How Light Works: Reflection, Refraction, and TIR
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Extra Light Topic Summary
Luminous Objects
Luminous objects emit or give out their own light or energy (e.g., the Sun, light bulbs).
Seeing Colour in Different Lights
Objects appear coloured because they reflect certain wavelengths (colours) of light and absorb the rest.
For example:
- A red object under red light looks red (it reflects red).
- A red object under blue light looks black (it absorbs blue, reflecting no light).
- A blue object under blue light looks blue, but looks black under red light.
Rods and Cones in the Eye
The human eye has rods and cones to detect light:
- Rods work in dim light and detect black, white, and grey. They are spread across the retina.
- Cones work in bright light and detect colour (red, green, and blue). They are mainly in the fovea, the center of the retina.
Total Internal Reflection (TIR)
Total Internal Reflection happens when light moves from a denser to a less dense medium (e.g., glass to air).
Two conditions must be met:
- The light must go from a denser to a less dense material.
- The angle of incidence must be greater than the critical angle.
At the critical angle, the refracted ray travels along the boundary. If the incidence angle is greater than the critical angle, the ray is totally reflected back into the medium.
Critical Angle
It is the angle of incidence when the refraction angle is 90°.
Different materials have different critical angles.
Uses of TIR
- Optical fibres: Used for communication, endoscopes, and decorative lamps.
- Prisms: Used in periscopes, binoculars, cameras, telescopes, and reflectors.
Prisms and Periscopes
A periscope uses two right-angled prisms for viewing over obstacles.
Prisms use TIR to reflect light at 90° or 180°:
- Single reflection: Light is turned 90°.
- Double reflection: Light is turned 180°.
Refraction
Light bends when moving between materials with different densities:
- Into a denser material: Bends towards the normal.
- Into a less dense material: Bends away from the normal.
Only speed and wavelength change — not frequency or colour.
Reflection
Occurs when light hits a boundary and does not pass through. It follows the law of reflection:
Angle of incidence = angle of reflection
Use arrows in diagrams to show direction:
- Towards the boundary: Incident ray.
- Away from the boundary: Reflected ray.
Light Transmission Properties
- Transparent materials allow all light to pass through clearly. You can see through them perfectly.
👉 Example: A clean glass window. - Translucent materials let some light pass through, but they scatter the light, so you can only see blurry shapes, not clearly.
👉 Example: Frosted glass or tracing paper. - Opaque materials do not let any light pass through. You cannot see through them at all.
👉 Example: A wooden door or a wall.
Example of Translucent Materials
A bathroom window made of frosted glass is translucent. You can see light coming through, but you cannot see people clearly. It gives privacy but still lets light in.
