CIE IGCSE Physics (0625) Dispersion of light Study Notes - New Syllabus
CIE IGCSE Physics (0625) Dispersion of light Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Dispersion of light
Key Concepts:
- Dispersion of Light by a Glass Prism
- Visible Spectrum – Colour Order by Frequency and Wavelength
Dispersion of Light by a Glass Prism
Dispersion of Light by a Glass Prism
White light is made up of a mixture of all the visible colours of the spectrum (ROYGBIV).When white light enters a glass prism, it is both refracted and dispersed.
- Refraction occurs because light slows down when it enters the denser glass material from air.
- Each colour in white light has a different wavelength, and therefore is refracted by a different amount:
- Violet light (shortest wavelength) is refracted the most.
- Red light (longest wavelength) is refracted the least.
- This separation of colours is called dispersion.
- As the light exits the prism, it refracts again, spreading the colours even more and forming a visible spectrum.
Result: A colourful band of light (spectrum) is produced, showing the different components of white light arranged from red to violet.
Important Note: Dispersion happens because the refractive index of glass varies slightly for different wavelengths (a property known as “material dispersion”).
Example:
White light passes through a glass prism and forms a spectrum on a screen. Why does red light refract less than violet light as it passes through the prism?
▶️ Answer/Explanation
Red light has a longer wavelength and a lower frequency than violet light.
The refractive index of glass is lower for longer wavelengths, so red light bends less than violet light, which has a shorter wavelength and is refracted more strongly.
This difference in refraction angles leads to the dispersion of white light into a visible spectrum.
Example:
A ray of violet light enters a glass prism with refractive index \( n = 1.53 \) for violet light. Calculate the speed of violet light in the glass. (Speed of light in vacuum = \( 3.0 \times 10^8 \, \text{m/s} \))
▶️ Answer/Explanation
Step 1: Use the formula: \( v = \dfrac{c}{n} \)
\( v = \dfrac{3.0 \times 10^8}{1.53} \approx \boxed{1.96 \times 10^8 \, \text{m/s}} \)
The speed of violet light in the glass is approximately \( \boxed{1.96 \times 10^8 \, \text{m/s}} \).
Visible Spectrum - Colour Order by Frequency and Wavelength
Visible Spectrum – Colour Order by Frequency and Wavelength
White light is made up of many colours, each with a different wavelength and frequency.
- The visible spectrum consists of the traditional seven colours, which can be remembered using the acronym: ROYGBIV.
Order of Colours by Increasing Wavelength (and Decreasing Frequency):
By Frequency: Violet → Indigo → Blue → Green → Yellow → Orange → Red
By Wavelength: Red → Orange → Yellow → Green → Blue → Indigo → Violet
Monochromatic Light:
- Monochromatic light is light of a single frequency or wavelength.
- It contains only one pure colour (e.g., a laser beam).
- Unlike white light, it does not disperse into a spectrum when passed through a prism.
Example:
A laser emits monochromatic red light. If this light is passed through a prism, will it produce a spectrum? Explain why or why not.
▶️ Answer/Explanation
No, it will not produce a spectrum.
Monochromatic light consists of a single frequency and wavelength, so all the rays bend by the same amount during refraction through the prism.
Since there is only one colour, there is no dispersion or splitting into multiple colours.
Example:
A monochromatic blue light has a frequency of \( 6.0 \times 10^{14} \, \text{Hz} \). Calculate its wavelength in air. (Speed of light in air ≈ \( 3.0 \times 10^8 \, \text{m/s} \))
▶️ Answer/Explanation
Step 1: Use the wave equation: \( v = f \lambda \)
Step 2: Rearranged: \( \lambda = \dfrac{v}{f} = \dfrac{3.0 \times 10^8}{6.0 \times 10^{14}} \)
Step 3: \( \lambda = 5.0 \times 10^{-7} \, \text{m} = \boxed{500 \, \text{nm}} \)
The wavelength of the monochromatic blue light is \( \boxed{500 \, \text{nm}} \).