P5.3 Flashcards
What can different substances do to electromagnetic waves
- absorb
- transmit
- refract
- reflect
They do these in ways that vary with wavelength
What types of diagrams are used to show reflection and refraction?
Ray diagrams
How do you draw a ray diagram
- using a ruler and pencil, draw lines to represent rays
- draw the normal at 90 degrees to the surface at the point the rays hits
- measure the angle between the normal and the rays
- add arrow to show direction
In reflection what is the angle of incidence equal to?
Angle of reflection
Why are electromagnetic waves refracted
When light goes from air into glass or a denser medium at an angle, it slows and bends towards the medium.
When in the change in direction higher
If the difference in density is higher
How is reflection used in terms of radio waves?
The earth is curved so to send radio waves long distances you have to reflect them from a layer of the atmosphere called the ionosphere. Itβs a low frequency wave so itβs less that 30MHz and has long wavelengths.
What are high frequency waves and what are they used for?
They have a frequency of over 30MHz and are used for satellite communications through the atmosphere due to their short wavelength.
What do wavelengths determine?
Itβs determines what happens to the electromagnetic wave
What do walls transmit and absorb?
They can transmit radio and microwaves BUT absorb visible light and ultraviolet (you donβt get sunburnt in a building and very slowly in a greenhouse)
What does a plastic bin bag absorb and transmit?
Itβs absorbs visible light BUT transmit infrared.
What does the atmosphere absorb ?
- absorbs X -rays and there are no natural X- ray sources
- absorbs Gamma rays from the Sun and space BUT gamma rays can be detected from rocks in your house
How do lenses form images?
They refract light which changes its direction
What is the focal length?
- Distance form centre of lens to the principal focus
- principal focus is on each side of lens
What is a concave lens, and where is its axis and virtual principal focus?
- concave (diverging) lens caves inwards
- causes the parallel rays of light to diverge (spread out)
- axis of lens = line passing though the middle of the lens
- principal focus (focal point) of concave = point where rays hitting the lens parallel to the axis appear to come from (all meet up at point behind lens)
Why can you not set fire to anything with a concave lens?
- concave lens = spreads light out
What is a convex lens, and where is its axis and principal focus?
- convex lens (converging) bulges outwards
- causes parallel rays of light to converge (move together)
- principal focus is where rays hitting the lens parallel to axis all meet
What are the two types of images that lenses can form?
- REAL image = light from an object comes together to form an image on a βscreenβ (e.g. image formed on an eyes retina)
- VIRTUAL image = rays are diverging so the light from object appears to be coming from a completely different place (e.g mirror = virtual image)
Give examples of convex lens are the type of image they form:
- magnifying glass = virtual,magnified , upright
- camera,eye = real, diminished, inverted
- projector = real,magnified, inverted
- microscope / telescope = lens near object - real and inverted/ lens near eyes - virtual and upright
Give examples of concave lens are the type of image they form:
- spy holes in door = virtual, diminished and upright
- back windows of coaches = virtual, diminished and upright
How do you draw a ray diagram for an image through a concave lens?
- pick point at to of image
- draw ray going from object to lens parallel to axis of lens
- draw ray from top of object to right through middle of lens
- incidence ray thatβs parallel to axis is refracted so appears to have come from principal focus (F)= dray ray from F to parallel line (make it dotted till lens = virtual here)
- ray passing through middle of lens doesnβt bend and mark where this ray meets virtual ray (dotted line) = top of image
- repeat for a point on the bottom of image (if bottom of object on axis, then image is also on axis)
What kind of image do concave lens produce?
- virtual image
- right way up, smaller than object and on same side of lens as object no matter where object is
How are concave and convex rays represented in ray diagrams?
Y = concave lens
β
β = convex lens
β
How do you draw a ray diagram for an image through a convex lens?
- pick point at top of object and draw ray from it parallel to axis
- draw another ray from top of object going through middle of lens
- incident ray thatβs parallel to axis is refracted through F
- ray passing middle doesnβt bend
- where rays meet = top of image
- repeat for bottom of object (if object bottom = axis then image = axis)
What does the distance form the lens to the object effect and how?
- distance from lens to object effects size and position of image
- an object 2F (2 focal lengths) from lens = real, inverted image same size as object and at 2F on other side of lens
- an object between F and 2F = real,inverted image bigger than object beyond 2F
- object nearer F makes virtual image the right way up, bigger than object on same side of lens
What happens in short sightedness?
- short sighted = canβt focus on distant objects
- eye has convex lens to focus incoming light onto back of retina = where image formed
- if eyeball too strong/long = lens canβt produce focuses image on refine and image of distant objects are brought into focus in front of retina = image is blurry
How can we correct short sightedness?
- concave lens
- correct it = put concave lens in front of eye = diverges light before enters the eye = focused onto the refine = produce sharp, clear image
What happens in long sightedness?
- long sighted = canβt focus on near by object
- lens is too weak / eyeball is too short
- images of near objects are brought into focus behind the back of the eye = images are blurry
How can we correct short sightedness?
- convex lens
- correct = convex lens put in front of eye = light starts to converge before entering the tee = can be focused on retina = clear, sharp image
What do triangular prisms disperse?
- White light
- Diff wavelengths (colours) of light travel at diff speeds so they refract by diff amounts
- So white light through prism = rainbow
How do triangular prisms disperse white light?
