Physics

PHS 5041 OpticsColours & Light


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


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


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


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


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”


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


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


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 + GreenMagenta: Red + BlueCyan: Green + Blue


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

A) Monochromatic?

B) Polychromatic?

Blue, Red, Green

Magenta, Yellow, Cyan


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

Example: Green & Magenta (Red + Blue)______& YellowRed & ______

Blue

Cyan (Blue + Green)

(Red + Green)


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


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

A) Green filter

B) Magenta filter

C) Yellow filter

Green

Red & Blue

Red & Green


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

A) A magenta light?

B) 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


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.


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

A) Blue pigment?

B) 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)


Primary pigments: Put together, they produce black colour, since they would absorb all colours and no light would be reflected. Primary pigments are Cyan, Yellow and Magenta

Secondary pigments: Product of the combination of two primary pigments

Example:_What secondary pigment would you obtain by mixinga) Cyan & Magenta

pigments?

b) Yellow & Cyan pigments?

Blue

Green


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 λv: speed of waves propagation (m/s)f : frequency of waves (Hz)λ : wavelength (m)


Waves reflect (same laws as for light):


Waves refract (same laws as for light):


Practice: A wave of red light has a frequency of 4.74 * 1014 Hz. CalculateA) Wavelength in vacuum

B) v in a medium with n = 1.52

C) λ in a medium with n = 1.52

c = f λλ = c / f

λ = 3 * 108 m/s / 4.74 * 1014 Hz λ = 6.33 * 10-7 m

n = c / vv = c / n

v = 3 * 108 m/s / 1.52 v = 1.97 * 108 m/s

n = λ / λx

λx = λ / nλx = 6.33 * 10-7 m / 1.52

λx = 4.16 * 10-7 m


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


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 fE = (6.64 * 10-34 J·s ) (1 * 1022 Hz)

E = 6.64 * 10-12 J

Physics

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