates an echo, and refraction can happen with sound.
• Waves, either transverse or longitudinal, are able to be reflected and refracted.
• Reflection occurs when a wave hits a boundary between two media and stays in the original medium.
• Medium refers to a material that transmits light; media means more than one medium in optics.

Law of Reflection

• States that the angle of incidence (i) is equivalent to the angle of reflection (r).
• Angles are measured between the wave direction (ray) and the normal, a 90-degree line to the boundary.
• The incoming wave's angle is the angle of incidence (i), and the outgoing wave's angle is the angle of reflection (r).
• In ray diagrams, arrows indicate wave direction
• An incident ray has an arrow towards the boundary; a reflected ray has an arrow away from it.

Refraction ray diagrams

• The incident and refracted rays directions are relative to the normal line.
• Density differences between two media affects refracted ray direction.
• Light bends toward the normal when moving from less to more dense (air to glass).
• Light bends away from the normal when moving from more to less dense (glass to air).
• Light does not bend when passing along the normal (perpendicular).
• The change in direction results from speed changes in different substances.
• Rays slow down and bend towards the normal when light enters a denser substance.
• Only speed and wavelength change during refraction; frequency remains constant.
• Different light colors have different frequencies; for example, red has low and blue has high frequency.
• Light's color doesn't change during refraction.
• When drawing ray diagrams for reflection, a straight line with an arrow can represent the wave.
• Accurately representing the angle is important.
• When light enters a block it bends toward the normal line, When it leaves, it bends the opposite way.

Core Practical: Investigating Refraction

• Uses transparent blocks, semi-circular blocks, and triangular prisms to investigate light refraction.
• Reflection & refraction revises the use of refraction.
• The independent variable is the shape of the block.
• The dependent variable is the direction of refraction.
• Control variables include light beam width and light frequency/wavelength.
• Equipment:
  • Ray Box: Produces a narrow, easily refracted light beam.
  • Protractor: Measures incidence and refraction angles.
  • Sheet of Paper: Marks incident and refracted lines.
  • Pencil: Draws incident and refracted ray lines on paper.
  • Ruler: Draws incident and refracted ray lines on paper.
  • Perspex blocks (rectangular, semi-circular, prism): To refract the light beam.
• Protractor and ruler resolution is 1° and 1 mm, respectively.

Refraction experiment setup:

• Place a glass block on paper and trace it.
• Direct a light beam at the block's side face using a ray box.
• Mark a point on the ray close to the box, the entry and exit points on the block, and a point on the exit light ray about 5 cm from the block.
• Draw a dashed normal line where points are located, remove the block, and connect the points with three lines.
• Repeat with different angles and perspex block shapes (prism and semi-circular), and observe light paths through different shapes.
• Multiple light rays with different incidence angles should be included in the final diagram.
• This will help show how the angle of refraction changes with the angle of incidence.
• Clear labels and laws of refraction can be used to analyze results
• Systematic errors include incorrectly drawn 90° lines, correct this by using a set square for perpendicular lines.
• Random errors include inaccurate incoming and reflected beam points, correct this by using a sharpened pencil.
• Inaccurate protractor resolution makes it difficult to get the angles accuretly, correct this by using a protractor with a higher resolution
• Handling Safety considerations:
  • Burns: Run under cold water for 5 minutes.
  • Eye Damage: Avoid direct looking, stand behind the box.
  • Keep liquids away from electrical equipment.

Snell’s Law

• The angles of incidence and refraction are related to the refractive index of a medium by an equation: n = sin i / sin r
  • n is refractive index, i is incidence angle (°), r is refraction angle (°).
  • 'Sin' is the trigonometric function 'sine' which is on a scientific calculator.
• Can revise the reflection and the concept of refraction
• Refractive index is related to the speed of light in a material (always less than in a vacuum): refractive index, n = speed of light in a vacuum / speed of light in material
• Always greater than 1, differs by material, higher in optically dense objects (e.g., 2.4 for diamond).
• Objects less dense have a lower index (e.g., 1.5 for glass); it is a ratio without units.
• The values of the angles aren't the same as the values of sines of the angles
• Can use the inverse sine function (sin-1) function (sin¯¹ on most calculators by pressing 'shift' then 'sine') to find the angle.
• Independent variable = angle of incidence, i
• Dependent variable = angle of refraction, r
• Equipment includes:
  • Ray Box: A narrow light beam that can be easily refracted
  • Sheet of Paper and Pencil and Ruler
  • Perspex rectangle: To refract the light beam Setup:

1. Place the glass block on a sheet of paper, and carefully draw around the block using a pencil
2. Draw a dashed line normal (at right angles) to the outline of the block
3. Use a protractor to measure the angles of incidence to be studied and mark these lines on the paper
4. Switch on the ray box and direct a beam of light at the side face of the block at the first angle to be investigatedmark on the paper.

• A point on the ray close to the ray box
• The point where the ray enters the block
• The point where the ray exits the block
• A point on the exit light ray which is a distance of about 5 cm away from the block

5. Remove the block and join the points marked with three straight lines
6. Replace the block within its outline and repeat the above process for a rays striking the block at the next angle
7. Place the block on a sheet of paper, and carefully draw around the block using a pencil

Snell's law analysis

• Snell's Law relates the angles of incidence and refraction.
• Covered in the Snell's law revision note.
• By plotting a sin i graph on the y-axis against sin r on the x-axis, the refractive.index is equal to the gradient of the graph
• Safety considerations
  • Burns: Run under cold water for 5 minutes.
  • Eye Damage: Avoid direct looking, stand behind the box.
  • Keep liquids away from electrical equipment.

Total Internal Reflection

• Total internal reflection refers to light being completely reflected instead of refracted when moving from a denser to a less dense medium.
• Total internal reflection (TIR) conditions:
  • The light is moving from a denser material to a less dense one.
  • The angle of incidence exceeds the critical angle
• Critical angle varies for different materials, refraction occurs when the incidence angle is lower and total internal reflection occurs when it is greater.
• Total internal reflection applications include optical fibers and prisms

Optical fibres

• Total internal reflection guides light along optical fibres used in communications, endoscopes, and decorative lamps, and light is internally reflected each time it hits the fiber's edge.

Prisms

• Prisms can have total internal reflections when light travels through them in periscopes, with the light reflects at right angles

Critical Angle

• The angle of incidence known as at which the angle of incidence is known as the critical angle, and from which the As the angle of incidence increases it will eventually surplus the critical angle and lead to total internal reflection of the light.