PHYSICS
13/10/22
Waves:
Key terms from lesson:
Critical angle
Total internal reflection
Refraction
Refractive index
Refraction:
● When light moves from one medium to another in changes speed
8
● The speed of light in a vacuum is 3 x 10 m/s
● When light pass through more dense mediums, it slows down by a small
amount
● Light waves change speed when they pass across the boundary between two
substances with a different density, such as air and glass. This causes them to
● change direction, an effect called refraction.
Refractive index:
𝑐
η= δ
𝑐 = 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑎 𝑣𝑎𝑐𝑢𝑢𝑚
η = 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑣𝑒 𝑖𝑛𝑑𝑒𝑥
Refraction continuing:
● At the boundary between 2 transparent substances:
● The light slows down going into a denser substance, and the ray bends
towards the normal
● The light speeds up going into a less dense substance and the ray bends away
from the normal.
Snells law
𝑠𝑖𝑛 𝑖
η= 𝑠𝑖𝑛 𝑟
η = 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑣𝑒 𝑖𝑛𝑑𝑒𝑥
𝑠𝑖𝑛 𝑖 = 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑖𝑛𝑑𝑒𝑥
, 𝑠𝑖𝑛 𝑟 = 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛
Refractive index:
● The speed of light is determined by the medium through which the light is
travelling. Light travels faster in a vacuum than it does in any other medium.
● Light changes speed as it passes from one medium to another. This is called
refraction.
● The frequency of light does not change as it refracts.
● The refractive index of a material is the measure of the change in the speed of
light as it passes from a vacuum into the material. It has the symbol n
● 𝑛 = 𝑐/𝑣
● 𝑛 = 𝑖𝑠 𝑡ℎ𝑒 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑣𝑒 𝑖𝑛𝑑𝑒𝑥 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙
● 𝑐 = 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑎 𝑣𝑎𝑐𝑢𝑢𝑚
● 𝑣 = 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑎 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙
● The bigger the refractive index, n, the slower the light travels in that material
● N is a ratio so does not have units
SNELL'S LAW:
When a ray of light is incident at normal incidence (at right angles), to the surface
between two optical materials, the ray travels in a straight line.
When the ray is incident at any other angle, the ray changes direction as it refracts
In refraction calculations, angles are always measured between rays and the normal.
The change in direction of a ray depends on the change in the speed of light being
used to calculate the refraction index.
Practice questions:
8
1. Light travels at 2.3 x 10 𝑚/𝑠 in water. What is the refractive index of water?
𝑐
η= δ
8
C = 3 x 10 ( the speed of light)
8
3 𝑥 10
η= 8 = 1.304
2.3 𝑥 10
8 3
*the 10 cancel eachother out so it becomes 2.3
*
2. The refractive index of glass is 1.47. What is the critical angle?
13/10/22
Waves:
Key terms from lesson:
Critical angle
Total internal reflection
Refraction
Refractive index
Refraction:
● When light moves from one medium to another in changes speed
8
● The speed of light in a vacuum is 3 x 10 m/s
● When light pass through more dense mediums, it slows down by a small
amount
● Light waves change speed when they pass across the boundary between two
substances with a different density, such as air and glass. This causes them to
● change direction, an effect called refraction.
Refractive index:
𝑐
η= δ
𝑐 = 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑎 𝑣𝑎𝑐𝑢𝑢𝑚
η = 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑣𝑒 𝑖𝑛𝑑𝑒𝑥
Refraction continuing:
● At the boundary between 2 transparent substances:
● The light slows down going into a denser substance, and the ray bends
towards the normal
● The light speeds up going into a less dense substance and the ray bends away
from the normal.
Snells law
𝑠𝑖𝑛 𝑖
η= 𝑠𝑖𝑛 𝑟
η = 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑣𝑒 𝑖𝑛𝑑𝑒𝑥
𝑠𝑖𝑛 𝑖 = 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑖𝑛𝑑𝑒𝑥
, 𝑠𝑖𝑛 𝑟 = 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛
Refractive index:
● The speed of light is determined by the medium through which the light is
travelling. Light travels faster in a vacuum than it does in any other medium.
● Light changes speed as it passes from one medium to another. This is called
refraction.
● The frequency of light does not change as it refracts.
● The refractive index of a material is the measure of the change in the speed of
light as it passes from a vacuum into the material. It has the symbol n
● 𝑛 = 𝑐/𝑣
● 𝑛 = 𝑖𝑠 𝑡ℎ𝑒 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑣𝑒 𝑖𝑛𝑑𝑒𝑥 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙
● 𝑐 = 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑎 𝑣𝑎𝑐𝑢𝑢𝑚
● 𝑣 = 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑎 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙
● The bigger the refractive index, n, the slower the light travels in that material
● N is a ratio so does not have units
SNELL'S LAW:
When a ray of light is incident at normal incidence (at right angles), to the surface
between two optical materials, the ray travels in a straight line.
When the ray is incident at any other angle, the ray changes direction as it refracts
In refraction calculations, angles are always measured between rays and the normal.
The change in direction of a ray depends on the change in the speed of light being
used to calculate the refraction index.
Practice questions:
8
1. Light travels at 2.3 x 10 𝑚/𝑠 in water. What is the refractive index of water?
𝑐
η= δ
8
C = 3 x 10 ( the speed of light)
8
3 𝑥 10
η= 8 = 1.304
2.3 𝑥 10
8 3
*the 10 cancel eachother out so it becomes 2.3
*
2. The refractive index of glass is 1.47. What is the critical angle?
