Ch1 reflection refraction

Ch1. Reflection & Refraction

Physics & Chemistry Now

3TTO Physics

1.1 What is this chapter about?

• Reflection in mirrors:

– Mirror image point – geometric construction

– Rain droplets

1.1 What is this chapter about?

• Refraction at a boundary between two materials (mostly with lenses)

– Refraction angles

– How does it work a magnifying glass and a camera

– Law of image formation in lenses

1.2 What do you still know about light?

• Natural vs. Artificial sources• Convergent, divergent and parallel beams• Light speed vs light year• Primary colours / spectral colours• Colour: spectral reflection• Shadows• Solar and moon eclipses• Reflection vs. refraction• Mirror reflection and glass refraction• Field of view• Focal length and lens power• Lens (converging, diverging) and its images• Eye accommodation• Eye illnesses: long-sightedness,…

1.2 What do you still know about light?

• Luminous and non-luminous objects

A luminous object is one that produces light.

A non-luminous object is one that reflects light.

StarsLight bulb…

PlanetsClothes…

• Which are natural and which are artificial?

1.2 What do you still know about light?

• Light beams:

– Divergent

– Convergent

– Parallel

• Light year:

– Is the distance (d) the light travels in 1 year:

– d = speed (m/s) x time (s)

– Can you calculate it?

1.2 What do you still know about light?

• Light travels in straight lines:

Laser

At this speed it can go around the world 8 times in one second.

• Light travels very fast–around 300,000 kilometres

per second.

1.2 What do you still know about light?

• Light travels much faster than sound. For example:

1) Thunder and lightning start at the same time, but we will see the lightning first.

2) When a starting pistol is fired we see the smoke first and then hear the bang.

1.2 What do you still know about light?

• We see things because they reflectlight into our eyes:

Homework

1.2 What do you still know about light?

• ShadowsShadows are places where light is “blocked”:

Rays of light

Colour

• White light is not a single colour; it is made up of a mixture of the seven colours of the rainbow.

We can demonstrate this by splitting (dispersion of) white light with a prism:

This is how rainbows are formed: sunlight is “split up”(dispersed) by raindrops.

The colours of the rainbow:

• Red

• Orange

• Yellow

• Green

• Cyan

• Blue

• Violet

Adding colours

• White light can be split up to make separate colours. These colours can be added together again.

• The primary colours of light are red, blue and green:

Adding blue and red makes magenta (purple)

Adding blue and green makes cyan (light blue)

Adding all three makes white

again

Adding red and green makes yellow

Seeing colour

• The colour an object appears depends on the colours of light it reflects.

For example, a red book only reflects red light:

White

light

Only red light is reflected

A white hat would reflect all seven colours:

A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue):

Purple light

White

light

Using coloured light

• If we look at a coloured object in coloured light we see something different. For example, consider a football kit:

White

light

Shorts look blue

Shirt looks red

• In different colours of light this kit would look different:

Red

lightShirt looks red

Shorts look black

Blue

light

Shirt looks black

Shorts look blue

Some further examples:

Object Colour of lightColour object seems to be

Red socks

Red Red

Blue Black

Green Black

Blue teddy

Red Black

Blue

Green

Green camel

Red

Blue

Green

Magenta book

Red

Blue

Green

Using filters• Filters can be used to “block” out different colours of light:

Red Filter

Magenta

(violet) Filter

Investigating filters

Colour of filter Colours that could be “seen”

Red

Green

Blue

Cyan

Magenta (violet)

Yellow

Red

Magenta

White

Yellow

Blue Green

Cyan

1.3 Point and mirror image pointMirror image point A'

mirrorn

orm

al

Point A

L

L’

L

L’

M

M’

O

Field of view

O'

L

mirrorn

orm

al

i

i angle of incidence

t

t angle of reflection

1.4 Two angles

Clear (regular) vs. Diffuse Reflection

Smooth, shiny surfaces have a clear (regular) reflection.

Rough, dull surfaces have a diffuse reflection.

Diffuse reflection is when light is scattered in different directions

1.5 The angle of refractionLight refracts, which means that it bends when passing from one

medium to another. When light enters a more dense medium from one

that is less dense, it bends towards a line normal to the boundary between the two media.

Three rules for calculating reflection and refraction.

1. All angles are measured from the normal. The normal is the line perpendicular to the surface at the point of reflection.

