SUPPORTING NOTES FOR REFLECTION AND REFRACTION PDF

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SUPPORTING NOTES FOR REFLECTION AND REFRACTION...

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Introductory Physics

SUPPORTING NOTES FOR REFLECTION AND REFRACTION 1.

REFLECTION LAWS

•

First Law of Reflection: Incident ray, reflected ray and normal are all in the same plane.

•

Second Law of Reflection: Angle of Incidence equals the angle of reflection.

DO: Draw a diagram of a light ray reflecting from a mirror. Show and label the rays and angles.

2.

IMAGES IN MIRRORS The image you see in a plane (flat) mirror has a number of properties.

• • • •

Same size (magnification of 1) Same distance behind mirror as object is in front of mirror. Laterally Inverted (turned around in a special way) Virtual - a technical term that means the image cannot be projected onto a screen (images that can are called real images). Curved mirrors can be treated as a series of small flat mirrors.

DO: Draw a diagram of how we see a point object in a mirror. Remember the eye is a lens system that focuses the light.

A. B. Chaplin 9/16

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Introductory Physics

3.

REFRACTION LAWS

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First Law of Refraction: Incident ray, refracted ray and normal are all in the same plane.

•

Second Law of Reflection (Snell’s Law): sine of the angle of incidence divided by the sine of the angle of refraction is a constant number for the particular material and is called the refractive index.

s i ni s i nr

=n

DO: Draw a diagram of a light ray passing through a block of glass. Show and label the rays and angles.

4.

MORE ABOUT REFRACTIVE INDEX

•

Should be written in the form 1n2 showing that the light goes from medium 1 into medium 2, e.g. airnglass means light is travelling from the air into glass.

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The refractive index values found in data books are measured using a vacuum as medium 1 and is called the absolute refractive index, they are often written in the form nglass etc. In practice there is very little difference if you use air rather than a vacuum.

•

Light travelling into a smaller refractive index material bends away from the normal. Light travelling into a bigger refractive index material bends towards the normal.

DO: Light travels from glass (n = 1.50) into water (n=1.33), if the angle of incidence was 20º would the angle of refraction be bigger or smaller? (no calculation necessary)

DO: Can you prove 1n2 = 1/2n1

A. B. Chaplin 9/16

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Introductory Physics

5.

REFRACTIVE INDEX AND THE SPEED OF LIGHT

•

Refractive index is connected to the speed that light travels through both mediums by the simple equation:

Spe e do fl i g h t i nav a c uu m

n dium = v a c u u m me

Sp e e dofl i g ht i nt h eme d i u m

DO: The refractive index of diamond is 2.42, does light travel faster or slower in diamond? (no calculation necessary).

DO: Calculate the speed of light in diamond. Take the speed of light in a vacuum as 2.9979 x 108 ms-1

DO: EXTENSION VERY OPTIONAL Prove that 1n2 = n2/n1 and hence calculate a value for oilnglass. (noil =1.28 and nglass = 1.50). (HINT: use the speed of light formula above)

A. B. Chaplin 9/16

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Introductory Physics

6.

TOTAL INTERNAL REFLECTION

a i r g l a s s

a i r g l a s s

a i r g l a s s

Most light is refracted. Some light of low intensity can be seen reflecting from the interface between the air and glass. Remember as the angle of incidence increases the refracted ray will bend further away from

As the angle of incidence increases there comes a point where the refracted ray grazes the glass surface, i.e. the angle of refraction is 90º. The angle of incidence that causes this is given a special name, it is called the critical angle. The light that is being reflected is

When the angle of incidence is greater than the critical angle all the light is reflected and none is refracted. This is called Total Internal Reflection. The glass air interface acts as a plane mirror. You only get TIR when the light is going from a high to low refractive index

CARE: The critical angle is often given the symbol c. Do not confuse it with the symbol for the speed of light which is also c (in A2 c is also used for the specific heat capacity).

7.

Connection between critical angle and refractive index Lets use the air glass interface as our example. The key formula is:

n

a i rg l a s s

A. B. Chaplin 9/16

= 1 s i nc

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Introductory Physics

DO: OPTIONAL You must learn the critical angle formula, but can you prove the formula? HINTS: Remember at the critical angle the angle of refraction is 90º and sin 90º is equal to 1.

DO

Calculate the critical angle for diamond.

8.

FIBRE OPTICS Uses

• • • •

Endoscopy Security fences etc Lighting Digital communications Construction

•

Pure glass core (9 to 60 micrometres depending on type of fibre) surrounded by another glass or plastic cladding of lower refractive index. The fibre is encased in a plastic sheath to protect the glass from scratching (glass is quite strong if it is not scratched). Fibre optic cables usually consist of bundles of fibre optics held together by a plastic outer jacket. Advantages

• • • • •

Small, light and flexible. High bandwidth compared to copper cables (technical terms that means they can carry more signals) No electromagnetic interference or crosstalk Hard to tap except at the junction boxes Can use digital regeneration to boost weak signals Disadvantages

• •

Hard to join. Simple fibre optics suffer from n effect called multi-path dispersion: in this the signal gets a little jumbled. Overcome by using more expensive monomode cables.

A. B. Chaplin 9/16

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