- Wavelength is the characteristic of colour, and light of the same colour from different sources has the same wavelength.
- In glass, due to different speeds of colours, μviolet > μred, so θviolet > θred, meaning violet deviates the most and red the least.
Definitions [5]
Define the term dispersion of light.
The phenomenon of the splitting of white light by a prism into its constituent colours is known as dispersion of light.
When a beam of white light or composite light is refracted through any transparent media such as glass or water, it is split into its component colours. This phenomenon is called ‘dispersion of light’.
Definition: Dispersion
The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion.
OR
The splitting of light into its component colours is called dispersion.
OR
The process of separation of light into its component colours while passing through a medium is called the dispersion of light.
Definition: Spectrum
On passing white light through a prism, the band of colours seen on a screen is called the spectrum.
or
The band of the coloured components of a light beam is called its spectrum.
Definition: Invisible Spectrum
The part of spectrum beyond the red extreme and the violet extreme is called the invisible spectrum.
Definition: Scattering of Light
Scattering is the process of absorption and then re-emission of light energy by the dust particles and air molecules present in the atmosphere.
Key Points
Key Points: Deviation Produced by a Triangular Prism
- Total deviation (δ) by a triangular prism is the sum of deviations at two surfaces:
δ = δ1 + δ2 - Angle of deviation δ depends on:
angle of incidence (i),
angle of prism (A),
refractive index (μ) of prism material. - Refractive index μ increases with a decrease in wavelength, so violet light (λ ≈ 4000 Å) deviates the most, and red light (λ ≈ 8000 Å) deviates the least.
- The speed of light in glass is lowest for violet light and highest for red light; hence, the deviation increases as the wavelength decreases.
Key Points: Colour in White Light with Their Wavelength and Frequency Range
Key Points: Dispersion of Light
- Dispersion is the splitting of white light into seven colours (VIBGYOR) when it passes through a prism or similar transparent medium.
- Human eyes can detect light with wavelengths ranging from 400 nm (violet) to 700 nm (red).
- Different colours travel at different speeds in a medium like glass, so each colour has a different refractive index.
- Violet light bends the most, and red light bends the least, as it passes through a prism, producing a spectrum.
- A rainbow is formed due to dispersion, refraction, and internal reflection of sunlight by raindrops acting as tiny prisms.
Key Points: Electromagnetic Spectrum
- The visible spectrum lies between violet and red and is only a small part of the electromagnetic spectrum, while regions beyond are called the invisible spectrum.
- The electromagnetic spectrum, in increasing order of wavelength, is: gamma rays, X‑rays, ultraviolet rays, visible light, infrared radiation, microwaves, and radio waves.
- All electromagnetic waves travel with the same speed (3 × 10⁸ m s⁻¹) in vacuum, and they are transverse waves that show reflection and refraction without a change in frequency.
Key Points: Properties and Uses of Different Radiations of the Electromagnetic Spectrum
| Radiation Type | Wavelength Range | Properties/Uses |
|---|---|---|
| Gamma Rays | Less than 0.1 Å | Most energetic radiation; used in radiotherapy and welding inspection. |
| X-Rays | 0.1 Å to 100 Å | Used in fracture detection, CAT scans, and the detection of concealed precious metals. |
| Ultraviolet Radiation | 100 Å to 4000 Å | Causes fluorescence; used for sterilisation and vitamin D production. |
| Infrared Radiation | 8000 Å to 10⁷ Å | Exhibits heating effect; absorbed by greenhouse gases; used in remote controls and therapeutics. |
| Microwaves & Radio Waves | Longest wavelengths in the spectrum | Used in satellite communication, radar, and TV/radio transmission. |
key Points: Scattering of Light
- Shorter wavelengths (violet and blue) are scattered more than longer wavelengths (red).
- The intensity of scattering follows Rayleigh’s law:
- Very small particles (smaller than the wavelength of light) scatter light more effectively than larger particles.
- The sky appears blue because blue light is scattered more due to its shorter wavelength.
- Red light scatters least, causing red/orange sunsets and making sunlight near Earth richer in red light.
Key Points: Applications of Scattering of Light
- Red colour of the Sun at sunrise and sunset is due to maximum scattering of blue light and least scattering of red light in the atmosphere.
- The blue colour of the sky is due to the greater scattering of blue (or violet) light by air molecules because of its short wavelength.
- The black colour of the sky in the absence of atmosphere occurs because there is no scattering of sunlight.
- Red light is used for danger signals because it has the longest wavelength and is scattered the least, so it can be seen from a far distance.
Important Questions [13]
- Why is the Colour Red Used as a Sign of Danger?
- Name the subjective property of light related to its wavelength.
- The Smoke from a Fire Looks White. Which of the Following Statements is True? Molecules of the Smoke Are Bigger than the Wavelength of Light and Molecules of
- A ray of light is incident at 45° on an equilateral prism in the diagram below. Name the phenomenon exhibited by the ray of light when it enters and emerges out of the prism.
- State an Additional Use of The X-rays Waves
- A wave has wavelength 50 Å. a. Name the wave. b. State its speed in vacuum. c. State its one use.
- Below is an incomplete table showing the arrangement of electromagnetic spectrum in the increasing order of their wavelength. Complete the table: Gamma ray X - ray UV rays Visible rays Infrared
- Assertion (A): Ultraviolet radiations is scattered more as compared to the microwave radiations. Reason (R): Wavelength of ultraviolet radiation is more than the wavelength of microwave radiation.
- Name the High Energetic Invisible Electromagnetic Waves Which Help in the Study of the Structure of Crystals
- Name the radiation which can be detected by a thermopile.
- What do you understand by the term Scattering of light?
- Why does sun appear red at sunrise and sunset?
- Which color of white light is scattered the least and why?
Concepts [8]
- Deviation Produced by a Triangular Prism
- Colour in White Light with Their Wavelength and Frequency Range
- Dispersion of Light
- Electromagnetic Spectrum
- Properties and Uses of Different Radiations of the Electromagnetic Spectrum
- Distinction between Ultraviolet, Visible, and Infrared Radiations
- Scattering of Light
- Applications of Scattering of Light
