CHAPTER - 6 SPECTRUM

DEVIATION The angle between the incident ray and the emergent ray is called the angle of deviation. It is represented by d

FACTORS AFFECTING ANGLE OF DEVIATION The angle of incidence at the first surface (i) The angle of Prism (A) The R.I. of the material of the prism (μ)

TYPES OF LIGHT White Light: Monochromatic Light: It’s a polychromatic light and has all the seven wavebands present in it. Monochromatic Light: A light which consists of one colour (one wavelength) only is called monochromatic light

POLYCHROMATIC LIGHT A light, which is a mixture of several colours (waveband) is called polychromatic light.

DISPERSION The phenomenon of splitting of white light into its constituent colours is known as DISPERSION

SPECTRUM The band of colours obtained on the screen, when a polychromatic light splits into component colours is called a Visible spectrum.

CAUSE OF DISPERSION When white light is incident on the first surface of a prism and enters it, light of different colours is refracted (or deviated) through different angles.

RECOMBINATION OF COLOURS

FREQUENCY AND WAVE LENGTH OF VISIBLE SPECTRUM C = λf C= speed of light. λ = wavelength of light f = frequency of light

COLOUR It is a sensation produced in the brain due to the excitation of the retina, by an E.M.Wave of some particular wavelength

violet blue green yellow orange red Color Frequency in 1014 Hz Wavelength Å violet 6.73–7.5  4000– 4460  blue 6.01–6.48  4640– 5000  green 5.19–6.01 5000–5780 yellow 5.08–5.19 5780–5920  orange 4.84–5.08  5920–6200 red 3.75– 4.84  6200–8000 

Electromagnetic Waves & the Electromagnetic Spectrum

Electromagnetic Waves Transverse waves without a medium! (They can travel through empty space)

They travel as vibrations in electrical and magnetic fields. Have some magnetic and some electrical properties to them.

When an electric field changes, so does the magnetic field When an electric field changes, so does the magnetic field. The changing magnetic field causes the electric field to change. When one field vibrates—so does the other. RESULT-An electromagnetic wave.

Electromagnetic Spectrum—name for the range of electromagnetic waves when placed in order of increasing frequency GAMMA RAYS ULTRAVIOLET RAYS RADIO WAVES INFRARED RAYS X-RAYS MICROWAVES VISIBLE LIGHT

Notice the wavelength is long (Radio waves) and gets shorter (Gamma Rays)

RADIO WAVES Have the longest wavelengths and lowest frequencies of all the electromagnetic waves.

Global Positioning Systems (GPS) measure the time it takes a radio wave to travel from several satellites to the receiver, determining the distance to each satellite.

A radio picks up radio waves through an antenna and converts it to sound waves. Each radio station in an area broadcasts at a different frequency. # on radio dial tells frequency.

(MAGNETIC RESONACE IMAGING) MRI (MAGNETIC RESONACE IMAGING) Uses Short wave radio waves with a magnet to create an image.

MICROWAVES Have the shortest wavelengths and the highest frequency of the radio waves.

Used in microwave ovens. Waves transfer energy to the water in the food causing them to vibrate which in turn transfers energy in the form of heat to the food.

RADAR (Radio Detection and Ranging) Used to find the speed of an object by sending out radio waves and measuring the time it takes them to return.

Shorter wavelength and higher frequency than microwaves. INFRARED RAYS Infrared= below red Shorter wavelength and higher frequency than microwaves.

PROPERTIES They are not affected by electric or magnetic field. They travel with the velocity of light. They obey the laws of reflection and refraction. They are least scattered by fog, mist, etc.

Their wavelength is between 0.8μm to 40μm. They do not affect ordinary photographic film. They can be detected by a thermopile or a thermometer.

You can feel the longest ones as warmth on your skin Warm objects give off more heat energy than cool objects.

Thermogram—a picture that shows regions of different temperatures in the body. Temperatures are calculated by the amount of infrared radiation given off. Heat lamps give off infrared waves. Therefore people give off infrared rays.

VISIBLE LIGHT Shorter wavelength and higher frequency than infrared rays. Electromagnetic waves we can see. Longest wavelength= red light Shortest wavelength= violet (purple) light

Shorter wavelength and higher frequency than visible light ULTRAVIOLET RAYS Shorter wavelength and higher frequency than visible light Carry more energy than visible light

PROPERTIES They are e.m.waves and are not effected by electric or magnetic field. They travels with the velocity light. The range of these radiations is between 4000A to 100A They produce chemical effect in sliver.

PROPERTIES They obeys the laws of reflection and refraction. They are absorbed by the atmosphere and convert oxygen to ozone. They produced fluorescence in substance like zinc sulphide, barium sulphide, etc.

Too much can cause skin cancer. Use sun block to protect against (UV rays)

Causes your skin to produce vitamin D (good for teeth and bones)

X- RAYS Shorter wavelength and higher frequency than UV-rays Carry a great amount of energy Can penetrate most matter.

PROPERTIES X-Ray are e.m.wave of very short wavelength ranging from 10-12m to 10-10m. They are not effected by electric and magnetic field. They effect the photographic plate intensely.

PROPERTIES They travel in vacuum with the speed of light. They ionize the gas through which they pass. X-ray undergo reflection and refraction.

Bones and teeth absorb x-rays Bones and teeth absorb x-rays. (The light part of an x-ray image indicates a place where the x-ray was absorbed)

Too much exposure can cause cancer (lead vest at dentist protects organs from unnecessary exposure)

Used by engineers to check for tiny cracks in structures. The rays pass through the cracks and the cracks appear dark on film.

GAMMA RAYS Shorter wavelength and higher frequency than X-rays Carry the greatest amount of energy and penetrate the most.

Properties Have velocity equal to that of light. Have high penetrating power. They can penetrate through several centimeters thick iron and lead blocks. They have got small ionizing power. They can effect a photographic plate.

Properties They can produce heating effect in the surface exposed to them. They are not deflected by electric and magnetic fields. They knock out electrons from the surface on which they fall.

Used in radiation treatment to kill cancer cells. Can be very harmful if not used correctly.

Exploding nuclear weapons emit gamma rays.

Rayleigh Scattering When a photon penetrates into a medium composed of particles whose sizes are much smaller than the wavelength of the incident photon, the scattering process is elastic and is called Rayleigh scattering. In this scattering process, the energy (and therefore the wavelength) of the incident photon is conserved and only its direction is changed. In this case, the scattering intensity is proportional to the fourth power of the reciprocal wavelength of the incident photon. The scattering of electromagnetic radiation by particles with dimensions much smaller than the wavelength of the radiation, resulting in angular separation of colors and responsible for the reddish color of sunset and the blue of the sky.

Light Scattering When light encounters matter, matter not only re-emits light in the forward direction (leading to absorption and refractive index), but it also re-emits light in all other directions. This is called scattering. Light scattering is everywhere. All molecules scatter light. Surfaces scatter light. Scattering causes milk and clouds to be white and water to be blue. It is the basis of nearly all optical phenomena. Scattering can be coherent or incoherent.

Mie Scattering .                                                                                                           

Figure 15.8: Because of the selective scattering of radiant energy by a thick section of atmosphere, the sun at sunrise and sunset appears either yellow, orange, or red. The more particles in the atmosphere, the more scattering of sunlight, and the redder the sun appears. Fig. 15-8, p. 420