Physics

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Optics

by Oren Lahav

The Physics of Optics!

Optics is a branch of physics that is concerned with behaviour and properties of light. It mostly cares about visible light, since it’s easiest to perceive and thus research and understand, but other electromagnetic waves are also involved- from x-rays through infrared and ultraviolet to radio and micro waves.

Light propagation:

The most basic concept of light is propagation- the way in which it travels. Light, being an electromagnetic wave, moves as a wave. Electromagnetic waves are self-propagating and thus can move through the vacuum of space and reach Earth from the sun. They are also transverse waves, which means that their oscillation is perpendicular to their direction of propagation and movement, which allows them to be polarized.

%{font-family:verdana}All wave motion can be measured with the equation $v=f\lambda$

, where v is the wave’s speed, $\lambda$ represents its wavelength, and f its frequency. The speed of light, as we all know, is constant at c = 3 * 10^8 m/s

, in a vacuum. Wavelength and frequency are thus related as $\lambda=\frac{c}{f}$ .%

Reflection and refraction:

Reflection is fairly simple- when a wave hits a surface with an incidence angle $\theta_1$ , it bounces back with a reflection angle equal to the incidence angle.

Refraction refers to the phenomena you observe when you see, say, something in water:

What happens is this: as light changes from a medium that’s not very dense (air), to a denser medium, it’s speed decreases. As a result, the angle of propagation will change according to Snell’s law: $\frac{v_1}{v_2}=\frac{n_1}{n_2}=\frac{\sin\theta_1}{\sin\theta_2}$ . The n’s represent the index of refraction, a measure that represents how much a light ray slows down in a specific medium.

Normally, when light moves from from a medium to a different medium with a lower reflective index, part of the light will be reflected and part refracted. However, if the angle is large enough, total internal reflection will occur, and all of the light will be reflected instead of refracted. This is used in fiber optics.

%{color:#a214da;font-family:verdana}*Dispersion:*% Every Pink Floyd fan knows this one! The phenomena occurs when light is refracted into a medium with a wavelength-dependent index of refraction (like a regular glass prism). This causes the different wavelengths of the light to refract at different angles, which separates light into visible colours.

Visible light is composed out of all the colours of the rainbow, each represented by a different wavelength/frequency. Separating the wavelengths results in colours. The colour spectra is a continuous spectrum, which expands well beyond the visible colours into the entire electromagnetic spectrum.

On a note of interest, rainbows happen for a similar reason- when the sunlight hits water droplets in the air, it dispreses into the different coloured wavelength which we can see.

Diffraction and polarization:

When waves hit a small slit or opening, they get diffracted:

When light waves hit a really really tiny opening (it has to be small enough to have an effect because the light’s wavelength is really small), it gets diffracted in a similar way. If there are 2 slits, the light be diffracted from the sources.

With diffraction, the circular waves interfere with each other, causes both destructive and constructive patterns where certain portions of them meet. This results in a distinct pattern of bright and dark nodes.

Another phenomenon in light is polarization. Since light waves are electromagnetic, they have a certain electric field associated with them at right angles to the direction of propagation. Polarizing filters are devices that interfere with the direction of polarization, reducing the amount of light that can pass through them- common polarizing filters are known as sunglasses.

In this picture, the left side uses a polarizing filter to eliminate some of the light and make the picture clearer.

Optical devices:

Optics is concerned with optical devices. Mirrors for reflection are one great example. Prisms used in dispresion and polarizing equipment are further examples. So are lenses.

Lenses are devices that can be used to transmit or reflect light, converging (causing the rays to come closer together) or diverging (moving rays further apart) it in the process. They’re used in cameras and glasses to help focus light.