Three rules for calculating reflection and refraction.

2. The reflected angle is equal to the incident angle.

ir

3. Snell’s Law for refraction

ffii nn sinsin

Three rules for calculating reflection and refraction.

• A ray of light strikes the surface of a beaker of hydrogen peroxide (n = 1.414) making a 30o angle with the surface normal.

• What angle does the reflected ray make with the normal?

• What angle does the transmitted ray make with the normal?

• a) The angle of the reflected ray is the same as the incident ray, 30o

• b)

7.20

354.0sin

sin414.130sin1

sinsin

f

f

f

fperoxideiair nn

• A ray of light inside a diamond encounters a boundary between the stone and air.

• The ray makes a 30o

angle with the normal. What is the angles of the refracted beam?

• Index of refraction of diamond = 2.42

Let’s review the definition of sine for a minute . . .

21.1sin

30sin42.2sin1

sinsin

f

f

idiamondfair nn

Sine function

• The sine of an angle is the ratio between the side of the triangle opposite the angle and the hypotenuse.

• The hypotenuse is the longest side of the triangle.• So, the sine of an angle must always be less than 1.

Back to the diamond . . .

• Consider a ray of light inside the diamond with an angle of incidence of 24.4o

• What is the angle of the transmitted ray?

90

1sin

2.24sin42.2sin1

sinsin

f

f

f

idiamondfair nn

Critical angle

• The critical angleof incidence results in a transmitted ray that is parallel to the boundary surface.

Total internal reflection

If the angle of incidence is greater than the critical angle, all the light is reflected and none is transmitted.

1

21sinn

ncritical

Rainbows

Rainbows are phenomena that involve refraction, dispersion (split up),

and internal reflection. In order to see a rainbow, it is necessary to look

at a portion of the sky containing raindrops with the Sun directly behind

you. White light from the Sun enters the raindrops, and gets refracted and dispersed inside the raindrop.

Maybe Too Much Information

When the dispersed light hits the back of the raindrop it gets internally

reflected, and when it emerges it gets dispersed even more. Because it refracts

more, blue light always emerges from the raindrop above the red light.

Consequently, only one color reaches your eye from any given raindrop. What

color you see depends on the angle at which you look.

In general you must look slightly higher up in the sky to see red light and lower

to see blue light. So you what you see is a band of color in the sky, with red on

top and blue on the bottom, and all the colors of the rainbow in between.

The reason rainbows appear as an arc in the sky is that the colors you see are

determined by the angle that your line of sight makes relative to the position of

the Sun behind your head. As your look along the blue arc of a rainbow, for

example, this angle remains constant.

Plane parallel plate (flat sheet)

Plane parallel plate (flat sheet)

Prism

Converging (convex) lens

F

Focus F

1.6 Applications of lenses: burning glass

SUN rays

Ray beams are always reversible

F

Focus F

1.6 Applications of lenses: magnifying glass

A ray beam through the optical centre of the lens doesn’tchange direction.

+

1. A ray beam parallel to the optical axis bends through the focus

2. A ray beam through the optical center of the lens keeps straight

3. A ray beam through the focus bends parallel to the optical axis

+

v b

v (voorwerp): object distance

b (beeld): image distance

Magnification (N)

N = Image length/ object lengthN = b/v = Image distance / object distance

+

+

+

+

+

+

+

+

+

+

2f 2f

If v = 2f, then b = v and Magnification (N) = 1

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

When an object is placed on the focus, the ray beams coming from it don’t cross each other after the lens: they become parallel:

There isn’t an image!

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When an object is placed between the focus and the lens, all the ray beams look like to come from a point-source before the lens and before the object

This is valid for any ray coming from the object

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1.7 Calculations with lenses

• di = distance from lens to image (b,u)

• d0 =distance from lens to object (v)

• f = focal length of lens0

111

ddf i

Lens Equation

• f is positive for converging lens

• f is negative for diverging lens

• Negative di is an image on the same side of the lens as the object

• Positve di is an image on the opposite side of the lens as the object

0

111

ddf i

Real and Virtual Images

• If the light rays actually pass through the point they appear to come from, the image is real.

• If the light rays are not actually coming from this position, the image is virtual.

Example of a virtual image

Ray tracing

Converging(convex)

lens

Diverging(concave)

lens