08 Electromagnetic Radiation Flashcards
What are the different types of radiation?
-radio waves
-microwaves
-infrared radiation
-visible light
-ultra violet
-x-rays
-gamma rays
What does visible light form part of?
The electromagnetic spectrum.
What is the only part of the electromagnetic spectrum that our eyes can detect?
Visible light
What are the similarities between the different parts of the electromagnetic spectrum?
-all travel at the speed of light in a vacuum
-all transverse waves
-all massless
-all can propagate through a vacuum
What is the law of reflection?
Angle of incidence = angle of reflection
What are the angles of incidence and reflection measured between?
The ray and the normal
When light travels from air into glass, where is the light refracted?
Towards the normal
When light travels from glass into the air, where is the light refracted?
Away from the normal
What happens if the light incident is perpendicular to a glass block?
It continues in a straight line without refracting.
What is refraction?
Light incident upon a glass block at an angle changes direction.
What happens when a light incident upon a glass block at an angle.
It changes direction (refraction).
How does light leave a rectangular glass block?
Parallel to the ray entering.
How do you find the refractive index of a substance?
(Speed of light in a vacuum) / (speed of light in the substance)
What is the speed of light?
3x10⁸m s‐¹
What is the unit for refractive index?
Nothing
What is the value for refractive index always between?
1.00 and 4.05
When light passes from a low refractive index to a high refractive index, how does it refract?
Towards the normal
When light passes from a low refractive index to a high refractive index, what happens to its speed?
It decreases
When light passes from a low refractive index to a high refractive index, what happens to its wavelength?
It reduces
When light passes from a low refractive index to a high refractive index, what happens to its frequency?
It’s constant
When light passes from a high refractive index to a low refractive index, how does it refract?
Away from the normal
When light passes from a high refractive index to a low refractive index, what happens to its speed?
It increases
When light passes from a high refractive index to a low refractive index, what happens to its wavelength?
It increases
Who discovered the relationship between angle of incidence and angle of refraction?
Willieboard snellius
Snell’s law of refraction:
When light travels through air, what should you assume the refractive index of air to be and why?
1.00 because light in air almost travels as fast as light in a vacuum.
What is the uncertainty in the angle measurement equal to?
The precision of the protractor
What is the definition of a critical angle?
The critical angle is equal to the incidence angle within a substance which results in a refracted ray having an angle of refraction of 90°.
The actual critical angle for a red light is lower than a blue light.
Explain why TIR is more likely to happen when using a red light?
There are more angles between the lower critical angle and 90° than the higher critical angles.
If the incident angle is equal to the critical angle, what is the angle of refraction?
90°
During TIR what does the reflected angle look like?
Identical to the angle of incidence
When the incident angle exceeds the critical angle, what occurs?
Total internal reflection (TIR)
What must happen for total internal reflection to occur?
- The angle of incidence must me greater than the critical angle.
- The ray of light must be travelling from a high refractive index to a low refractive index.
How can you calculate refractive index using the critical angle formula?
Sin 90 = 1 so
Sin θc = n2/n1 or
Sin θc = 1/n1
What is an optical fibre?
A core of glass of a high refractive index surrounded by cladding made of glass of a lower refractive index.
How does light travel along the optical fibre?
By undergoing total internal reflection.
What is an optical fibre made up of?
Cladding (low refractive index) and a core (high refractive index).
What is the purpose of a core in an optical fibre?
The light travels along the core undergoing total internal reflection at the boundary between the core and the cladding.
What is the purpose of the cladding in an optical fibre?
- the cladding prevents light from crossing over between two touching fibres
- it prevents damage to the core (which could cause light to escape).
- if the optical fibre is being used for communication the cladding keeps the information secure so preventing fraud.
-increases the critical angle which reduces moral dispersion.
What does this graph show?
How the refractive index of a step index optical fibre changes.
Describe the refractive index of the core and the cladding using the graph.
- The core of the fibre has a constant refractive index.
- The cladding has a lower, but also constant, refractive index.
Explain what is modal dispersion (multipath dispersion)?
The extra distance that some light beams take.
What is pulse broadening?
Modal dispersion increasing the duration of a light pulse traveling along an optical fibre.
What does modal dispersion and pulse broadening cause?
The pulses to overlap therefore smearing which distorts the information.
How can modal dispersion be reduced?
By making the core of an optical fibre very thin.
Why does material dispersion occur?
As not all frequencies are refracted by the same angle in the fibre.
Why does chromatic dispersion occur?
As different frequencies travel at different speeds.
Which travels faster? Blue light or red light?
