Also see Simulation program 3.1 in Physics at Work Multimedia CD-ROM 1.
11 4 refraction-of_light
Refraction of light
1 John says, ‘When hunting a fish under water, you should aim your spear directly at the fish.’ Do you agree? Yes, of course. No, because the fish is actually located somewhere else. No, because size of objects changes when they are put under water.
2 When sunlight falls on the water surface, which of the following occur(s)? It is reflected back to the air. It refracts into the water. It is absorbed by water and turned into heat.
IntroductionRefraction is the bending of lightwhen the light passes from one mediumto another. air glass
IntroductionUseful words to describe refraction of light angle of incidenceincid ent ray normalairglass re f ra c te d ra angle of y refraction
Introduction From a less dense to a denser medium• e.g. from air to glass incid normal ent ray air glass re f ra c te d ra y• Light is bent towards the normal.
Introduction From a denser to a less dense medium• e.g. from water to air in cid normal en tr ay water air refra cted ray• Light is bent away from the normal.
Relation between angle of incidenceand angle of refraction sin i i = angle of incidence r = angle of refraction sin r O straight line passing through the origin ⇒ sin i is directly proportional to sin r.
Laws of refraction• The incident ray, the refracted ray, and the normal all lie in the same plane. incid ent normal ray air glass re glass fra ct ed ra y
Laws of refraction• The ratio of the sin i to sin r is constant. i.e. sin i = constant sin r – This is called Snell’s law. In general, n1 sin θ1 = n2 sin θ2
Refractive index• refractive index n = sin i sin r• e.g. for glass, ang=sin θa sin θg where a: air g: glass
Refractive indexRefractive indices of some materials Material Refractive index Glass 1.5 – 1.7 Water 1.33 Perspex 1.5 Diamond 2.42
Refractive index and speed of light distan ce tha travel t lightvacuum (or air) in 1 s 3 × 108 mwater (n = 1.33) 2.25 × 108 mglass (n = 1.5) 2 × 108 m diamond (n = 2.42) 1.25 × 108 m
3 Refractive index and speed of light distan ce tha travel t light vacuum (or air) in 1 s 3 × 108 mwater (n = 1.33) fastest 2.25 × 108 m glass (n = 1.5) 2 × 108 mRefractive indexSpeed of light n = 2.42) diamond (slowest 1.25 × 108 m
4 Examples of refraction of lighta Bent chopstick • The chopstick appears bent because of refraction
4 Examples of refraction of lightb Shallower in water • The depth that the object is actually at is called the real depth. real I depth O Simulation
4 Examples of refraction of lightc Flickering objects in hot air • The object you see through the unstable hot air appears blurred and flickering.
Example 3A ray of light passes from air into water.Angle of incidence = 30°What is angle of refraction in water? nwater = 1.33 30° air water
Example 3Applying Snell’s law, nw sin θw= na sin θa⇒ 1.33 × sin θw = 1 × sin 30°⇒ sin θw = sin 30°/1.33 = 0.376 30° ∴ θw = 22.1° air water 22.1°
Example 4 A ray of laser hits a rectangular block. The table gives a set of results. Notes: θa = angles of incidence, θg = angles of refraction. θa 15° 30° 45° 60° 75° θg 11° 18° 28° 34° 40°(a) Is the refracted ray bent towards or away from the normal? Towards
Example 4(b) Find out how sin θa and sin θg are related by plotting a graph of sin θa against sin θg. θa 15° 30° 45° 60° 75° θg 11° 18° 28° 34° 40° sinθa 0.259 0.500 0.707 0.866 0.966 sin θg 0.191 0.309 0.470 0.559 0.643
Example 4(c) Find the refractive index ng of the glass block in used in the experiment. sin θaRefractive index = = slope of graph sin θg = 1.52∴ The refractive index of the glass block used is 1.52.
Q1 A boy shines a torch under…A boy shines a torch under water as shown.Which one shows the correct path of the lightray?A Path X.B Path Y.C Path Z.D All of them.
Q2 A beam of light travels air to… A beam of light travels from air to water. Which of the following is equal to the ratio sin r : sin i ? normalA nwater : 1 iB nair : 1 airC 1 : nwater waterD 1 : nair r
Q3 A light rays is incident on a…A light ray is incident on a glass prism.Which equation can beused to find θ ? 30° sin 30° sin θ 20° 40° θ A = sin 20° sin 40° B sin 30° sin 40° = sin 20° sin θ C sin θ = 1.5 sin 40° D sin 40° = 1.5 sin θ
Q4 True or false: When light is…True or false: When light is incident on asurface along the normal, only refractionoccurs; there is no reflection. (T/F)
Q5 Statements:... 1st statement 2nd statementLight is bent Glass has a greatertowards the normal refractive index thanwhen it passes from air.air to glass. true? true? Yes Yes
Q1 True or false: Light slows…True or false: Light slows down when it entersa material from air. (T/F)
Q2 True or false: The refractive…True or false: The refractive index cannot besmaller than 1. (T/F)
Q3 True or false: If light travel…True or false: If light travel at the same speedin all materials, refraction would still occurwhen it passes from air to water. (T/F)
Q4 True or false: If the speed of…True or false: If the speed of light in raindropis equal to that in air, there would be norainbows. (T/F)
Q5 Sketch a ray diagram for the...Sketch a ray diagram for the tip of thechopstick to show why the chopstick looksbent when dipped into water.