- light bends towards the normal as it enters the prism as glass is denser than air = diff wavelengths/colours of light bend by different amounts (red bends least and violent bends most)
- light bends away from the normal as it leaves a prism and different colours bend by different amounts and due to the prismβs shape this spreads the wavelengths out even more
- on far side of prism= spectrum (rainbow)
How can you investigate refraction of a light using a prism?
- need light source, coloured filters and triangular glass prism on piece of paper
- place red filter in front of ray box and shine a thin light beam into prism at an angle to the normal (some light will be reflected)
- trace the incident and emerging rays onto paper and remove prism
- draw refracted ray by joining ends of the other two rays with a straight line (could measure incidence and refraction angle)
- repeat with blue filter (keep incidence angle same)β> blue light refracts more at each boundary
- repeat with more colours/wavelengths (shorter wavelength = more refraction) OR repeat with white so it disperses
- could try changing the shape or material of prism
What happens to opaque objects when light waves hit them
- donβt transmit light and when visible light hits them, they absorb some wavelengths of light and reflect others
What does the colour of an opaque object depend on?
- depends on which wavelengths of light are reflected
- red apple = red as wavelengths corresponding to the red part of spectrum are reflected
What colours canβt you make?
- red, green and blue (primary colours)
- banana looks yellow because itβs reflecting yellow or red and green light
What happens when wavelengths hit white and black objects?
- white objects = reflects all wavelengths of visible light equally
- black objects = absorbs all wavelengths of visible light (black = lack of visible light p)
What happens when wavelengths hit transparent and translucent objects?
- transparent (see through) and translucent (partially see through) transmit light
- not all light hitting surface is absorbed/reflected = some can pass through objects
What happens when waves are reflected or absorbed off transparent/translucent objects?
- some wavelengths that may be absorbed or reflected by translucent/(more less likely) transparent objects = these objects appear to be the colour of light that corresponds to the wavelength most strongly transmitted by objects
How do colour filters work?
- only let particular Ξ» through and filter out diff Ξ» of light = only certain colours (Ξ») are transmitted and the rest are absorbed
What colour does a primary colour filter transmit?
- only transmits that colour (white light on blue = only blue light let through and rest of the colours are absorbed)
- look at blue obj through blue filter = looks blue as blue is reflected from objectβs surfer and transmitted by filter
What happens when a blue filter is placed on a red object?
- obj appears black through blue filter = all light reflected by object will be absorbed by the filter
What colour does a non primary colour filter transmit?
- let through both Ξ» of light corresponding to that colour and the Ξ»of the primary colours that can be adde together to make that colour
- e.g. cyan = blue + green so cyan colour filter lets through all Ξ» of light corresponding to cyan, blue and green
What do filters do to white light?
- they take away colours from light NOT add colours to light
What happens in specular reflection?
- light bounces off objects into our eyes
- Specular reflection = light rays reflect off smooth surfaces all in the same direction = giving a clear reflection (light on mirror)
What happens in scattered reflection?
- light reflects off rough surfaces in all directions = each day hits surface at different angles so if scatters the light
- because normal is different for each incident ray
- if light reflected by rough = surface looks Matt = no clear reflection
How can you investigate reflection using a ray box and a mirror?
- take piece of paper and draw solid line across then dotted at 90 degrees (normal)
- place plane (flat) mirror lining with solid line
- using ray box, shine thin beam of white light at the mirror so light hits mirror where normal meets mirror
- trace incident and reflected light rays
- measure angle between incident and normal and the angle between reflected ray and normal
- repeat using varying angle of incidence (incidence always = reflection)
- repeat for diff colours by using colour filters (any colour always should stay same)
- keep test fair by keeping same mirror, same width, brightness of beam
Why does: Milk look white Ink looks black Clouds look white or black Sky looks blue
- milk = white because particles scatter all Ξ»
- ink = black beacuse particles absorb the Ξ»
- clouds = some particles absorb and some scatter Ξ»
- sky = molecules in atmoshere are very small and scatter light with short wavelengths such as blue
For the investigation including a ray box and a mirror, why may a systematic error occur and why may someoneβs results be more consistent and how will different colours have an affect on the results?
- surface of the mirror may not be flat or the normal may be drawn incorrectly
- using a sharper pencil or the ray box producing a narrower ray of light causes more consistent results
- coloured light has no affect as angle of incidence is equal to angle of reflection
Why happens when u shine red light on a blue cube?
- appears black
- red wavelengths only transmitted by red filter and rest absorbed
- only blue filter reflected by cube and rest absorbed
- so therefore all light reaching the cube is absorbed = looks black
What colour comes out of the ray box when you place a red and blue filter?
None as both together would absorb the light
What does the focal length depend on?
Thickness of the lens
- fatter lens = shorter focal length as light spends more time inside the lens and thus refracts more
How do you know if an image is virtual?
- itβs virtual if the top of the image is where the rays appear to come from
When you shine a torch on a wall in a dark room, why can everyone see it?
- wall = rough so the light from the torch is scattered from the surface (diffused scattering)
- thus wherever you are in the room, torch will reflect from each part of the rough surface into your eye
What are the advantages of optical fibres?
- rapid transmission rate of data
- multiple signals at once
- output signal is clear
- noise is not recognised
- interference is not recognised
