Light and Sound Flashcards
what type of wave is visible light?
TRANSVERSE
what can visible light be?
reflected and refracted
what does reflection of visible light allow us?
to see most objects. light bounces off them into our eyes.
what happens when light is reflected from an uneven surface?
when light is reflected from an uneven surface, such as a piece of paper, the light reflects off at all different angle and you get a DIFFUSE REFLECTION
what happens when light is reflected from an even surface?
when light is reflected from an even surface (smooth and shiny like a mirror) then its all reflected at the same angle and you get a clear reflection.
what is the law of reflection?
- angle of incidence = angle of reflection
- this applies to EVERY REFLECTED RAY.
what is the normal?
- an imaginary line thats perpendicular (at right angles) to the surface at the point of incidence (the point where the wave hits the boundary)
- the normal is usually shown as a dotted line.
where is the angle of incidence?
the angle of incidence is the angle between:
- the incoming wave and
- the normal
where is the angle of relfection?
the angle off reflection is the angle between:
- the reflected wave and
- the normal
when are virtual images formed?
virtual images are formed when the light rays bouncing off an object onto a mirror are diverging, so the light from the object appears to be coming from a completely different place.
how is a light wave refracted?
- waves travel at different speeds in mediums of different densities.
- EM waves travel more slowly in denser medium usually. Sound waves travel faster in denser substances.
- so when a wave crosses a boundary between two substances, e.g. from glass to air, it changes speed.
what happens if a wave hits a boundary “face on”?
it slows down but carries on in the same direction
what happens if a wave hits a boundary at an angle?
- it will hit the denser layer and slow down. this leads to the wave refracting and bending TOWARDS the normal.
- the wave changes direction - its been refracted.
how do you draw a ray diagram for a refracted ray?
- start by drawing the boundary between the two materials and the normal
- draw an incident ray that meets the normal at the boundary.
- the angle between the ray and the normal is the angle of incidence (if you’re given this ray draw it carefully with a protractor)
- now draw the refracted ray on the other side of the boundary. if the second material is denser than the first, the refracted ray bends towards the normal. the angle between the refracted ray and the normal (angle of refraction) is smaller than the angle of incidence
- if the second material is less dense, the angle of refraction is larger than the angle of incidence.
what happens to rays passing through a glass block?
they are refracted twice.
PRACTICAL: refraction, using a light source and a rectangular block of a particular material resting on top of a piece of paper.
- shine a light ray at an angle into the block. some of the light is reflected, but a lot of it passes through the glass and gets refracted as it does so.
- trace the incident and emergent rays onto the paper and remove the block. you can draw in the refracted ray through the block by joining the ends of the other two rays with a straight line
- you should see that as the light passes from the air into the block (a denser medium), it bends towards the normal. this is because it slows down.
- when the light reaches the boundary on the other side of the block it passes into a less dense medium. so it speeds up and bends away from the normal. (some of the light is also reflected at the boundary).
- the light ray that emerges on the other side of the block is now travelling in the same direction it was to begin with, its been refracted towards the normal and then back again by the same amount.
what do triangular prisms do?
DISPERSE WHITE LIGHT.
- different wavelengths of light refract by different amounts, so white light (which is a mixture of all visible frequencies) disperses into different colours as it enters a prism and the different wavelengths are refracted by different amounts.
- a similar effect happens as the light leaves the prism, which means you get a nice rainbow effect.
what does every transparent material have?
a refractive index.
what does the refractive material of a transparent material tell you?
how fast light travels in that material.
what is the refractive index of a material defined as?
refractive index (n) = speed of light in a vacuum (c) / speed of light in that material (v)
n = c / v
what does Snell’s Law say?
when an incident ray passes into a material:
n = sin i / sin r
so if you know any two of n, i or r, you can work out the missing one.
PRACTICAL: find the refractive index of glass using a glass block.
- draw around a rectangular glass block on a piece of paper and direct a ray of light through it at an angle. trace the incident and emergent rays, remove the block, then draw in the refracted ray.
- you need to then draw in the normal at 90° to the edge of the block, at the point where the ray enter the block
- use a protractor to measure the angle of incidence (i) and the angle of refraction (r). remember, these are the angles made with the normal.
- calculate the refractive index (n) using Snell’s Law n = sin i / sin r
PRACTICAL: use semicircular blocks to show Total Internal Reflection
- light going from a material with a higher refractive index to a material with a lower refractive index speeds up and so bends away from the normal - e.g. when travelling from glass into air.
- if you keep increasing the angle of incidence (i), the angle of refraction (r) gets closer and closer to 90°
- eventually i reaches a critical angle C for which r = 90°. the light is refracted right along the boundary.
- above this critical angle, you get total internal reflection - no light leaves the medium.
- an experiment to demonstrate this uses a semicircular block instead of a rectangular one. the incident light ray is aimed at the curved edge of the block so that it always enters at right angles to the edge. thus means it doesn’t bend when it enters the block, only when it leaves from the straight edge.
- to inestigate the critical angle, C, mark the positions of the rays and the block on paper and use a protractor to measure i and r for different angels of incidence.
(in semicircular blocks) if the angle of incidence is LESS THAN CRITICAL ANGLE…
most of the light passes out but a little bit is internally reflected