Angle of deviation depends on three factors:
- Angle of incidence at first surface
- Angle of prism
- Refractive index of material
Refractive index of glass increases with the decrease in the wavelength of light
violet(wavelength = 4000 A) deviates most
red(wavelength 8000 A) deviates the least
Violet: 4000-4460 A
indigo : 4460 - 4640 A
Blue: 4640-5000 A
Green: 5000 - 5780 A
Yellow: 5780 - 5920 A
Orange: 5920 - 6200 A
Red: 6200 - 8000 A
The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion
On passing white light through a prism, the band of colours seen on a screen is called the spectrum
The cause of dispersion of white light is that light of different wavelengths travels with different speeds in a medium. When white light enters the first surface of the prism, lights of different colours due to their different speeds inglass get deviated through different angles towards the base of the prism
The part of spectrum beyond the red extreme and the violet extreme is called the invisible spectrum
Complete electromagnetic spectrum in increasing order of wavelengths:
- Gamma rays
- X rays
- UV rays
- Visible light
- Infrared rays
- Microwaves
- Radio Waves
Speed = frequency x wavelength
Gamma rays:
Frequency: above 10^19 Hz
Wavelength: Shorter than 0.1 A
X rays:
Frequency: between 3 x 10^19 to 3 x 10^16 Hz
Wavelength: 0.1 to 100 A
UV rays:
Frequency: between 3 x 10^16 to 7.5 x 10^14 Hz
Wavelength: 100 A to 4000 A
Visible light:
Frequency: between 7.5 x 10^14 to 3.75 x 10^14 Hz
Wavelength: 4000 A to 8000 A
Infrared rays:
Frequency: between 3.75 x 10^14 to 3 x 10^11 Hz
Wavelength: 8000 to 10^7 A
Microwaves:
Frequency: between 3 x 10^11 to 3 x 10^8 Hz
Wavelength: 10^7 A to 10^10 A
Radiowaves:
Frequency: Below 3 x 10^8 Hz
Wavelength: Above 10^10 A
Properties common to all electromagnetic rays:
- Do not require medium for propagation
- Travel with same speed in vacuum and air
- Exhibit properties of reflection and refraction
In refraction, when an em ray passes from one medium to another, there is change int its direction of travel, speed and wavelength, but its frequency remains unchanged
- Not deflected by electric and magnetic fields
- Transverse in nature
Gamma Rays:
(i) Most energetic electromagnetic radiations
(ii) Obtained in radioactive emissions when the nuclei of radioactive atoms pass from the excited state to the ground state. Also found in cosmic radiations
(iii) Cause fluorescence when in contact with fluorescent material(zinc sulphide). Easily penetrate through thick metallic sheets and pass through human body and cause great biological damage
(iv) Used in medical science for radiotherapy
X rays
(i) Obtained from heavy metal target of high melting point when highly energetic electron beam(cathode rays) are stopped by it
(ii) Chemically more active than UV radiation
(iii) Strongly affect photographic plate
(iv) Cause fluorescence when in contact with fluorescent materials
(v) pass through human flesh, don’t pass through bones
(vi) Used in detection of fractures in bones and teeth, CAT scans, atomic arrangement in crystals, detection of concealed precious metals
UV radiation
(i) First detected by Prof. J. Ritter in 1801
(ii) detected when silver chloride solution is exposed to electromagnetic rays starting from the red to the violet end and then beyond it, it is observed that from the red to the violet end, the solution remains almost unaffected, but beyond the violet end, the solution first turns violet and then dark brown/black
Uv rays are also known as actinic rays
(iii) UV rays can be detected by their chemical activity on dyes and photographic plates
(iv) The spectrum of UV radiations is obtained by passing the radiations through a quartz prism instead of a glass prism because glass absorbs radiations
(v) The electric arc and sparks give uv radiation. Mercury vapour lamps emit UV radiation. Sun is a main source of uv radiation
Properties of uv:
(i) UV bulbs have an envelope made of quartz instead of glass, because glass absorbs radiation
(ii) They travel in a straight line with a speed of 3 x 10^8 m/s in air/vacuum
(iii)scattered by dust particles
(iv) Obey laws of reflection and refraction
(v) affect photographic plate
(vi) produce fluorescence on striking zinc sulphide screen
Harmful effects of uv: Health hazards and skin cancer
Useful UV:
- Sterilising air, surgical equipments
- Detecting purity of germs, eggs, ghee
- Producing vitamin d in food of plants and animals
Infrared radiation:
(i) first detected by William Hershel in 1800
Detection : if a thermostat has its bulb blackened from the violent end towards the red end of the spectrum of visible light, it is observed that there is a very slow rise in temperature. But when it's moved beyond the red extreme, a rapid rise in temperature is noticed
(ii) The spectrum of ir radiation is obtained by using a rock alt prism, because a rock salt prism does not absorb infrared radiations
Sources of ir radiation: red hot objects
Uses of ir radiation:
- Therapeutic purposes
- Night photography and in foggy/misty conditions where it can penetrate easily
- Dark rooms for photograph developing
- War signal
- Remote control
Microwaves
(i)Wavelength 10^7 A to 10^10 A(1mm to 1m)
(ii) Sources: electronic devices like crystal oscillators
(iii) uses - satellite communication, analysis of molecular and atomic structures, radar communication, microwave ovens
Radio Waves
- Longest wavelength(above 1 m/10^10 A)
- Frequency below 3 x 10^8 Hz
- Uses - radar communication, television transmission
Scattering: Process of absorption and re-emission of light energy by dust particles and air molecules of atmosphere
Air molecules of size smaller than wavelength of incident light absorb energy of incident light and re-emit without change in wavelength.
I ∝ 1/wavelength^4
Violet - least wavelength, most scattering
Red - most wavelength, least scattering
Violet scattered 16 times more than red
Air molecules of size bigger than the wavelength scatter all light to same extent
Applications of scattering:
- Red colour of sunrise and sunset(scattered blue, unscattered red)
- White colour of sky at noon(directly overhead, no distance or time to scatter)
- Blue colour of sky(blue scattered more than red)
- White colour of clouds(particles bigger than wavelength of visible light)
- Red for danger signals(less deviation)