# Physics- Unit 2- Optics

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• Created by: FireDwarf
• Created on: 19-03-14 17:48
When does light bend towards the normal?
when it travels from a rare to dense material.
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What is an experiement that can be carried out to find the refractive index of a material?
Use a ray box to direct a light ray onto a a rectangular glass block at an angle of incidence.Find the angle of refraction then by drawing where the line is reflected and measuring from the normal.
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What is snells law?
n1 sini = n2 sinangler
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why does diffraction occur?
Because when the wave passes a boundary and into a new substance, the speed of the light wave is diffrent.
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What is another formula for refractive index? What do the c's mean?
The first c indicates the substance it is currently in, while the cs indicates the substance it is moving into.
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How else can we calculate refractive index (talking about speeds)
speed of light in vaccum/speed of light in that substance.
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Relationship between speed of light and refractive index?
Smaller the speed, greater the refractive index.
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What happens if the angle of incidence is less then the critical angle?
It refracts.
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What happens if the angle of incidence the same as the critical angle?
Then the light is refracted along the boundary.
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What happens if the angle of incidence is higher then the critical angle?
Light ray undergoes total internal refraction at the boundary.
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What are the two conditions needed for it to occur?
Incident substance must have a larger refractive index then the other substance. Angle of incidence exceeds the critical angle.
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What is the angle of refraction in total internal reflection?
90 degrees
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Therefore what is the relationship between the diffrent substances refractive indexs and angle of incidence?
n1sini= n2sin90 (sin 90= 1) therefore sini = n2/n1
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How does a endoscope work?
Light totally internally reflects along the boundary, even when it bends. At the boundary, the incidence angle always exceeds the critical angle. Allows for medical personale to view inside the body.
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Communication optic fibre?
Pulses of light that enters at one end, from a transmitter, to reach a reciever at the other end. Highly transparent to reduce light absorbtion.
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What is the optic fibre made up of?
core surrounded by a layer of cladding of lower refractive index.Takes place at this cladding boundary,
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Why is cladding of a lower refractive index good?
causes larger reflections of light which increases the speed it moves at.
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Why must the core be narrow?
To prevent multipath dispersion where light travelling along the axis would merge with the next pluse because it becomes longer then it should.
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What does the double slit experiement demostrate? therefore proving?
The interference of light & therefore that light is a wave.
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What does the light source have to be in the experiement?
Coherent- so that the wavelengh and frequency are simular (same phase diffrence)
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How does it work?
Light passes from the source through a single slit. It then passes through a double slit, where on the screen we observe alternate bright and dark fringes (Youngs Fringes).
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What is special about these fringes?
They are evenly spaced and parallel to the double slits.
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What is happening when a bright fringe is formed?
The light from one slit is reinforcing the light from the other slit. They are in phase and undergoing constructive interference.
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dark fringe?
Out of phase by 180 , therefore cancelling each other out.
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How do we measure the fringe seperation?
Distance from the center of a bright fringe to the center of the next bright fringe.
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What does w stand for? D? Wavelengh? s?
Fringe seperation, distance from slits to screen, wavelengh of coherent source, slit spacing.
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What is a coherent source?
Constant phase diffrence
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What happens to the fringe speration in diffrent colours of light?
It changes because the colours are of diffrent wavelenghs. Red is the largest spacing (largest wavelengh) and violet the smallest.
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Vapour lamps and discharge tubes?
Produce a monochromatic source of light because it is of one colour and therefore one wavelengh.
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Filament lamp/sun?
Large wavelengh range
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Laser light?
Highly monochromatic- can specifify eavelengh to the nanometer.
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What dont we need in the double slit experiment with a lazer?
A single slit beforehand, because the source is very monochromatic.
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WHATS THE WORD YOU NEED TO REMEMBER!?
Monochromatic
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What is the fringe pattern observed with white light?
Centeral fringe is white. Inner fringes are blue on the inner side and red on the outerside.
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What is diffraction? How is it increased?
Spreading out of waves due to passing through a slit or arund an object. Increase wavelengh or narrow the gap.
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Single slit diffraction, how do we do it?
Direct a parrellel beam of light at a single slit. Observe the patterns on a white screen.
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What does the fringe pattern look like?
Centeral fringe with further fringes either side. The intesnity of the fringes decreases away from the center. The centeral fringe is also twice as large.
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Formula for fringe seperation?
Wavelengh/width of the slit
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What is a diffraction grating?
Plate with many closely spaced parrell slits ruled onto it.
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What happens if a parellel beam is projected at it?
Light gets transmitted by the grating in certian directions only. This is because the light passing through each slit is diffracted & they only reinforce each other in certian directions and cancel each other in all others.
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What are each beams known as?
Orders. the Centeral beam is known as the zero order, with them increasing in number as we go away from it.
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How do we increase the angle of diffraction?
Increase wavelengh or decrease slit spacing.
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Equation?
dSin(angle)= n(wavelengh)
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Whatv is n?
Number of slits per meter. Calculated with 1/d (d = grating spacing).
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How do we calculate the maximum number of orders?
d/wavelengh (ROUNDED DOWN TO THE WHOLE NUMBER)
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## Other cards in this set

### Card 2

#### Front

What is an experiement that can be carried out to find the refractive index of a material?

#### Back

Use a ray box to direct a light ray onto a a rectangular glass block at an angle of incidence.Find the angle of refraction then by drawing where the line is reflected and measuring from the normal.

### Card 3

#### Front

What is snells law?

### Card 4

#### Front

why does diffraction occur?

### Card 5

#### Front

What is another formula for refractive index? What do the c's mean?