unit 10 light refraction discover physics for gce ‘o’ level science
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Unit 10 LightRefraction
Discover PHYSICS for GCE ‘O’ Level Science
10.4 Refraction at Plane SurfacesLearning OutcomesIn this section, you’ll be able to:• Understand the terms used for refraction:
• Normal• Angle of incidence• Angle of refraction
• Recall that , and solve related problems
• Define refractive index of a medium in terms of ratio of the speed of light in vacuum to that in the medium
sin i = constant
sin r
10.4 Refraction at Plane Surfaces
In Figure 10.28, we can observe that the pencil seems to be bent. What causes this effect?
Figure 10.28
10.4 Refraction at Plane Surfaces• Light can travel through transparent materials such as glass,
water or plastic.• Figure 10.29 shows a light ray traveling from air into glass,
and then into air again.
• We can see the bending effect on the light ray when it passes from one medium into another. We call this refraction.
Figure 10.29
10.4 Refraction at Plane Surfaces
RefractionRefraction of light is the bending of light ray as it passes from one medium into another.
10.4 Refraction at Plane Surfaces
What causes refraction?• Light travels at different speed in different
media.• For example, in air its speed is 3.0 × 108 m s-1,
while in glass it is 2.0 × 108 m s-1.• At the boundary of the two media such as air
and glass, there is a sudden change in speed of the light. This change in speed causes the path of the light to bend, resulting in refraction.
10.4 Refraction at Plane Surfaces
Laws of Refraction1. The incident ray, the normal and the
refracted ray all lie in the same plane.
2. For two particular media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, i.e.
sin i = constant
sin r
10.4 Refraction at Plane Surfaces
Snell’s LawThe equation
where n = constant, is also known as Snell’s Law.
sin i = n
sin r
10.4 Refraction at Plane SurfacesRefractive IndexFor the case of a light ray traveling from airinto a medium such as glass, then
sin i = n
sin rwhere n is the refractive index of the medium
Medium (glass)
i
r
incident ray
refracted ray
air
10.4 Refraction at Plane SurfacesRefractive Index, n
speed of light in vacuum or airRefractive index of medium =
speed of light in medium
cn =
v
For light passing from air or vacuum into a medium, then
sin i c = n =
sin r v
10.4 Refraction at Plane Surfaces
Table 10.3 Refractive indices of some transparentmaterials
10.4 Refraction at Plane SurfacesRefractive indexGlass (nglass = 1.50) a higher refractive index than water
(nwater= 1.33). When a light ray enters glass, it will bendtowards the normal more than compared to when it enterswater.
Glass, nglass = 1.50water, nwater = 1.33
air airi i
rg
rw
10.4 Refraction at Plane SurfacesCan reflection and refraction occursimultaneously?
Light is both reflected and refracted at air-glass interface. However the amount of light reflected is usually smaller than the amount of light refracted.
Figure 10.36
10.4 Refraction at Plane Surfaces
One-way MirrorsA one-way mirror is made of a sheet of glass coated with a thin reflecting layer of metal. This mirror reflects half the light and allows the other half to pass through.
10.4 Refraction at Plane SurfacesPhenomena of Refraction• ‘Bent’ Objects – when a rod is placed in
water, it appears ‘bent’.
Figure 10.39 Ray diagram of the ‘bent’ image of a rod in a glass of water
In Figure 10.39, the light rays traveling from water to air bend away from the normal.However, our brain tends to tell us that the light rays travel in straight line.Thus, we tend to visualize the rod as ‘bent’.
10.4 Refraction at Plane Surfaces
Phenomena of Refraction• Misperception of Depth – The effect of
refraction can make a swimming pool seem shallower than it really is.
Figure 10.40 Ray diagram of the image I of a point O at the bottom of a swimming pool.
10.4 Refraction at Plane SurfacesThe Secret of the Archer Fish
How is the Archer fish able to overcome the visual distortion caused by refraction?
Answer: The Archer fish position itself directly under the prey. This way, the prey appears the least distorted, as the light rays entering the water surface perpendicularly are not refracted.
http://www.naturia.per.sg/buloh/verts/archer_fish.htm
10.4 Refraction at Plane SurfacesNews Reader Prompter
Have you wondered how a news reader is able to read the script and yet maintain eye contact with the camera lens?
Answer: The news reader reads the script off a partially reflected image, which is formed on a one-way mirror. The camera is positioned behind the mirror. A news reader prompter
10.4 Refraction at Plane Surfaces
Key Ideas1. Refraction occurs because the speed of light
changes when travelling through different optical media.
2. The two Laws of Refraction are:a. The incident ray, the normal and the refracted
ray all lie in the same plane.b. For two particular media,
where i is the angle of incidence in air.
sin i = constant
sin r
10.4 Refraction at Plane SurfacesKey Ideas3. The refractive index n of a transparent
medium is cn =
vwhere c is the speed of light in vacuum or air,
v is the speed of light in the medium
4.sin i
= nsin r
where i is the angle of incidence of light in air or vacuum
5. Light is both reflected and refracted at the boundary of two optical media.
10.4 Refraction at Plane SurfacesTest Yourself 10.41. Draw a diagram to show how the direction of light
changes when it travels from air into water.
water
airi
r
normalincident ray
refracted ray
i = angle of incidence,r = angle of refraction
Answer:
10.4 Refraction at Plane SurfacesTest Yourself 10.42. How is the speed of light in glass related to the
angle of incidence and angle of refraction?
sin i c = n =
sin r v
Answer:
where i = angle of incidence in vacuum or air,r = angle of refraction in glassc = speed of light in vacuum or airv = speed of light in glass
glass
airi
r
10.4 Refraction at Plane SurfacesTest Yourself 10.43. At what angle of incidence in air will light pass
through another transparent medium without being refracted?
Answer:When the angle of incidence, i = 0° from the normal, the light ray passes straight through with refraction or bending.
air
medium
10.4 Refraction at Plane Surfaces
Test Yourself 10.44. Draw a diagram to show how the eye sees a coin
at the bottom of a bucket of water.
water
air
coin
image of coin