5.3 - Optics Flashcards
What is refraction
When waves pass from one medium to another, there is a change in speed. The frequency remains CONSTANT, so the change in speed causes a change in wavelength. If the waves are approaching the interface between 2 media at an angle then the change in speed causes a change in DIRECTION as well
When do the waves bend towards the normal
When the wave travels more slowly in a medium, the wave moves towards the normal, so denser materials mean slower speeds so they bend towards the normal
What happens when a ray is crossing the interface along the normal line
It does not change direction at all, wavefronts are parallel to the edge and so wavelength is equally changed along the length of the wavefront.
What happens if the wavefront is not parallel to the interface, wavefront is at an angle
Then the part that hits the interface first will change speed first, and wavefront becomes bent because different parts of it are travelling at different speeds.
Change in direction caused by refraction are the basis for the functioning of lenses and can lead to optical illusions 😎
What’s the refractive index (absolute refractive index)
A measure of the amount of refraction causes by different materials is called the refractive index, n , it’s equal to the ratio of the speed of light in a vacuum to the speed of light in the material
In lesson we called thus ABSOLUTE refractive index
n= c/v
What is Snells law
The relationship between direction and refractive index
n1 x sin theta1 = n2 x sin theta2
The values of n1 and n2 are the refractive indices of each medium. The values of theta 1 and theta 2 are the angles that the ray of light makes to the normal to the interface between the two media at the point the ray meets the interface
How can we investigate refractive index practical
Using the equation for refractive index with experimental measurements will allow us to measure the refractive index of a material, as long as we know n for the other material. As the speed of light In air is virtually unchanged in comparison to a vacuum, we take the refractive index of air to be 1
Using a prism and laser, we can take several different measurements of the angle of incidence theta 1 and corresponding angle of refraction Theta 2. The prism/ block used should be an exact semicircle and we aim the ray to meet the glass at exaclty the midpoint of the flat side, this means ray will travel along a radius of the semi circle and will leave the semicircle along the normal to the circular edge. Only change in direction occurs along the flat side of the block
We can rearrange equation of snells law so
We plot sintheta 2 against sintheta 1 that should produce a line of best fit through the origin, the gradient is the reciprocal of n, refractive index for the glass (since semi circle was made from glass here)
What is dispersion
Splitting up of white light into a rainbow of colours by a prism
n = c/v
V = f x lamder
n = c/f x lamder
If a ray enters a prism from air (n = 1)
Our equations for snells law become
Sintheta 1 = (c x sin theta 2)/ f x lamder
Sin theta 2 = (f x lamder x sin theta 1)/ c
As the frequency stays constant through out refraction, and speed of light in vacuum must be constant, if we keep the same angle of incidence then the sine of the angle of refraction will be proportional to the wavelength. Smaller wavelengths (violet) will be closer to the normal in the glass.
On emergence from the glass prism, take care to continue with theta 1 as the angle in air
Sin theta 1 = (c x sin theta 2) / f x lamder
The sine of the angle of refraction (theta 1 on emergence) will now be Inversely proportional to wavelength. The smaller wavelengths will be further from the normal in air.
