8. Waves - optics

Question 1

What type of wave are light waves?

Answer 1

Transverse

Question 2

Why does aluminium reflect light well?

Answer 2

Smooth, shiny surface, gives clear image as reflect light regularly

Question 3

What is the law of reflection?

Answer 3

angle of incidence = angle of reflection

i=angle of incidence
r=angle of reflection

Question 4

What is a normal?

Answer 4

imaginary line 90’’ to surface of object

Question 5

what is lateral inversion?

Answer 5

when left appears to be right and right appears to be left in a mirror

Question 6

what is refraction?

Answer 6

• change of direction of a wave as it enters different medium
• due to change in speed of wave

the frequency of a wave never changes!

Question 7

what happens when light enters a more dense medium?

how does it refract?

Answer 7

bends towards the normal

denser substance -> rays slow down

Question 8

what happens when light leaves the glass block and enters the air (less dense medium)?

how does it refract?

Answer 8

bends away from the normal
- paralell with ray of incidence

this is because the medium is less dense, light rays speed up

Question 9

Why do birds not aim at the fish they see in the water, but instead a location next to it?

Answer 9

refraction!

Question 10

why does the speed change?

refraction

Answer 10

medium changes

Question 11

why does the frequency stay constant?

refraction

Answer 11

same number of waves pass a point each second

Question 12

why does the wavelenth change?

refraction

Answer 12

waves travel a smaller distance between when they are created

Question 13

How is white light split by a prism?

Answer 13

different colours refracted by different amounts, as slowed down in varying amounts by the glass

Question 14

What is refractive index?

equation

Answer 14

refractive index = speed of light in a vaccuum/air / speed of light in a substance
No units! should be a small number (1-2.5)

n = C/Cs

eg speed of light (300 000 000 m/s) divided by speed in water (225 000 000 m/s) = 1.33

Question 15

when you plot a graph of sin i (y) sin r (x), what is represented by the gradient of this directly proportional relationship?

Answer 15

refractive index

Question 16

what is the equation linking refractive index, sin (i) and sin (r)?

Answer 16

n = sin (i) / sin (r)

n = refractive index

Question 17

what is total internal reflection?

Answer 17

when light moves from a denser to less dense medium, instead of refracting, all the light is reflected

Question 18

what are the conditions for total internal reflection?

Answer 18

1. light must be travelling from a higher to lower refractive index (from dense to less dense material, eg glass to air)
2. Angle of incidence is larger than critical angle

Question 19

what is the equation that links refractive index with critical angle?

total internal reflecton

Answer 19

sin c = 1/n

sin critical angle = 1/refractive index

eg glass critical angle = sinc = 1/1.5
c = sin-1 (1.5)
c= 41.8”

Question 20

what is the critical angle?

Answer 20

angle of incidence (which is critical angle when the angle of refraction is 90”)

Question 21

what are uses of total interal reflection?

Answer 21

• optical fibres (communication, beam of light enters optical fibres, refracted as enters the fibre, bu then travels down the fibre through repeated total internal reflection)
• endoscopy

Question 22

why are diamonds so sparkly?

Answer 22

have high refractive index, means a small critical angle, thus large proportion of light is totally internally reflected -> v sparkly (if light ray larger than critical angle)