Light

• electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight.

• People make and use light for many other purposes than to see by

• Using scientific instruments, people can study light itself and learn much about the universe

Early Ideas about Light

Corpuscular Theory

• Discovered by Isaac Newton• Using a prism, he found that each color in

a beam of white light could be separated• A light consists of tiny particles that

travel in straight lines through space

Early Ideas about Light

Wave Theory

• Discovered by Christiaan Huygens• Behavior of light• In 1678, Dutch physicist, Christiaan Huygens, believed that light was made up of waves vibrating up and down perpendicular to the direction of the light travels, and therefore formulated a way of visualising wave propagation.

Early Ideas about Light

Electromagnetic Theory

• Proposed by James Clerk Maxwell• The influence that changing electric fields

and magnetic fields have on one another allows for the travel of waves

Early Ideas about LightQuantum Theory of Light

• Max Planck discovered an equation that matched experimental data about emission of light by a hot surface

• Tiny emitters of light can have only certain values of energy

• Einstein(1913) revealed that light itself is quantized

• Quanta – a light comes in a tiny packet of energy• Photons - a particle of light

Sources of Light• Natural light- comes from sources that we do not

control

• Artificial light- comes from sources that we control

Other Sources of Light• Luminescence – is emission of light by a substance

not resulting from heat; it is thus a form of cold body radiation

• Bioluminescence- chemicals within the organisms combine to produce a different chemical that has exited atoms

• Aurora- caused by the collision of energetic charged particles with atoms in the high altitude atmosphere

• Laser – produces a powerful, narrow beam of light in which all the photons have the same energy and travel in the same direction

• Wavelength- distance along a straight line from one crest of the wave to the next

• Frequency- the number of times each second that crests pass a stationary checkpoint

• Amplitude- the greatest distance of a crest from the ray

Remember!the higher the frequency, the

shorter the wavelength

Features

A wave’s energy corresponds to it’s amplitude. Thegreater the amplitude, the more energy the wave has. Theenergy of a light wave also corresponds to it’s frequency.The wavelength determines the color of the light. Just like

a wave..

c =fλWhere c – the speed of light

f – the frequency in Hz λ – the wavelength in m

Example: Find the wavelength of yellow light whose frequency is 5 x 10 Hz and of radio waves whose frequency is 1 MHz. (1MHz = 1 megahertz = 10 Hz)

Givenfrequency of yellow light (fyellow) 5 x 10 frequency of radio waves (frw) 1 MHz

Finda. Wavelength of yellow light (λyellow)b. Wavelength of radio waves (λrw)

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Solution The wavelength of the yellow light is given by the formula λ=c/f . For (a) :

λyellow = c/f = (3 x 10 m/s) / (5 x 10 Hz)

Since Hz unit is per second or s

λyellow = c/f = (3 x 10 m/s) / (5 x 10 Hz) = 6.00 x 10 m = 600 nm

Which is less than 1/1000 of a millimeter, (1nm = 10 )

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Solving for (b), let us first convert 1 MHz to Hz,Using the formula λ = c/f

λrw = c/f = (3 x 10 m/s) / (1 x 10 Hz) = 3m

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Is light wave or particle?

• The light is strictly neither

Transverse wave – vibrations are perpendicular to the motion and speed of the wave

Is light wave or particle?

• Electromagnetic radiation – refers to it’s generation from alternately oscillating electric and magnetic fields

• Light consists of electric and magnetic fields

• Violet light- has the shortest wavelength that is visible

• Red light- has the longest• Ultra Violet- waves that have wavelengths

slightly too short to be seen• X-rays – waves that somewhat shorter than

ultraviolet rays

Electromagnetic Waves

• Infrared rays – waves with wavelengths slightly longer than those of red light

• Continuous spectrum – creates from the spread of sunlight into it’s different colors by prism

• Microwaves and radio waves – have longer wavelengths than infrared waves

• Spectrometer – a device used to separate colors.

Electromagnetic Waves