8th grade chapter 9 Dispersion of Light and Color
Understanding the Dispersion of Light and Color
White light, though seemingly uniform, is actually made up of a spectrum of colors. When it undergoes dispersion, we can observe these colors separated based on their wavelengths. This concept is foundational to understanding how we perceive color and how light interacts with objects around us.
Dispersion of Light
Dispersion is the process in which white light splits into its component colors when it passes through a prism. Each color of light travels at a different speed in the medium, resulting in varying angles of refraction.
Each wavelength bends at a different angle. Red light, which has the longest wavelength, bends the least. Violet, with the shortest wavelength, bends the most. This separation results in a visible spectrum ranging from red to violet.
Color and the Spectrum
After dispersion, white light becomes a spectrum of seven visible colors: red, orange, yellow, green, blue, indigo, and violet. These colors fall into two main categories:
- Primary Colors: Red, Green, Blue
- Secondary Colors: Yellow, Cyan, Magenta
These colors can interact in different ways:
- Added: Colors combine to form new colors (e.g., red + green = yellow)
- Absorbed: Objects take in some wavelengths and not others
- Reflected: The remaining wavelengths are reflected to our eyes, creating the color we perceive
Young’s Double-Slit Experiment
While dispersion reveals how colors are separated, Young’s experiment confirms the wave nature of light. When light passes through two slits, it creates an interference pattern that supports the idea that light behaves as a wave.
9.2 Seeing Colours Emitted by a Light Source
How do we perceive the colors that come directly from light-emitting sources, such as lamps, screens, or the Sun? These sources, known as luminous objects, emit light that consists of multiple wavelengths.
White light—the type emitted by most natural and artificial sources—is composed of three primary colors of light: red, green, and blue. These colors can be combined through a process known as color addition to form new colors, called secondary colors.
This additive mixing happens because different wavelengths stimulate different types of cone cells in the retina. When two or more cone types are stimulated simultaneously, our brain interprets the combination as a different color.
Here's how the primary colors of light combine:
Red + Green = Yellow
Red + Blue = Magenta
Green + Blue = Cyan
Red + Green + Blue = White
This process is the basis of modern visual display technologies. For example, LED TVs and computer monitors emit varying amounts of red, green, and blue light to create all visible colors.
Venn Diagram
9.3 Seeing Colors in Non-Luminous Objects
Non-luminous objects do not emit light. We see them because they reflect certain wavelengths of white light and absorb the others. This process is called the subtraction of colors.
To better understand this, scientists often use color filters transparent materials that absorb some colors and allow others to pass through. The color of light that emerges after passing through a filter depends on what that filter allows and blocks.
Individual Filter Examples:
Red Filter – transmits red, absorbs green and blue
Yellow Filter – transmits red and green, absorbs blue
Magenta Filter – transmits red and blue, absorbs green
Cyan Filter – transmits green and blue, absorbs red
Combined Filters:
When two filters are placed over one another, only the overlapping colors that both filters allow will pass through:
Yellow + Magenta Filters → Only Red light can pass
Cyan + Yellow Filters → Only Green light can pass
These examples show that the colors we see from non-luminous objects depend on what wavelengths are reflected or transmitted, and which are absorbed or subtracted.
Another interesting case occurs when a magenta filter is placed over a cyan filter. In this arrangement, only blue light is able to pass through both filters. The cyan filter absorbs red light, while the magenta filter absorbs green light. Since blue is the only primary color that both filters allow to pass, it is the only color visible on the screen.
It’s also important to note that the resulting light appears dimmer than the original white light. This is because each filter absorbs a portion of the spectrum, reducing the overall light intensity. What reaches the screen is just a small segment of the original light, which explains the reduced brightness.
How Do We See Colors of Non-Luminous Objects?
Non-luminous objects do not emit light on their own. Instead, we see them because they reflect certain wavelengths of light from an external source usually white light like sunlight or a bulb. These objects behave much like light filters: they absorb some colors and reflect others.
The color we perceive is determined by the wavelengths that are reflected into our eyes. For example, when white light shines on a red apple, the apple absorbs all other colors green, blue, yellow, etc. and only reflects red light. This reflected red light is what enters our eyes, and we perceive the apple as red.
But what happens if we shine a different color of light on the same apple?
Let’s say we shine green light onto the red apple. Because the apple can only reflect red light and there is no red light in the green beam all the green light is absorbed. With nothing reflected back to our eyes, the apple appears black. This illustrates that if an object cannot reflect the color of light it is illuminated with, it will appear dark or black.
This concept helps explain why the appearance of objects can change dramatically under different lighting conditions. Let’s now explore how other colors behave when illuminated by different colored lights.
