Flashcards in Light Y11 Deck (11)
What is refraction?
When a light ray changes direction because it crossed the boundary between two substances, causing it to change speed
What happens when light goes into/leaves a denser substance?
Goes into: slows down and bends towards the normal
Leaves: speeds up and bends away from the normal
What is snell's law?
Sin i = n x sin r
∴ sin I /sin r = n
(N is the refractive index)
What is an experiment to investigate the refraction of light?
• produce a narrow beam of light from the Ray box
• place glass block on piece of paper and draw around it
• aim ray at point half way along longer side of block
• mark the path of ray by making two small crosses on the paper
• do for both incident and emergent ray
• remove block. Where ray enters, draw normal line, and measure angles of incident and refraction
• repeat for different angles of incidence, 0 ° to 90 °.
• from here work out refractive index
What is total internal reflection?
When a beam of light entering a semicircular prism goes out at the same angle it came it (image as if it were mirrored)
What is the critical angle? How do you work it out?
The angle at which refraction stops and TIR starts. You see both.
Sin Critical angle = 1/n
How is TIR used in optical fibres?
• light enters bunch of fibres as infrared or visible light.
• it totally internally reflects, as angle incidence > critical angle as light slower in glass
• ∴ trapped inside the fibre and not refracting, until it reaches the end, when it comes out
• the fibres are made of glass w a high refractive index, but coated with glass in a lower refractive index to stop light crossing from one fibre in the bundle to another
Pro: cheaper than copper. Don't need to boost as often, as less resistance
What are uses of TIR in prisms?
• reflectors, e.g. On bikes: light is totally reflected twice, and so reverses direction
• periscope: arrange tw prissm so the light is reflected twice at 90° ∴ can see over corners, walls
• camera viewfinder: pentaprism TIRs light twice before entering eye
What is the law of reflection?
Angle of incidence = angle of reflection
What are properties of a reflected image?
• same size as object
• image as far behind mirror as object
• image laterally inverted
• image is virtual