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Physics. PHS 5041 Optics Colours & Light. PHS 5041 Optics Colours & Light. Dispersion of light: Decomposition of white light into the colours of the rainbow. PHS 5041 Optics Colours & Light.

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PHS 5041 OpticsColours & Light

Dispersion of light: Decomposition of white light into the colours of the rainbow.

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PHS 5041 OpticsColours & Light

Primary colours: Blue, green and red. Each of them is light of only one value of wavelength (monochromatic light)

White light is polychromatic light because it is composed of all colours (primary and secondary), therefore, more than one value of wavelength

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PHS 5041 OpticsColours & Light

Visible spectrum: Band of colours or radiation produced by the dispersion of white light.

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PHS 5041 OpticsColours & Light

Each color is refracted differently as white light travers a prism. Thus, is like the prism (crown glass) would have a different refraction index for every colour.

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PHS 5041 OpticsColours & Light

Light recomposition: Dispersed white light that is “put together” once again, by using a second prism.

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PHS 5041 OpticsColours & Light

Chromatic aberration: Blurred image, composed of several images of different colours, as a result of light dispersion.

_It is corrected using a system of lenses (converging & diverging lenses), which is in turn called “achromatic lens”

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PHS 5041 OpticsColours & Light

Primary rainbow:

  • Incident ray is refracted (1st)
  • Refracted ray suffers total internal reflection (TIR)
  • TIR-ed ray refracted (2nd), which favors light dispersion (colours visible)
  • Observer must be located between Sun and rainbow
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PHS 5041 OpticsColours & Light

Colour perception:

  • Relative to the observer (need of adjectives: sky blue, cherry red, etc)
  • Rods & Cones (light sensitive cells in retina)
  • Rods detect whether or not there is light (images in black & white)
  • Cones distinguish colours

Colour blindness:Absence of, or faulty response of certain cones, thus impossible to distinguish colours

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PHS 5041 OpticsColours & Light

Secondary colours: Colours obtained by the combination of primary colours of equal intensity.

Each of them is light of more than one value of wavelength (polychromatic light)

Yellow: Red + Green

Magenta: Red + Blue

Cyan: Green + Blue

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PHS 5041 OpticsColours & Light

Practice:Of the colours blue, magenta, red, yellow, green and cyan; which are:

  • Monochromatic?
  • Polychromatic?

Blue, Red, Green

Magenta, Yellow, Cyan

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PHS 5041 OpticsColours & Light

Complementary colours: Primary and secondary colours that can combine to produce white light.

Example:

Green & Magenta (Red + Blue)

______&Yellow

Red & ______

Blue

(Red + Green)

Cyan

(Blue + Green)

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PHS 5041 OpticsColours & Light

Light filters: Absorb all colours, and transmit only the desired one/s.

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PHS 5041 OpticsColours & Light

Practice:What primary colour/s do the following filters transmit when white light is directed through them?

  • Green filter
  • Magenta filter

C) Yellow filter

Green

Red & Blue

Red & Green

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PHS 5041 OpticsColours & Light

Practice:What colour does a cyan filter transmit if we direct through it:

  • A magenta light?
  • A yellow light?

C) A red light?

Blue. Cyan only transmits Blue & Green, so the Red component of Magenta would be absorbed

Green. Cyan only transmits Blue & Green, so the Red component of Yellow would be absorbed

Black. Cyan only transmits Blue & Green, so the Red light would be absorbed

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PHS 5041 OpticsColours & Light

Colour of objects: An opaque object would appear of a given colour because its surface absorbs all other colours, and reflect only the one we “see”.

Pigment: Actual “painting” that covers an object. Absorbs all other colours, except that of the “painting”.

*The colour of an object, since it is a “perception”, depends on the pigment that covers it as well as the light that falls on it.

Example: An object coated with green pigment would appear green if yellow light falls on it, or black if magenta light falls on it.

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PHS 5041 OpticsColours & Light

Practice:What colour does the eye see if a cyan light is used to illuminate an object coated with:

  • Blue pigment?
  • Red pigment?

C) Yellow pigment?

Blue. Cyan light made of Blue & Green, and Blue pigment absorbs all but Blue, Blue is the only light is reflected (Green light is absorbed)

Black. Cyan light made of Blue & Green, and Red pigment absorbs all but Red, NO light is reflected (Blue & Green lights are absorbed)

Green. Cyan light made of Blue & Green, and Yellow pigment absorbs all but Red and Green, Green is the only light reflected (Blue light is absorbed)

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PHS 5041 OpticsColours & Light

Primary pigments: Put together, they produce black colour, since they would absorb all colours and no light would be reflected. Primary pigments areCyan,YellowandMagenta

Secondary pigments: Product of the combination of two primary pigments

Example:

_What secondary pigment would you obtain by mixing

  • Cyan & Magenta pigments?
  • Yellow & Cyan pigments?

Blue

Green

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PHS 5041 OpticsColours & Light

Light: Electromagnetic wave whose velocity is maximum in vacuum and decreases as the optical density of the medium through which it propagates increases.

Periodic waves: Waves emitted at regular intervals

v = f λ

v: speed of waves propagation (m/s)

f : frequency of waves (Hz)

λ : wavelength (m)

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PHS 5041 OpticsColours & Light

Waves reflect (same laws as for light):

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PHS 5041 OpticsColours & Light

Waves refract (same laws as for light):

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PHS 5041 OpticsColours & Light

Practice:A wave of red light has a frequency of 4.74 * 1014 Hz. Calculate

  • Wavelength in vacuum
  • v in a medium with n = 1.52

C) λin a medium with n = 1.52

c = f λ

λ = c / f

λ= 3 * 108 m/s / 4.74 * 1014 Hz

λ = 6.33* 10-7 m

n = c / v

v = c / n

v = 3 * 108 m/s / 1.52

v = 1.97* 108 m/s

n = λ / λx

λx= λ / n

λx= 6.33 * 10-7 m / 1.52

λx= 4.16* 10-7 m

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PHS 5041 OpticsColours & Light

Electromagnetic radiation consists of particles that are consistent with both models of light (particle & wave). These particles are called photons.

Quantum: Energy associated to a given photon

E = h f

E: energy of the photon (J)

f : frequency of wave associated with the photon (Hz)

h: Plank’s constant (6.64 * 10-34 J·s)

*Photons are produced as excited electrons “jump back” to their energy levels

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PHS 5041 OpticsColours & Light

Energy of a wave increases in the following order:

Radio waves < IR < Visible (red < orange < yellow < green < blue < violet) < UV < X-rays < Gamma rays

Practice: Calculate the energy of a photon of a gamma ray (f = 1022 Hz)

E = h f

E = (6.64 * 10-34 J·s ) (1 * 1022 Hz)

E = 6.64 * 10-12 J