Refraction and diffraction - WordPress.com · 5/16/2016 · Refraction and diffraction Book page 109 ... • Place a coin at the bottom ... •If your incident ray is just a little

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Refraction and diffraction

Book page 109 – 110

Syllabus 3.17, 3.21 16/05/2016 ©cgrahamphysics.com 2016

Can you name each phenomena?

16/05/2016 ©cgrahamphysics.com 2016

Starter

• Get a beaker with water and a cup

• Place a coin at the bottom of the empty cup.

• Position your head so that the coin is just out of sight.

• Now without moving your head pour the water from the other beaker into the one with the coin.

• Q. What happens? • Q. Can you describe what is

happening to the light?


Aim • Draw refraction diagrams • Describe an experiment to investigate the refraction of

light using a rectangular and a semicircular glass block and a triangular prism

• Know how waves diffract Key words • Angle of refraction • Refraction • Total internal reflection • Critical angle • Dispersion • Diffraction


The big picture

Have you ever wondered

What are some cool effects of refraction?


Refraction in a glass block


If an incident ray enters

glass at an angle, then it

is refracted, and bends

towards the normal.

A material which light passes through,

such as glass or air, is known as a medium.

Refraction – labelling diagrams

When the light leaves the

glass, the opposite

happens: it bends away

from the normal.

normal

incident

ray

refracted

ray

air glass

i1

r1

r2

normal

The angle of incidence (i) is greater than the angle of refraction (r).


Revision tip

Remember the word: TAGAGA

• Towards (normal)

• Air

• Glass

• Away (from normal)

• Glass

• Air


In the water

Light moves more slowly through

denser media.

Fast and slow

If you were running along a

beach and then ran into the

water when would you be

moving slower, in the

water or on the beach?

In a similar way as light moves from one medium to another of different density the speed of light changes.

Do you think light moves faster or slower as the density of the medium it travels through increases?


Understanding why light refracts


Why is there no refraction?

• If the ray strikes the boundary between two media at 900, the ray continues without a change of direction

• The angle of incidence is zero, therefore there will be no reflection or refraction


What happens when using a semicircular block? • When a light ray passes from a more dense

medium into a less dense medium, for example from glass into air, something interesting happens

• Go to PhET simulation and chose the “Bending light” investigation.

• Set the top medium to glass and the bottom one to air.

• Turn on the angles

• Move the laser and observe what happens

• This will only work when the light goes from a more dense to a less dense medium


Total internal reflection TIR

• Using a semicircular glass block TIR can easily be demonstrated

• The incident ray will always be at 900 with the surface and therefore not refract

• Can you find the angle, at which no light is refracted?

• How could you describe this angle?


The critical angle • The critical angle is the

angle at which the refracted ray changes from refraction to reflection

• It depends on the material used

• If your incident ray is just a little bit larger than the critical angle, TIR will occur.

• The critical angle occurs at the incident angle when the refracted ray makes a 900 angle with the normal

𝜃1 < 𝜃𝑐 𝜃1 = 𝜃𝑐 𝜃1 > 𝜃𝑐 16/05/2016 ©cgrahamphysics.com 2016

Using a triangular prism

Two effects can be observed:

• TIR

Remember: - draw normal

- Add arrows to show direction

• Dispersion

• White light is composed of all colors

• As white light is refracted at the surface, the different colors refract at different angles

• They travel at slightly different speeds through the prism

• Red light refracts the least and violet the most


What are the properties of reflection, refraction and diffraction?

Task 1: Copy and fill out the table below

Definition Properties Examples

Reflection e.g. the law of reflection states that …

Refraction The change in _________ of ________ when they travel across a b_______

S_______ pool appearing s_______than it is

Diffraction • The n_______ a gap is, the g_________ the diffraction

• The w_______ a gap is, the _________ the diffraction

*if extra speedy: we do not normally see diffraction of light in everyday life. Suggest a reason for this


What are the properties of reflection, refraction and diffraction?

Definition Properties Examples

Reflection The change in direction of a wave, such as a light or sound wave, away from a boundary the wave encounters

• e.g. the law of reflection states that the angle of incidence = the angle of reflection

• occurs when light changes direction as a result of "bouncing off" a surface like a mirror

• Mirrors e.g. mirror sign in an ambulance

Refraction The change in direction of waves when they travel across a boundary

• Occurs in all forms of waves (e.g. light, sound)

• When a light ray travels from air into glass, the angle of refraction is smaller than the angle of incidence

• Swimming pool appearing shallower than it is

• water waves • White light

Diffraction The spreading out of waves when they pass through a gap or round the edge of an obstacle

• The narrower a gap is, the greater the diffraction

• The wider a gap is, the greater the diffraction

• Diffraction of light (e.g. telescope)

• Diffraction of ultrasonic waves (ultrasound)

• TV/radio reception 16/05/2016 ©cgrahamphysics.com 2016

Refraction summary


What is diffraction? Diffraction is a property of all

waves, which can be shown by

water waves in a ripple tank.

The waves pass through

the gap and spread out

on the other side.

This is diffraction.

Here, the waves travel as normal

until they reach the gap.

The width of the gap is similar to

the wavelength of the waves.


Gap size


Large gaps and obstacles The amount of diffraction depends on how

the wavelength compares with the size

of the gap.

Maximum diffraction occurs when

wavelength is similar to the gap size.

If the wavelength is much smaller than

the size of the gap, only the edges of the

wave front are diffracted.

What happens when waves pass an

obstacle on only one side?

Only the edge of the wave on that side

is diffracted.


Diffraction summary


Review check 1: A, B or C?

Q1. What is shown in the diagram? A. Reflection B. Refraction C. Diffraction




Review check 1: A, B or C?

Q1. What is shown in the diagram?

A. Reflection B. Refraction C. Diffraction

Q2. What is shown in the diagram? A. Reflection

B. Refraction C. Diffraction

Q3. What is shown in the diagram? A. Reflection B. Refraction

C. Diffraction 16/05/2016 ©cgrahamphysics.com 2016

Key words • Angle of refraction – the angle between the refracted ray and the

normal • Refraction – the bending of light as it enters a different medium • Total internal reflection – when a ray passes from a more dense

into a less dense medium and all of it is reflected instead of refracted

• Critical angle – the incident angle at which the refracted ray makes a right angle with the normal or the incident angle when the refracted angle lies on the surface

• Dispersion – the spreading out of white light into the composite colors

• Diffraction – the spreading out of waves around obstacles or though a gap


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Refraction and diffraction - WordPress.com · 5/16/2016 · Refraction and diffraction Book page 109 ... • Place a coin at the bottom ... •If your incident ray is just a little