Fiat Lux Let there be Light Kingshuk Majumdar Berea College, Berea, KY

Overview Part I:  Light and some of its properties.  Sources of light.  Different phenomena: Reflection, Refraction, Dispersion, Polarization, Interference  Blackbody Radiation Part II:  Strange properties of light  Wave-particle duality Part III:  Food for thought

 Light is an electromagnetic wave. First predicted by James Clark Maxwell (1831 – 1879). Experimentally detected by Heinrich Hertz (1857 – 1894). What is light? What are its properties?

 Light wave is transverse (wave propagation is perpendicular to electric and magnetic field oscillations).

 Light does not need a medium to propagate (different from sound wave for example). Experimentally verified by Albert A. Michelson and E. W. Morley (1880). First American to receive a Nobel prize in physics in 1907 E. W. Morley

 Speed of light c is maximum in vacuum (= 3 x 10 8 m/s) First precise measurement made by Albert Michelson.  No information can be sent at a speed more than the speed of light - one of the fundamental postulate of Einstein’s special theory of relativity in  c is a fundamental constant of nature. Examples of other fundamental constants are charge of an electron, mass of an electron, Planck’s constant, etc. Light can only be studied indirectly, that is, in terms of how it behaves.

Sources of Light: Luminous:  When something produces light it is said to be luminous. Examples: Sun, Stars, light bulbs, burning materials. Incandescent:  When light is given off as a result of high temperatures an object is said to be incandescent. Examples: Sun, a flame from a burning source, light bulbs Black Body Radiation:  The radiation given off by an object at any temperature.

Betelgeuse, the brightest star in the constellation Orion. (Produced with ESA's Faint Object Camera (FOC), Hubble Space Telescope.) A luminous, incandescent source

Light interacts with Matter: A ray of light travels in a straight line from a source until it encounters some object or particles of matter. Light that interacts with matter can be reflected, absorbed or transmitted through transparent materials.

Reflection (A) Rays reflected from a perfectly smooth surface are parallel to each other (specular reflection). (B) Diffuse reflection from a rough surface causes rays to travel in many random directions.

Law of Reflection The law of reflection states that the angle of incidence  I is equal to the angle of reflection  R. Both angles are measured from the normal, a reference line drawn perpendicular to the surface at the point of reflection.

Application of Reflection – Image Formation Light rays from the block are reflected according to the law of reflection, and reach your eyes. You see a virtual image of the block, because light rays do not actually come from the image.

Refraction When a light ray moves from one transparent material to another, such as from air to water, the ray undergoes a change in the direction of travel at the boundary. This change in direction is called refraction.

Law of Refraction Refractive index (n) of a medium c = speed of light in vacuum (= 3 x 10 8 m/s) v = speed of light in the medium (Note: v < c) Law of Refraction: n = c/v n 1 sin  I = n 2 sin  R

Two cases possible: (A) A light ray moving to a denser material is refracted toward the normal (  I >  R ). (B) A light ray moving to a less dense material from a denser material is refracted away from normal (  I <  R ).

An application of refraction

Total Internal Reflection The angle of incidence for a material that results in an angle of refraction of 90 o is called the critical angle. When the incident ray is at this critical angle or greater, the ray is reflected internally. Then it is called total internal reflection. The critical angle or water is about 49 o, and for a diamond it is about 25 o.

Application of Total Internal Reflection - Mirage Mirages are caused by hot air near the ground refracting, or bending light rays upward into the eyes of a distant observer. The observer believes he is seeing an upside down image reflected from water on the highway.

Dispersion and Color  Different colors of light have different frequencies and wavelengths.  The speed of light (c) is related to the wavelength ( ) and frequency (f) by  Visible light is the part of the electromagnetic spectrum visible to humans.  Ordinary light (white light) is composed of 7 colors: Red, Orange, Yellow, Green, Blue, Indigo, Violet. c = f

Dispersion of light with Prism

The color of an object depends on the wavelengths of light the object reflect. Each of these flowers absorbs the colors of white light and reflects the color that you see.

The flowers appear to be red because it reflects light in the 7.9 x m to 6.2 x m range of wavelengths.

Electromagnetic spectrum

Polarization For unpolarized light, the electric field vectors oscillate in all transverse directions. Light can be polarized – electric field vector oscillates only in one plane (plane polarized)

Picket Fence Analogy Polaroid Filter

Circularly and Elliptically polarized light

Optical Rotation and Polarization 1.Plane of a polarization of a plane polarized light rotates as it passes through a optically active (chiral) solution. 2. Rotation angle depends on (a) path length of the medium and (b) concentration of the chiral substance.

Interference Question: What happens when we superpose two waves? Case I: A wave of maximum intensity. (Constructive interference) Case II: A wave of minimum intensity. (Destructive interference) Young’s (Thomas Young, 1773 – 1829) double slit experiment: Animation

Blackbody Radiation Blackbody: a body that would absorb all the radiation falling on it. Blackbody radiation: Radiation emitted by a blackbody. Spectrum of light emitted by a body depends on it’s temperature. Animation Question: A red star and a blue star – which one is hotter?

Part II: Strange Properties of Light Photoelectric effect (A. Einstein got Nobel prize on this): Animation Compton effect (discovered by A. H. Compton in 1923): Animation Question: What can we learn?

Wave - particle duality Answer: Light behaves as a particle (photon) and also as a wave but not both at the same time. This is known as wave-particle duality (essence of quantum physics) Question: Is this wave-particle duality just special for light? Answer: NO!!

Wave-particle duality continued…. In 1923 de Broglie proposed (de Broglie’s hypothesis) : Matter (for example electrons) also have wave like properties. Or, This hypothesis was confirmed several years later. Wavelength = Planck’s constant / Momentum of the particle = h / p

Thought experiments with electrons Experiment 1:

Watching the electrons!! Experiment 2:

Part III: Food for thought !! Question 1: Light travels in a straight line. But, in presence of strong gravity light bends (Bending of light was first proposed by A. Einstein in his theory of gravity). So is there a contradiction? Hint: How do you define a straight line? A line that appears curved can be a straight line!!! Physics of current interest: Gravitational lensing in Astronomy

Question 2: Holograms are three dimensional images. When we see an object, we actually see the hologram (three dimensional image) of that object embedded in two dimensions (in our retina)!! Note: The correspondence between the image in our retina and our brain’s interpretation of the image is not one to one So is it that we perceive a holographic universe? Are there extra spatial dimensions (more than 3) that our brain fails to interpret? Physics of current interest: Holographic Universe, Extra dimensions

Question 3: Fundamental constants of nature: Speed of light c, Gravitational constant G, mass of an electron m e, charge of an electron e, Planck’s constant h, mass of up and down quarks m u and m d, QCD Mass  QCD, etc. A fundamental theory of our universe should explain 1.Are these constants fundamental? 2.If they are fundamental, why there are so many or so few?

Thank you all !!