Notes on Light: Spectrum, Prism, and Material Interaction

Electromagnetic spectrum overview: energy spans from very high to very low energy regions. High-energy end includes cosmic rays, gamma rays, X-rays; low-energy end includes FM radio and radar.
The visible light region is the narrow band in the center of the spectrum. Everything you see around you uses this small window of energy, i.e., visible light.
Key takeaway: what you perceive with your eyes comes from visible light interacting with objects in the environment.

Isaac Newton showed that passing light through a prism causes refraction, splitting the beam into multiple colors (dispersion).
Important observations:
- Light travels at the same speed through a given medium, but the beam changes direction when entering a different medium due to refraction.
- In the original beam, light is treated as white light, which looks like a single beam of light but contains a mix of colors.
- Sunlight appears white because it is composed of many colors; sunlight actually contains all the colors of the spectrum.
The prism demonstrated that white light is not a single color but a mixture of colors that can be separated.
Significance: dispersion shows that white light is composed of multiple wavelengths, each color corresponding to a different wavelength within the visible spectrum.

Materials interact with light in three main ways: transmission, absorption, and reflection.

When light hits a material and most of it passes through, the material is said to transmit the light.
Example: glass – a material that allows light to pass through to some extent (high transmission).
Conceptual note: transmission means light continues on through the material with little attenuation.

When a material absorbs light, the light energy is taken up by the material rather than passing through or reflecting.
Example: a sponge-like material that soaks up light; it does not transmit or reflect much light.
Absorption removes certain wavelengths from the incident light, contributing to the color you perceive if some wavelengths are absorbed more than others.

Reflection involves light bouncing off a material’s surface.
Example: a piece of plastic that is partially transparent, showing some transmission, some absorption, and some reflection.
If the plastic has a red tint, it absorbs other wavelengths more and reflects red light, contributing to the perceived color.
Real-world materials typically do not exhibit perfect transmission, absorption, or reflection; they exhibit a combination of these interactions to varying degrees.

An object appears red because it reflects red wavelengths and absorbs other wavelengths in the visible spectrum.
The pigments in the apple’s skin absorb most wavelengths except red, which is reflected to our eyes.
This is a direct consequence of how materials modify light through transmission, absorption, and reflection.

Color perception depends on how objects modify the spectrum of light and how our eyes/brain interpret the reflected wavelengths.
The same principles explain how sunglasses, tinted windows, and color filters work: they alter transmission and absorption to change which wavelengths reach our eyes.
Real-world relevance includes imaging, photography, display technologies, and color science.