Optics Flashcards
Review key terms, formulae, experiments and typical exam questions for the optics module.
What is the refractive index of a material?
The ratio of sini/sinr for light travelling from a vacuum into that material.
Why does refraction occur?
Refraction occurs because the speed of light is different in each substance, the amount of refraction depends on the speed of the wave in each substance.
The refractive index is given by ns=c/cs what does this show?
The smaller the speed of light in a substance the greater the refractive index.
Define the terms in n1sin1=n2sin2.
refractive index of material 1, sine of incident angle, refractive index of material 2, sine of refracted angle
How does light refract when entering a more optically dense material?
It refracts towards the normal.
How does light refract when entering a less optically dense material?
It refracts away from the normal.
What is total internal reflection?
When light reflects inside a material.
What are the requirements for total internal reflection?
1.The incident substance has a larger refractive index than the other substance. 2.The angle of incidence exceeds the critical angle.
How do we calculate the critical angle of a substance?
The refracted angle is 90 degrees using Snell’s Law: n1sin1=n2sin2 sin1 = n2/n1 (as sin2=sin90 = 1)
How do optical fibres work?
The light ray is totally internally reflected each time it reaches the fibre boundary even when the fibre bends. At each point where the light ray reaches the boundary the angle of incidence exceeds the critical angle.
Why do optical fibres need to be narrow and have core cladding?
Total internal reflection occurs at the core cladding boundary, if there was no cladding the light would cross from one fibre to another leading to a signal not being secure. The core has to be narrow to prevent multipath dispersion - this is when the light ray can reflect a different amount of times before reaching the end of the fibre.
What is monochromatic light?
Light of a single wavelength
What is coherent light?
Light waves with constant phase difference and the same frequency.
Describe Young’s Double Slit experiment.
Light was passed through two slits and then an interference pattern (areas of light and dark) was viewed on a screen opposite.
Why are bright fringes formed in Young’s Double Slit Experiment?
The light from each slit reinforces and the light arrives at the screen in phase.
What interference pattern is produced when white light is shone through double-slits instead of monochromatic light?
1) A central white fringe where every colour contributes. 2) Further fringes are a spectrum with blue on the inner side and red on the outer side where wavelengths diffract by different amounts and wavelengths do not overlap.
What pattern does a diffraction grating give?
Bands of light opposite the grating starting with a central band and then moving outwards. It can also split visible light into its constituent colours (CDs do this to light as they act like a diffraction grating).
How is a line emission spectra formed?
A glowing gas emits light at specific wavelengths. The wavelengths of the lines are characteristic of the chemical element that produced the light. The emitted light is shone through a prism and displayed as a spectrum with fine lines corresponding to the wavelengths.
How is a line absorption spectra formed?
A continuous spectrum of light is shone through a glowing gas. The gas absorbs light of specific wavelengths so the transmitted light is missing these wavelengths. The light is shone through a prism and a spectrum shows dark lines where the elements have been absorbed.
