The Electromagnetic Spectrum 3

Year 09 Topic 1: The Electromagnetic Spectrum.
12 Learning Opportunities From Monday the 30th of March until Friday the 1st of May

Learning Opportunity 3: Refraction
This Learning opportunity has 6 tasks to do.
Target Time: 60 minutes.

Task 1: Use an introduction and a video to complete some sentences.
Read the text below, watch the video, and then copy the sentences and fill in the gaps.

Refraction introduction
When light travels through a medium that is more dense than air, e.g. water, it will slow down. Imagine trying to run through custard! It would be harder to do so! When the light slows down it ‘bends’ as it changes direction slightly. We say it ‘bends towards the normal’. When the light leaves the more dense medium it speeds back up again, ‘bending’ away from the normal.

http://www.bbc.co.uk/learningzone/clips/what-is-refraction/7912.html

Now copy the sentences and fill in the gaps
Remember to check your work at the end of the blog.

When waves enter a different medium the wave speed changes.
This is called ________ and happens at the ________ between two media.
A line at _______ _______ to the interface is called the ________ line.
Light travelling along the ________ does not change __________ when it goes into a different ________.

Key words: medium. refraction. interface. normal. 90 degree. direction

Task 2: Draw an accurate ray diagram.
Copy the blank diagram.
Then Read the instructions.
Then watch the video and create your own ray diagram to show what happens to light as it is refracted through a Perspex block.
Check your answers at the end of the blog.
Diagram:

Instructions:
1) Label the incident ray and refracted ray.
2) Colour code the angles to show which are the same and which are different.
3) Label the relative speed of light at each point.
4) Ensure you have shown the normal lines.
5) Write a safety report for this practical: what are the hazards and how can you ensure you remain safe?

Video:

Task 3: Write a method.
Write a method using the equipment given here to determine the relationship between the incident and refracted rays of light as they travel through a Perspex block. (6 marks)
Perspex block   Ray box   Protractor   Paper   Pencil
Check your answers at the end of the blog.

How to write a scientific method: In an exam if you are asked to ‘explain how’ then you should expect to write a method, a set of instructions of what to do.
1) Write down what pieces of equipment are needed for your investigation.
2) Name your independent, dependent and control variables.
3) Describe the setup of the experiment, how you will use the equipment.
4) Describe how you will take your measurements, for example if you are measuring the pH:  ”using the pH probe record the pH to one decimal place”.
5) Finally, ensure to include any repeats (and average of your results) that you expect to take.

Task 4: Plotting a graph and writing a conclusion.
Use the data below to plot a graph. Then write a conclusion by answering all the questions underneath.

Here is a set of data to show the incident ray of light as it enters the perspex block and the angle of the refracted ray inside the Perspex block.

Plot a graph of the data. Plot ‘i’ on the x axis ‘r’ on the y axis.
For help with graphs, click here: http://www.shodor.org/interactivate/activities/SimplePlot/

Write a conclusion to explain what this data shows by answering these questions.
Check your answers at the end of the blog.
1) When the light enters the block, does it refract (bend) towards or away from the normal?
2) When the light leaves the glass block, does it bend towards or away from the normal?
3) Is there a relationship between the angle of incidence & the angle of refraction?
4) Why do you think the light refracts when it enters & leaves the glass block?

Task 5: Check your answers, and then try the Key Questions.

Answers.
Task 1 refraction, interface, 90 degrees, normal, normal, direction, medium.
Task 2 The speed inside the block is slower.

refraction


Task 3
1) Get a Perspex block,   Ray box,   Protractor,   Paper,   Pencil, Ruler.
2) We will change the angle of incidence so this is the independent variable.
We will measure the angle of refraction.
We will keep the same material, same light box set to the same power, and have the same person measure the angles. We will turn off the lights to make the experiment easy to see.
3) Place the block on your paper and shine the beam of light into it at an angle.
Then draw two dots on the beam coming into the block and two dots on the beam coming out.
Using a ruler connect the dots and add an arrow to show the direction of the light.
Finally, draw the line joining the light beams inside the block using a ruler.
4) On the top surface of the block, draw the normal line at 90-degrees. Make sure the line goes inside the block. Measure the angle of incidence and angle of refraction using the normal line.
5) Finally, repeat the whole experiment at a different angle.
Task 4


1) When the light enters the block, it refracts (bends) towards the normal.
2) When the light leaves the glass block, it bends away from the normal.
3) As the angle of incidence gets bigger, the angle of refraction gets bigger.
The rate of increase gets less for bigger angles.
4) Light slows down when it enters the block.

Now answer the Key questions
1) What speed does light travel through a vacuum?
2) What speed does sounds travel through a vacuum?
3) Describe what happens to light when it is refracted.
4) How are normal lines drawn?
5) How does the speed of light change when entering or exiting a more dense material?
6) CHALLENGE: Compare the angles of incidence and the angles of refraction on a ray diagram, which are the same and which are different?
7) SUPER CHALLENGE: Explain how rainbows are formed.

Task 6: Now complete the following exam questions and mark your work.

Exam questions

(iii) here is a graph of the student’s results. 

Use the graph to calculate a value for  (angle Y ÷ angle X)………………………………… (2)
(iv)  The student concludes that angle Y is directly proportional to angle X.
Explain what the student must do to test this conclusion in more detail.