Light Y11

Question 1

What is refraction?

Answer 1

When a light ray changes direction because it crossed the boundary between two substances, causing it to change speed

Question 2

What happens when light goes into/leaves a denser substance?

Answer 2

Goes into: slows down and bends towards the normal

Leaves: speeds up and bends away from the normal

Question 3

What is snell’s law?

Answer 3

Sin i = n x sin r
∴ sin I /sin r = n
(N is the refractive index)

Question 4

What is an experiment to investigate the refraction of light?

Answer 4

• 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

Question 5

What is total internal reflection?

Answer 5

When a beam of light entering a semicircular prism goes out at the same angle it came it (image as if it were mirrored)

Question 6

What is the critical angle? How do you work it out?

Answer 6

The angle at which refraction stops and TIR starts. You see both.
Sin Critical angle = 1/n

Question 7

How is TIR used in optical fibres?

Answer 7

• 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

Question 8

What are uses of TIR in prisms?

Answer 8

• 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

Question 9

What is the law of reflection?

Answer 9

Angle of incidence = angle of reflection

Question 10

What are properties of a reflected image?

Answer 10

• same size as object
• image as far behind mirror as object
• image laterally inverted
• image is virtual

Question 11

Are light waves longitudinal or transverse?

Answer 11

Transverse