1680s

Dutch physicist Christian Huygens captured this propagation mechanism MATHEMATICALLY

Every point on a wavefront behaves as a point sourcefor waves generated in the direction of the wave’spropagation (each point is the center of its own wavelet)

A train of waves of fixed frequency is traveling along a thin, light cord toward the juncture to a heavy, thick cord. Both cords are stretched taut and under the same tension.

The waves that travel across the heavy cord will have

1) greater speed than along the lighter cord.2) the same speed as along the lighter cord.3) smaller speed than along the lighter cord.

A train of waves of fixed frequency is traveling along a thin, light cord toward the juncture to a heavy, thick cord. Both cords are stretched taut and under the same tension.

The waves traveling across the heavy cord have

1) larger wavelength than along the lighter cord.2) the same wavelength as along the lighter cord.3) smaller wavelength than along the lighter cord.

A train of waves of fixed frequency is traveling along a thin, light cord toward the juncture to a heavy, thick cord. Both cords are stretched taut and under the same tension.

The waves will continue across the heavy cord

1) at lower frequency than along the lighter cord.2) with the same frequency as along the lighter cord.3) at higher frequency than along the lighter cord.

Answers to 3 previous slides

3) smaller speed than along the lighter cord.

Speed of mechanical waves,

Here the tension remains unchanged, but the density increases.

Tv

3) smaller wavelength than along the lighter cord.Slower speed means the waveforms will bunch up as they pass the juncture.

2) with the same frequency as along the lighter cord.

vacuum

A ray of light travelling in a vacuum encounters an interface with some medium as shown above. In which direction will the ray of light bend ?

1) ray 1

2) ray 2

3) could be either, depending on the

medium

1

2medium

1

air

2

air

Parallel light rays cross interfaces from air into two different media, 1 and 2, as shown in the figures above. In which of the media is the lighttraveling faster?

1) 1 2) 2 3) same speed in both

Consider a light ray which traverses a thick slabConsider a light ray which traverses a thick slab

ray bends ray bends towards the normaltowards the normal upon entering the glass upon entering the glass

ray bends ray bends away from the normalaway from the normal when it exits from the glass when it exits from the glass

exiting light ray is at exiting light ray is at same anglesame angle as original ray, but is as original ray, but is shiftedshifted over to one side over to one side

An observer views two closely spaced lines through an An observer views two closely spaced lines through an angled piece of glass. To the observer, the lines appear:angled piece of glass. To the observer, the lines appear:

1) shifted to the right

2) shifted to the left

3) spaced farther apart

4) spaced closer together

5) no change -- exactly as before

Answer to 3 previous Concept Question slides:

1) ray 1

As the text describes, light is impeded (slowed) in any mediumso travels slower than it does in a vacuum.

1) Medium 1Light will bend more the greater its change in speed . It slowed down less upon entering the 1st medium.

2) shifted to the left

Total Internal ReflectionTotal Internal Reflection When light goes from a medium with high n into a

medium with low n, rays bend away from the normal.

n1

n2 ( < n1)air

water

c

At angles greater than c there is no refracted ray at all. The incident rays are completely reflected !!

this is total internal reflection

At a particular incident angle (critical angle c), the refracted angle becomes exactly 90°.

Total Internal ReflectionTotal Internal Reflection

What is the condition for total internal reflection?

when i = c refracted angle is 90°

221 90 nnn o

c sinsin1

2n

nc sin

Remember: this only works when the incident medium has the higher index of refraction.

n1

n2 ( < n1)air

water

c

For glass with n = 1.5 we find that:

sin c = 1.0/1.5 = 0.67 c = 41.8°

so for i = 45°, the light is totally reflected

Example: binoculars use 45° prisms to reflect light