Red light
What can light travelling through a fibre be reflected by?
Impurities
What are the advantages of a narrow core in a fibre optic?
•loses less light
•increases the probability of TIR
•light is less likely to fall below the critical angle.
•improves data transfer rate.
Give 2 examples of uses of optical fibre?
- Cables made of optical fibres are used for Internet communications. This allows data to be transmitted at higher rates.
- An endoscope uses a bundle of optical fibre to produce images inside of the body. This enables a more accurate medical diagnosis.
What are coherent sources of light?
They both have the same constant frequency and constant phase angle.
What safety precautions should you use when working with lasers?
•don’t shine lasers into anyone’s eye
•wear laser safety glasses.
What is monochromatic light?
Light of only one specific wavelength (frequency).
What is fringe spacing (w)?
The distance from the centre of one bright fringe to the centre of the next .
In youngs double slit experiment, what would be seen on the screen?
Fringes
In the fringe spacing formula, what is ‘D’?
Distance to screen
Wn the fringe spacing formula, what is ‘s’?
Slit spacing.
What is the wavelength of red light?
700nm
What is the wavelength of blue light?
400nm
How will the fringe pattern change if the red light was replaced by a blue light in a double slit experiment?
•fringes will move closer together.
•central fringe doesn’t move
•fringes will now be blue.
What is this graph? (Double slit)
Intensity graph for double slit experiment.
What do you notice about the width of each fringe in an intensity graph for the double slit experiment?
They are equal in width.
In the double slit experiment, why is the central fringe brighter than all the other fringes?
The further the light has to travel , the more it attenuates. The central fringe is when the path difference is 0 so it has to travel the least.
In the double slit experiment, explain why in reality the minimas of an intensity graph does not reach the x-axis?
One ray of light has to travel further, which means it attenuates more, they don’t cancel eachother out perfectly so the destructive interference isn’t perfect.
What does attenuate mean?
To make something less or weaker.
What property of laser light makes the double slit experiment possible?
Coherent
Is a white light from an LED a coherent beam?
No.
When using a white LED in the double slit experiment, what must you do to create a single point source?
Position a single slit after the lamp.
When using a white LED in the double slit experiment, why must the single slit be narrow?
•it allows the wavefront to hit the double slit sources so that they emit light in phase (coherent).
•it produces a wide diffracted beam which lights up both slits.
Describe the appearance of interference fringes produced by a white light source?
- the central bright fringe is white
- the side fringes are continuous spectra
- the side of a fringe nearest the centre is blue/violet
-the side of a fringe furthest from the centre is red.
What is the beam going straight through the grating?
The zero order.
How is the zero order labelled?
n = 0
At the zero order (n=0) what is the path difference?
0λ
On either side of the central maxima, what is there?
The first order diffracted maxima (n=1), then the second order (n=2), etc.
What type of number are the visible maxima always?
Odd
If there are 7 maxima visible on the screen, how many visible orders are there? Why?
- N=0 doesn’t count as a visible order and up to n=3 is visible.
What is a diffraction grating?
A piece of optical equipment that creates a diffraction pattern when it diffracts.
When shining light through a grating, for the rays of light shown to produce a constructive interference beam, what must the path difference be?
It must be equal to an integer number of wavelengths. eg. 0λ, 1λ, 2λ … nλ.
The beams of light must be coherent.
What is total internal reflection? (TIR)
When the incident angle exceeds the critical angle.
What does each element produce with coloured lines at specific wavelengths?
Emmission spectrum
Explain an emission spectrum?
An element emits certain specific colours. The grating spreads these colours out depending on their wavelength. Hence the line spectra appear as a series of coloured lines at specific wavelengths.
Explain absorption spectrum?
Shine a white light through a container of the element. The exact same wavelengths shown on the emission spectrum are now absorbed producing dark lines on a continuous spectrum background.
What is like an elements finger print?
A line spectrum.
When high density, hot matter goes through a diffraction grating what does it create?
A continuous spectrum.
When hot gas goes through a diffraction grating, what does it create?
An emission spectrum
When cold gas goes through a diffraction grating, what does it create?
Absorbtion spectrum
Why can a diffraction grating split light into the colours that it is made up of?
Because light of different wavelengths are diffracted by different angles.
What does the spectra of stars appear as?
An absorption spectra
Why does a spectra of stars appear as an absorption spectra?
Because specific colours are absorbed by atoms in the outer atmosphere.
How can someone determine a stars chemical composition?
By comparing the stars spectrum with the spectra of elements.
Which of these elements are in the star?
•Helium
•Hydrogen
