lect 4 physics for computer science

8/3/2019 Lect 4 Physics for Computer Science

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https://sites.google.com/site/phs1019pfc/

Clayon Harrison
https://sites.google.com/site/phs1019pfc/https://sites.google.com/site/phs1019pfc/


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Facts about Light:

Light travels in a straight line

Light travels through a vacum

Light can be thought of as a wave or as a

stream of particles

Light changes direction when it leaves one

medium and enters another


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We see objects because light bounces offthem. This bouncing is called reflection.


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The normal is an imaginary linewhich is drawn perpendicular top themirror.

A ray which is normal to the mirror isreflected along its original path .

The LAWS OF REFLECTION:

The angle of incidence equals theangle of reflection

The incident ray ,normal andreflected ray all lie on the sameplane.


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Features of the image formed by a planemirror

The image is virtual. Rays of light do not pas

through the image they only appear to do so.

The image is the same size as the object. The image is the same distance behind the

mirror as it is in front of it.

The image is upright

The image is latterally inverted.EG your right

hand when viewed in the mirror looks like a

left hamnd


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Light bends or changes direction if it leaves

one medium and enters another of a

different optical density. This bending or

change in direction is called refraction


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Rays of light travelling from one medium intoan optically denser medium are bent

towards the normal. AIR GLASS

Rays travelling into an optically less densemedium are bent away from the normal.

Rays normal to the surface are not deviated.


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The laws of refraction :

The incident , the normal and the refracted

rays all lie in the same plane.

The sine of the angle of incidence divided by

the sine of the angle of refraction is a

constant known as the refractive index. This

is know as SNELLS LAW.


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The refractive index is represented by the

symbol1n

2, where the 1 refers to the

incident medium and 2 refers to the

refracting medium.

1n2 = sini/sinr

= real depth/apparent depth

= speed of light in the incident medium

speed of light in the refracted medium


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The principle of reversibility of light states that the

paths of light rays are reversible .This means that

if a ray of light is sent in the exact opposite

direction it will follow the same path.

From medium 1 to 2 we use 1n2 for the refractive

index.

1n2 = sin isin r

Where angle i is in medium 1 and angle r is in

medium 2


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For a ray travelling in the opposite directionfrom medium 2 into medium 1. We use 2n1

2n1 = sin r

sin i

From the two relations we can see that

2n1= 11n2


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The conditions for total internal reflection tooccur are:

Light must be travelling to a less dense

medium

The angle of incidence must be greater than

the critical angle.

The angle of incidence for which the angle of

refraction is 90 is known as the critical

angle


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Applications :

Right angled prisms are

used in periscopes


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Right angled prisms are used in binoculars to give an image that is theright way up


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Interesting!!!!!!!!!! Total internal reflectionaccounts for the sparkle of diamonds


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An OPTICAL FIBRE is a thin glass or plastic

fibre with diameter between .01 and .05mm.

The critical angle for glass is 42.


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For a ray of light going from glass to air , the angle ofincidence i equals the critical angle c when the angle ofrefraction r is just 90 :

i= c and r =90

The refractive index from glass to air gna is given by :

gna = sin i = sin c = sin csin r sin 90 1

Therefore sin c = gna = 1 (principle of reversibility of light)

ang

Or sin c = 1

refractive index of glass


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Thus if the refractive index of glass ang = 1.5 ,we have:

sin c = 1 = 0.67

1.5

Therefore critical angle of glass = 42

In general the critical angle c between a

medium of refractive index n and the air isgiven by: sin c = 1 or n= 1

n sin c


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Light entering at 42 undergoes a series oftotal internal reflections before emerging.

Bundles of fibre are used to send information

especially in telecommunications

A laser beam moving through a single optical

fibre can carry thousands of telephone

conversations and several TV programmes atthe same time.


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The principle of Superposition states that at apoint where two waves meet, the

instantaneous displacement is the vector sum

of the individual displacements due to each

wave at that point.

Where superposition takes place the waves are

said to be superposed.


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Diffraction is the spreading of light waves when

they encounter obstacles or pass through

openings which have widths close to the

wavelength of the wave. The narrower the

opening the greater the extent of diffraction.


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The ability of wave motions to combinetogether is known as the principle of

superposition. What happens when they

combine is called interference.

There are two types of interference

Constructive waves are in phase

Destructive

waves are in antiphase

Other phase differences will produce

amplitudes between the two extremes.


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Conditions for two source interference.

1. Similar (almost identical frequency)

2. Equal or nearly equal amplitude

3. Coherent Sources which means they must

have zero or a constant phase difference.


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Some degree of coherence is necessary toobserve interference.

If two waves originate from independentsources, such as incandescent bulbs or gasdischarge lamps, the waves will be mutuallyincoherent.

Laser sources, though independent, can

possess sufficient mutual coherence forinterference to occur.


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L

d

Note: L >>d and d >>


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x>>d

Note: L >>d and d >> Tan = yL


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md sin

)2

1(sin md

Where d is slit separationm is a whole number


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lect 4 physics for computer science

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