P5.3 Wave Interaction Flashcards
Ray diagrams
Show refraction of a wave at a boundary
To draw a ray diagram:
Draw a ray from the object to the lens that is parallel to the principal axis
Once through the lens, the ray should pass through the principal focus
Draw a ray which passes from the object through the centre of the lens
Some ray diagrams may also show a third ray
Lens
Shaped piece of transparent glass or plastic which refracts light
When light is refracted it changes direction due to the change in density as it moves from air into glass or plastic
Lenses are used in cameras, telescopes, binoculars, microscopes and corrective glasses
A lens can be convex or concave
Convex lens
Thicker in the middle than it is at the edges
Parallel light rays that enter the lens converge
They come together at a point called the principal focus
In a ray diagram, a convex lens is drawn as a vertical line with outward facing arrows to indicate the shape of the lens. Distance from the lens to the principal focus is called focal length
Concave lens
Thinner in the middle than it is at the edges - causes parallel rays to diverge - they separate, but appear to come from a principle focus on the other side of the lens
In a ray diagram, a concave lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens
Concave lenses always produce images that are:
Upright
Diminished (smaller than real object)
Virtual
Real image vs virtual image
Real image = image that can be projected onto a screen: forms on opposite side of lens as the object
Virtual image appears to come from behind the lens: forms on same side of lens as object
Correcting short sightedness
People who are ‘short sighted’ cannot see things that are far away
Can only see things that are close to them
This is because eye refracts the light and brings it to a focus before it reaches the retina
This is corrected by using a concave or a diverging lens
Correcting long sightedness
People who are ‘long sighted’ can see distant objects clearly, but they cannot clearly see objects that are nearby
This is because the eye refracts the light rays and they are brought to a focus having passed the retina
Corrected by using a convex or converging lens
Visible light range of the electromagnetic spectrum
Continuous range of colours
In order of increasing frequency (and decreasing wavelength) these are given as:
red, orange, yellow, green, blue, indigo, violet
(Richard of york gave battle in vein)
Each colour within the visible light spectrum has its own narrow band of wavelength and frequency
Reflection and absorption of colour
Waves can be absorbed at the boundary between two different materials
When waves are absorbed by a surface, the energy of the wave is transferred to the particles in the surface
This will usually increase the internal energy of the particles
When white light shines on an opaque object, some wavelengths or colours of light are absorbed
These wavelengths are not detected by our eyes
The other wavelengths are reflected, and these are detected by our eyes
White light is a combination of all of the colours in the visible light spectrum
For example, grass appears green in white light:
red, orange, yellow, blue, indigo and violet are absorbed by the grass
green light is reflected by the grass and detected by our eyes
Why some objects appear to be black
They absorb all wavelengths of visible light
For example, an object that appears blue in white light will appear black in red light
This is because the red light contains no blue light for the object to reflect - object only reflects blue light
Colour filters
When white light passes through a coloured filter, all colours are absorbed except for the colour of the filter
For example, an orange filter transmits orange light but absorbs all the other colours. If white light is shone on an orange filter, only the orange wavelengths will be observed by the human eye
Scattering of light
Light can be scattered from particles that are present in liquids or in the air
For example, in milk, particles will scatter the wavelengths of light in all different directions, which is why milk looks white
Very small molecules in the atmosphere scatter blue light the most - this is why the sky appears to be blue
Small water droplets in clouds scatter all of the wavelengths in white light equally in all directions - this causes some clouds to appear white
However, larger droplets in rainclouds can absorb all of the wavelengths in light so that no light gets transmitted - this is why some clouds appear black