As the angles of incidence on emergence are not all the same, due to dispersion when they first entered the glass, the colour spreading effect is amplified, creating the familiar spectrum of colours
Define refraction
Refraction is a change in wave speed when the wave moves from one medium to another. There is a corresponding change in wave direction, governed by snells law
What’s the refractive index (absolute refractive ind3x)
n, can be defined in several ways, but it is fundamentally a result of the change in wave speed
n = c/v
Define snells law
n1 x sin theta 1 = n2 x sin theta 2
Can we used snells law for total internal reflection
No bro - this is not refraction as that would require a change in medium, snells law cannot apply
What happens as incidence angle changes
If i, incidence angle is less than critical angle, refraction occurs
If i, incidence angle is equal to critical angle, the ray emerges to travel right along the interface, 90 degrees to the normal
If angle of incidence is greater than the critical angle, then Total internal reflection occurs
How can we calculate critical angle for total internal reflection
From snells law,
n1 x sin theta 1 = n2 x sin theta 2
If we take medium 1 to be the optically more dense material, then theta 2 must be 90 degrees when the light is at critical angle, theta 1 = theta c in medium 1
The equation can be rearranged so
Sin theta c = n2/ n1
If the situation involves a ray emerging into air, the equation becomes
Sin theta c = 1/n1
If we know the critical angle, then that will give us the refractive index for the material
n1 = 1/sin theta c
How can we investigate total internal reflection
We can use a semi circle glass block and laser, we can steadily increase the angle of incidence within the more dense glass and observe the emerging angle of refraction along the flat/ straight side of the interface. We shall see partial reflection of the ray within the glass growing stronger in intensity, until the critical angle is reached inside the glass
The ray should hit the midpoint if the flat side of the semicircle so it’s travelling a radius in length to it and then along interface so the only change in direction occurs at the flat side of the block. By carefully recording the critical angle, when the light emerges, we can calculate the refractive index for glass
nglass = 1/sin theta c
What are some of the applications for TIR
Periscopes , reflective signs
Fibre optics - a thin glass fibre can guide light along its length by the repeated TIR at the internal edges. This may just be used for decorative lighting
But on a larger scale can be used to guide sunlight to the interior of large buildings. Alternatively, optical fibres can be used to carry information as light pulses (as in fibre broadband) or as actual images such as medical endoscope. Endoscopes send light along one optical fibre and the reflection is carried away along the other for view by medical staff
Define the critical angle
The critical angle is the largest angle of incidence that a ray in a more optically dense medium can have and still emerge into less dense medium, beyond this angle, the ray will be totally internally reflected
Define total internal reflection (TIR)
Requires two conditions to be met
The ray is attempting to emerge from the more dense medium
The angle between the ray and the normal to the interface is greater that the critical angle
What is a lens
A lens is an object made of clear material that has curved faces so that it changes the direction of light rays. The two most common types of lenses have convex or concave profiles
A convex lens causes the rays to..
Converge
A concave lens cause the rays to..
Diverge
Rays of light from a distant object will arrive at the lens parallel to eachother, what do the lenses do
A converging lens will bring these rays together at a point, called the focus, or focal point.
A diverging lens will spread these rays apart so they will not meet at a point - back tracing the rays shows that they appear to have all come from a focal point, in this case it’s referred to as a virtual focus.
But in each cause, the distance from the lens/ optical centre to the focal point is called the focal length
As a diverging lens produces a virtual focus on the same side of the lens that the rays come from, the focal length is recorded as a negative value
For symmetrical lenses, there’s a focal length of equal distance from the lens on either side depending on which direction rays are coming from
If a ray passes through the exact centre of a lens, regardless of its approach angle, it will pass through the lends undeviated, continues a straight line as if lens was not there
What is the power of the lens and how is it calculated
The distance from a lens to its focus is a measure of its strength. More powerful lenses bring the light rays together in a shorter distance - shorter focal length, power can be calculated if we know focal length
Power (dioptres, D) = 1 / focal length(m)
If focal length negative, power can be negative
P = 1/f
Eg fatter convex lenses generally have shorter focal lengths, more powerful
What happens if twin lenses are placed one after another in combination
The overall power of the combination is equal to the sum of the individual power of the lenses
P = p1 + p2 + p3 ….
Practical : how can we investigate the power of lenses
A distant object outside a window will provide parallel rays of light. If we focus an image of this object on a screen, we can easily measure the distance between the lens and screen, the focal length
By carefully recording the focal length, f, of the lens we can calculate it’s power
Define convex lens
Converge parallel rays to a focus, at the focal length from the lens
Define concave lens
Diverge parallel rays to appear to have come from a virtual focus, at the focal length back from the lens
A converging lens will…
Bring light rays closer together
A diverging lens will..
Spread light rays further apart
What’s the focus / focal point
The focal point of the lens is the point where rays incident on the lens and parallel to the principal axis will be made to meet by the refraction of the lens
Define virtual focus
The virtual focus of a diverging lens is the point where rays incident on the lens and parallel to the principle axis will appear to have come from an emergence from the lens
Define focal length
Is the distance from the centre of a lens to its focal point
What are the rays doing when an object is placed on the opposite side of a peice of paper between a convex lens (order is: object, convex lens, peice of paper)
A ray hitting the centre of the lens continues in a straight line
A ray parallel to the principal axis will pass through the focal point on the other side
A ray passing through the focus on it’s way to the lens will emerge parallel to the principal axis
The image will be real and inverted
How many rays are needed to find the position of an image
Only 2 rays are needed to find the position of an image, the third can be used for confirmation.
