 # Do Now Reflection Question

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Do Now Reflection Question

Refraction

Today’s lesson Refraction of light
Refraction of light by a prism is called dispersion Please remember to remain in you seats unless given permission Mirror Island Homework due in

The mug trick! Demonstrate the “Mug-trick”. Students put a polystryrene cup in front of them with a coin/mass/ink dot on the opposite side to them. They move their heads until the coin/mass/ink dot is JUST out of sight. Keeping their heads still they then slowly and carefully pour water into the cup from another cup and lo and behold the coin/mass/ink dot appears! Explain that the lesson aims to show why!

Refraction of light Beam of light Beam of light Glass block
Students set up the above and do in their books.

WHY?

Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
A model for refraction Teacher notes This animation is designed to clearly explain the link between wave speed, direction and wavelength. At the end of stage one students could be challenged to predict what will happen to the column. Encourage them to explain why they have predicted this, and to describe the soldiers as a wave, using appropriate terms.

Refraction in a glass block
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Refraction in a glass block Teacher notes This exercise should act as revision from KS3 Science. By activating the angle measurements in stages, students are given a chance to identify the various elements of the experiment. Initially they should try to define the normal, name the rays and angles shown and describe the relative angle size. Revealing the angles at the air to glass boundary will act to clarify these points to students. Students can then be challenged to identify i2 and r2, before revealing them to the class.

Refraction Beam of light Glass block
Light going from air to glass bends towards the normal Glass block Light going from glass to air bends away from the normal

Do Now Can you carefully add this diagram & notes to your book? Refraction Ray of light A light ray entering along the normal continues in a straight line. Glass block

Today’s lesson Refraction of light
Refraction of light by a prism is called dispersion Please remember to remain in you seats unless given permission Mirror Island Homework due in today

Please stick into your book; use a double page and then fill in.

Light waves Light slows down as it goes from air to glass/water
its speed changes from 3 x 108 m/s to 2 x 108 m/s or from km/s to km/s

Water waves Water waves travel slower in shallow water

Wavelength and speed effects
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Wavelength and speed effects Teacher notes Draw the students attention to the way in which a change in direction of the wave coincides with a change in speed and wavelength. Ask students to identify the type of wave produced (transverse) and highlight that this is the same as a light wave. What other similarities can the students note between the behaviour of water waves and light waves?

Refraction – a few notes
The wavelength changes, the speed changes, but the frequency stays the same. when a waves direction is changed it is deviated (deviation) Ripple Tank Simulation

Refraction experiment
Typical results air to glass: angle of incidence / ° angle of refraction / ° deviation / ° 15 10 5 30 19 11 45 28 17 60 35 25 75 40 No deviation occurs when the angle of incidence is zero. Increasing the angle of incidence increases the deviation.

Refraction experiment Air to Perspex
Typical results air to Perspex : angle of incidence / ° angle of refraction / ° deviation / ° 15 30 45 60 75

Complete the paths of the RED light rays:
B C D E F

Let’s try some drawing! Suitable questions from their textbook

Why a pool appears shallow
observer Why a pool appears shallow normals object at the bottom of a pool AIR WATER image

Refraction by a prism – copy please
A prism splits the colours of white light into the spectrum. This is called dispersion. Violet light deviates the most it has the shortest wavelength, red the deviates the least it has the longest wavelength. prism white light spectrum

Choose appropriate words to fill in the gaps below:
Refraction occurs when a wave changes ______ as it crosses the boundary between two regions. The _________ of the wave also usually changes. Light rays deviate ________ the normal when they pass from less dense air to more dense _________. The greater the angle of incidence the greater is the _________. Different ______ of light deviate by different amounts. Violet deviates the _____. A prism can be used to split the colours of white light into a spectrum. This is called _________. speed direction towards perspex deviation colours most dispersion WORD SELECTION: towards deviation most dispersion direction perspex speed colours

Refraction When a wave changes speed (normally when entering another medium) it may refract (change direction) Ripple Tank Simulation

Do Now and finish reflection and refraction sheet
Choose appropriate words to fill in the gaps below: Refraction occurs when a wave changes ______ as it crosses the boundary between two regions. The _________ of the wave also usually changes. Light rays deviate ________ the normal when they pass from less dense air to more dense _________. The greater the angle of incidence the greater is the _________. Different ______ of light deviate by different amounts. Violet deviates the _____. A prism can be used to split the colours of white light into a spectrum. This is called _________. speed direction towards perspex deviation colours most dispersion WORD SELECTION: towards deviation most dispersion direction perspex speed colours Solar bottle Do Now and finish reflection and refraction sheet

Today’s lesson Refraction of light Refraction index Snell’s Law

Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Refractive index Refractive index is a measure of how much a substance slows down light. The higher its value, the more a medium slows light. The more the light is slowed, the more it bends towards the normal. Refractive index is calculated by comparing speed of light in a vacuum to that in a given medium: refractive index = speed of light in vacuum speed of light in medium Teacher notes There are no units for refractive index. The speed of light in a vacuum is 300,000,000 m/s, and the speed of light in water is 225,000,000 m/s. What is the refractive index of water? refractive index = 300,000, ,000,000 = 1.33

Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Snell’s Law The refractive index can also be calculated using Snell’s Law, which uses the angle of incidence (i) and angle of refraction (r) to establish how much a medium slows light. refractive index (n) = sin i sin r normal Use the information in the diagram to find the refractive index of glass. Teacher notes Students should note that the equation can be easily rearranged to find both the angles of incidence and refraction if required. Students could measure the angle of incidence and refraction in a range of different materials and calculate the refractive index of these themselves. refractive index = sin 45° sin 28° refractive index = 1.5

Using Snell’s Law Use Snell’s Law to answer the following:
If a ray of light enters water at an angle of 15° and has an angle of refraction of 11.2°, find its refractive index. refractive index = sin 15 sin 11.2 refractive index = 1.33

Let’s try some questions!
Suitable questions from their textbook

Using Snell’s Law Use Snell’s Law to answer the following:
Perspex has a refractive index of 1.5. If a ray of light passing into a perspex block has an angle of refraction of 24°, find the angle of incidence. sin i = sin r × refractive index sin i = (sin 24°) × 1.5 = 0.61 i = sin i = 37.6°

Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Using Snell’s Law Use Snell’s Law to answer the following: Diamond has a refractive index of 2.4. If light passes into a diamond crystal at an angle of 15°, find the angle of refraction. sin r = sin i refractive index sin r = sin 15° = 0.1 2.4 r = sin-1 0.1 r = 6.2°