RRefraction ooff LLight

form 3

Physics Project陳達權 chan tat kuen f.3d (5)

Outline

Bending Light

Apparent Depth

Inside reflections

LightLight Light is not only what our eyes can see. In general we

call everything in the electromagnetic spectrum – gamma rays and X-rays down to microwaves and radio waves – light:

We can think of light as wave or particles – it has wavelengths and frequencies and consists of photons.

bending lightlight

Why does the pencil / chopstick appear bent?

Light ray is bent as it passes from one medium to another.

It is due to light travelling at different speeds in the two media.

It is called refraction of lightrefraction of light.

bending light

As a result, the ray keeps the same direction as before but is laterally displaced.

The ray is not bent only when it hits the block at right angle.

density of medium

•Medium–A transparent material through

which light travels

• When light passes from a less dense to a denser medium, it is bent towards the normal.

density of medium

• When light passes from a denser to a less dense medium, it is bent away from the normal.• sin isin i is directly proportional to sin rsin r

while i = angle of incidence & r = angle of refraction

laws of refraction

When a ray of light passes from one medium to another,

the incident ray, the refracted ray and the normal all lie in the same plane, sin i

sin r

which is called Snell’s Law

Snell’s Law was discovered by Snell, a Dutch scientist, in 1621 and so was named after him.

is a constant

refractive index

The refractive index of a medium is

sin i

sin r

where i and r are the angle of incidence and

angle of refraction respectively when a ray of

light enters the medium from air.

Refractive indices of various materials

Material Refractive index

Glass 1.50 – 1.70

Water 1.33

Perspex 1.50

Diamond 2.42

Example 1

A ray of light passes from water into glass. Given angle of incidence is 30º.

1.33 sin 30º = 1.5 sin r

where r = angle of refraction

r = 26.3º

Example 2 A light beam enters a

piece of glass from the vacuum at 60º from the normal as shown and bends to 45º from the normal once inside the glass.

n sin(45º) = sin60º where n = index of

refraction of the glass

n = 1.22

A prismprism is a block of glass or other transparent material, usually having a triangular base.

When a beam of narrow white light passes through a prism, the beam is refracted and splits into all the ccoolloouurrss of the rainbow.

The effect is called dispersiondispersion.

Refraction by a Prism

red orange yellow green blue indigo violet

彩虹從那裡來 ?

太陽發出的強大的高速帶電粒子，流射入地球外圍高空大氣層時，與稀薄空氣分子發生猛烈碰撞，產生發光現象，這就是光極。

有時暴雨後，天空中飄著許多小水滴；太陽光通過時，部分太陽光不僅改變了前進的方向，而且還被分解成赤、橙、黃、綠、藍、靛、紫色光，如角度合適，天空中就會出現彩虹彩虹。

Apparent Depth

Why does a swimming pool appear shallower than it really is?

Real Depth vs ApparentApparent Depth Illustrated by the figure, the cone of rays from O

at the bottom of the swimming pool bend away from the normal as it enters the air. When it appears to come from I which is higher than O.

Real Depth vs ApparentApparent Depth An object placed under water or glass

appears to be higher than it actually is.

the depth that the object appears = ApparentApparent Depth

The depth that the object is actually at = Real Depth

Inside reflections

the inside surface of water as a perfect mirror.

Total internal reflection A ray of light hits the inside face of a

semicircular glass block.

The incident ray splits into two rays. The angle of refraction is 90º and the refracted ray only just manages to leave the glass surface.

Total internal reflection When light travelling from a denser to a less

dense medium is incident on the interface at an angle greater than the critical angle C

where C = sin-1 (1/n) and n = refractive index of the material

Medium Refractive index Critical angle

Glass 1.50 – 1.70 36º - 42º

Water 1.33 49º

Perspex 1.50 42º

Diamond 2.42 24º

Fish-eye views

A fish or a diver under water can see everything above the surface, but their view is squeezed into a cone with an angle of 98º.

Fish-eye views Outside this cone, the surface looks like a

mirror reflecting light from under the surface.

A photograph taken underwater looks like a mirror.

Prisms as perfect mirrors A glass prism behaves like a perfect mirror if

light rays strike the inside face at an angle > 42º (critical angle for glass). Prisms are used as mirrors in periscopes and binoculars.

Example 3 The figure shows a pentaprism made of flint

glass of refractive index 1.58. Its upper face is inclined at an angle of θ to the base.

critical angle C = sin-1 (1/1.58) = 39.3º It is also the minimum angle of θ required for

light to be totally internally reflected by the upper face.

Example 4 A diver carries a lamp below the surface of a

still lake (index of refraction of water is 1.33) and circle of light is observed coming out of the surface with a radius of 16 m. How far is the diver below the surface?

critical angle C = sin-1 (1/1.33) = 48.8º R/h = tan C 16m/h = tan 48.8º h = 14.0 m

Refracting Telescopes These are the first kind of telescopes used, for

example, by Galileo.

Refracting telescopes use lenses and the principle of refraction, as shown above.

Reflections inside a diamond The facets of a diamond have to be cut at

carefully chosen angles so that it can reflect back light that goes into it.

refractive index of diamond = 2.42 critical angle for diamond-air interface = sin-1(1/2.42) = 24.4º

Mirage 海市蜃樓海市蜃樓 On a hot day in a desert, a driver may

see a pool of waterpool of water at a distance.

What the driver sees is in fact a miragemirage. It is just the reflection of the skysky.

Mirage

It meets a layer of air near the ground at an angle greater than the critical angle. Then total internal reflection takes place. This reflection of the sky looks like a pool of water to the driver.

Layers of air near the ground are hot. While layers higher up are cooler and denser, and have higher refractive indices. Light from the sky is gradually refracted more towards the horizontal.

晴朗的天空為甚麼是蔚藍色的 ?

太陽的光可分解成赤、橙、黃、綠、藍、靛、紫七色光。

空氣分子對波長較短的陽光，散射作用 較大。在晴朗的天氣裡，當太陽位於天頂時，藍光被散射 50% 以上；波長短的紫、藍等色光，很容易被空氣分子和懸浮在空氣中的微粒向四面八方散射開去，使天空顯出蔚藍色分子。

OOptical FFibre Light can pass along thin glass fibre even the

fibre is bent. The ray of light bounces from one side to the other and will not be lost to the air from the sides.

An optical fibre cable is made up of a bundle of thin glass fibre. It can be used for various applications, such as data communication for computers and TV programmes.

OOptical FFibre

Advantages of using fibre cables include:

¤ low transmission loss and wide bandwidth ¤ small size and weight ¤ immunity to interference ¤ electrical isolation ¤ signal security ¤ abundant raw material

http://www-cgi.informatik.uni-oldenburg.de/~da/sirohi/refraction.html

[calculating unknown in Snell’s Law]

http://wigner.byu.edu/LightRefract/LightRefract.html

[refraction of light in different media]

http://micro.magnet.fsu.edu/primer/java/refraction

[wave length, angle of incidence & medium]

http://www.icsd.k12.ny.us/highschool/swirt/science/physics/lesson/otherpub/wfendt/refraction.htm

[refraction of light simulation from one medium to another]

Useful links