What lens produced a real image
A real image is common with convex lenses
What are the properties of an image
Orientation - is it inverted or upright
Diminished or magnified
Real or virtual image?
What is the equation that links the positions of the image and object with the focal length of a thin lens (what’s the lens formula)
1/object distance(m) + 1/image distance(m) = 1/focal length(m)
1/u + 1/v = 1/f
This uses the concept “real is positive” - if a diverging lens is used, it would have a negative focal length value. If a virtual image were formed on the same side of lens as object, value for V would be negative
Practical: how can we investigate the lens formula
We can alter the position of an object and measure distance u, we move the screen back and forth until the clearest possible image is formed on screen. We then measure the distance, v.
For each u and v value, we need to calculate the reciprocal - this will allow us to draw a graph of 1/v on the y axis against 1/u on x axis, which means c (y intercept) is equal to 1/f
What is magnification
The magnification is a numerical value given to measure a size comparison
It’s a ratio, no units
How can we calculate magnification (there r 2 ways)
Magnification = image size/ object size
m = h(subscript i) / h(subscript o)
How else can we calculate magnification
m = v/u
V = image distance U = object distance
What is a real image
Can be projected onto a screen
Is on the other side of the lens from the object
What is a virtual image
Cannot be projected onto a screen
Is on the same side of the lens as the object
What is magnification
Magnification is a numerical value given to measure the size comparison between image and original object
Magnification = image size/object size
What is plane polarisation
Transverse waves have oscillations at right angles (perpendicular) to the direction of motion. In many cases, the plane of these oscillations might be in one fixed orientation. - plane polarised. For electromagnetic waves, the plane of the electric fields oscillations is the one that defines its plane of polarisation.
What’s unpolarised waves
Often, Many waves travel together, with oscillations in a variety of planes. In this case, light from this source is said to be unpolarised. This is how light emerges from a light bulb, a candle and the sun.
Polarisation is only possible….
With TRANSVERSE WAVES
If a wave is polarised, it must be transverse
How do polarising filters work
Unpolarised radiation can be passed through a filter that will transmit only those waves that are polarised in a particular plane.
How can waves on a string be polarised
Waves on a string could be polarised simply by passing the string through a card with a slit in it, which will then only allow oscillations to pass through if they are in line with the slit.
How can light waves be plane polarised
For light waves, the polariser is a piece of plastic impregnated with chemicals with long chain molecules, called a Polaroid sheet.
The Polaroid filter will only allow light waves to pass if their electric field oscillations are orientated in one direction
What are crossed Polaroid’s
When two polarising filters are at right angles, so all light is blocked out - it’s polarised
Practical: how can we investigate structural stresses
We can use crossed Polaroid’s to observe stress concentrations in clear plastic samples
The first Polaroid produces polarised light, which passes into the plastic sample. Stressed areas have their molecules in slightly different orientations, and this will affect the passage of the light through the plastic. This affect varies with the colour of the light.
When the second Polaroid acts on the emerging light, some of the light will have travelled slightly more slowly through the plastic and will destructively interfere with other light waves of the same colour. Thus, depending on the degree of stress in the plastic, the colours that emerge vary.
Differently stressed areas appear as different colours through the second Polaroid.engineers use this to see stress concentrations in models of structures, and to observe how the stress concentrations change when the amount of stress changes. This allows them to alter the design to strengthen a structure in regions of highest stress.
What is polarisation by reflection and refraction
When unpolarised light reflects from a surface, such as a road, the waves will become plane polarised. The degree of polarisation depends on the angle of incidence, but it is always tending towards horizontal plane polarisation.
Light waves incident on a surface into which they can refract, such as a pond, will reflect partially horizontally polarised light, but will also transmit partially vertically polarised light into the new medium.
How does polarisation by chemical solutions work
Different parts of the plastic model have different effects on polarised light. This is the case with some chemicals, such as sugar solution. The amount of the concentration of the sugar solution varies the angle to which it rotates the polarisation of the light.
We can use Polaroid filters to analyse the strength of sugar solution, by measuring the angle at which the light polarisation emerges after passing through the solution. The colour of light depends on concentration of solution.
Define polarisation
Refers to the orientation of the plane of oscillation of a transverse wave. If the wave is (plane) polarised, all its oscillations occur in one single plane.
