Refraction

what is refraction?

the change in direction of a wave when it passes through a boundary between mediums of different density.

what is the change in direction in refraction caused by?

the change in speed of the different parts of the wavefront as they hit the boundary.

regarding refraction, what happens if the second material is more optically dense?

the waves travel slower, the wavelength decreases, and the light bends towards normal.

regarding refraction, what happens if the second material is less optically dense?

the waves travel faster and the light bends away from the normal.

what happens when a wave refracts?

its speed and wavelength change but its frequency stays the same.

what is the refractive index of air?

1

what is the refraction index equation?

n=sin(i)/sin(r) or n=C/Cs Cs= speed of light in substance

what is snells law?

n1sin(i)=n2sin(r)

how are refractive index and critical angle related?

the larger the refractive index the smaller the critical angle.

what happens when the angle of incidence equals the critical angle?

the angle of refraction will be 90 degrees, the refrcated ray is refracted along the boundary between the two materials.

what happens when the angle of incidence is smaller than the critical angle?

the ray is refracted and exits the material.

what happens when the angle of incidence is bigger than the critical angle?

the ray undergoes total internal reflection.

what is the equation for critical angle?

sin(critical angle)=n2/n1

or sin θ(incidence) / sinθ(refraction)

how do fibre optics work?

light refracts when it enters at one end, then undergoes repeated total internal reflection against the sides of the fibre until it reaches end and is refracted out.

what are some uses of optical fibres?

communications, medical imaging.

what is an optical fibre made up of?

optically dense core tube, lower optically dense cladding, outer sheath.

when does total internal reflection occur in fibre optics?

when refractive index of cladding is smaller than refractive index of core.

what is the role of the cladding?

protect core, prevent signal degradation, keeps signals secure.

what is material dispersion?

when white light is used in optical fibres instead of monochromatic, the white light seperates into colours, disperses and beam gets wider. Violet has the shortest wavelength so it travels the slowest in the fibre.

what is modal dispersion?

when monochromatic light pulses spread out, since each wavefront has a different angle of reflection so undergoes total internal reflection a different number of times.

when is modal dispersion more prominent?

when the optical fibre core is wider. to prevent it core needs to be very narrow.

how can we reduce pulse absorption?

use an extremely transparent core.

how can we reduce pulse broadening?

narrow core, monochromatic source, use single mode fibre.