(3.14-3.22) Light Waves, reflection + refraction

What kind of waves are light waves?

Transverse waves

How would a light wave be shown on a graph?

A wave with peaks and troughs

What is the law of reflection?

Angle of incidence (i) = Angle of reflection (r)

What is the angle of incidence?

The angle of the wave approaching the boundary

What is the angle of reflection?

The angle of the wave leaving the boundary

How are angles i/r measured?

The angle between the wave and a line perpendicular to the boundary called the normal

Describe how to draw a ray diagram for reflection.

• Perpendicular line to boundary (normal)

• Incident ray to the corner between boundary + normal, pointing towards boundary

• Reflected ray pointing from boundary

Why does refraction occur?

When light travels between mediums of different densities at an angle

its speed changes for the part of the wave in the different medium

as part of it speeds up but the other part stays in the slower medium

the wave changes direction.

Describe a ray diagram for refraction.

• Same principle as reflection

• less → more dense = light ray bends towards the normal

• more → less dense = light ray bends away from the normal

• along the normal = no bending

Describe the experiment to investigate the refraction of light with

• rectangular blocks

• semi-circular blocks

• triangular prisms

1. Place block on paper + draw around Perspex.

2. Direct beam through the side. Measure angle of incidence.

3. Mark on the paper

   • point on ray close to source

   • point on ray near block

   • point of ray’s exit

   • further point of exit

4. Use set squares to draw the normals of the ray’s entrance + exits. (at 90°)

5. Remove block + join points. Measure angle of refraction.

6. Replace block + repeat, for different angles.

7. Repeat for each shape.

What is the refractive index?

shown by ‘n,’ always >1

• shows how much the speed of light changes in a specific medium, compared to in a vacuum

a ratio:

speed of light in a vacuum/speed of light in material

What is the link between the refractive index of a material and its density?

optically denser → higher index

optically less dense → lower index

What is Snell’s Law?

n = sin i / sin r

Describe the experiment to investigate the refractive index of glass.

1. Place the block on paper + outline it

2. Draw 90° normals to the outline.

3. Use a protractor + measure angle of i to be studied. Mark angles as guiding lines.

4. Direct a beam of light at the the first angle.

5. Mark:

   • point on ray close to source

   • point on ray near block

   • point of ray’s exit

   • further point of exit

6. Remove block + join points. Measure + record angle of refraction

7. Replace block + repeat with each angle.

Analysis of the previous experiment.

The i angles are the angle between the beam and the normal.

The r angles are angles between the normal within the block and the refracted rays.

Plot a graph of sin i on y-axis and sin r on x.

refractive index = gradient

What is the refractive index of glass?

1.4-1.7

What is total internal reflection?

when light goes from dense → rare medium

all the light is reflected instead of being refracted

When does TIR occur?

angle of incidence > critical angle

+

incident material is denser than second

What is the critical angle?

incidence angle at which the reflected ray is at 90 degrees

and travels along the boundary of the surface

Describe what happens to an emergent ray as the angle of incidence increases.

as i increases, r increases

until i = critical + ray travels along boundary

as r = 90 degrees

when i > critical, ray is totally internally reflected

Where is TIR used?

optical fibres

prisms

How is TIR used in optical fibres?

light carries energy and information w/o any matter

when it hits the edge of the fibre, the light is TIRd

so no data is lost through refracted light

How does an endoscope work?

light from light source travels up to objective lens

the object being viewed reflects the light down the fibre to the eye

which can see the image through the eyepiece lens

Where are prisms used?

periscopes - 2 right angled prisms used to see over tall objects

telescopes

cameras

binoculars

How can the critical angle (c) be calculated?

sin c = 1 / n

What does that formula mean for the quantities?

> n = < c

light rays in a material with high n are more likely to be TIR’d