6.4 Bending Light February 17, 2011 What must light be doing to make these images?
Explain that light bending at the surface of glass or water can deceive us. [Level 4]
Observe and describe what happens when objects are viewed through water. [Level 5]
Draw a ray diagram to describe what happens when a ray of light is refracted on entering and leaving a glass block. [Level 6]
normal normal i r
The beam changes direction as it goes into the block and then back again as it exits
The angle of incidence and angle of refraction are important to notice
When it goes into a denser material the ray bends towards the normal and vice versa
Light does bend!
Although light travels in straight lines, it can change direction when it moves from one material to another. Scientists who make refracting telescopes need to have very accurate information about the way that light changes direction as it moves from air into glass and glass into air.
Equipment: ray box, paper, glass block, protractor 2 Carefully mark the angles on the sheet of paper, making sure that they are lined up with the centre of the flat side of the glass block. 3 Shine the light accurately along the first line so that the light ray hits the centre of the glass block. SAFETY: the ray box will get hot enough to burn you… be careful incident light (light coming in from light source) semi-circular glass block mark refracted light paths in this region paper 80 ° 70 ° 60 ° 50 ° 40 ° 30 ° 20 ° 10 ° 0 °
Mark the line of light that comes out of the curved side of the glass block accurately, using a ruler, as shown in the diagram below. 5 Repeat steps 3 and 4 for each of the lines. 6 When you have completed the experiment, remove the glass block and draw a line from the centre of the flat side of the block where the light ray came in to each of the light ray paths that you drew coming out of the curved side. 7 Measure the angle between the straight through line and each light ray. incident light ray semi-circular glass block refracted light ray angle measured here normal
Presenting your results Now draw a graph of your results incident angle / o refracted angle / o Refracted angle ( ) 80 70 60 50 40 30 20 10 0 Incident angle ( )
Considering, Evaluation and Extension
1 What do you notice about the angle of the refracted light when the incident angle is O?
2 From your graph, describe the relationship between the incident angle and the refracted angle.
3 Is there an easy mathematical link that you can see between the two sets of angles?
4 Glass is used to make lenses for people suffering from short and long sight. Use either the text book or some other source of information to explain how your results could help an optician make the right lenses for a customer?
5 Try working out the difference between the two sets of angles. Although you may be able to predict what the next angle might be, is there an easy mathematical link that you can see between the two sets of angles?
‘If you move the straw, does it ever look bent? When?’, ‘
Does the straw ever look straight? When?’,
‘Does the straw ever look broken into two parts? When?’,
‘What differences can you see between the sections of straw above and below the water?’
6.4 Bending light Questions
Look at the table showing angles of refraction in different materials on page 93. Which of these experiments produces more refraction?
a. Ray travels from air into water with an angle of incidence of 15°.
b. Ray travels from air into glass with an angle of incidence of 15°.
Look at the table showing angles of refraction in different materials on page 93. A ray of light falls onto a transparent material. The angle of incidence is 60°. The angle of refraction is 21°. What is the material?
1 of 3 (Level 4) (Level 4)
6.4 Bending light Questions
Look at the table showing angles of refraction in different materials on page 93. Does glass always produce more refraction than water for rays travelling from air?
Jeff has a mysterious transparent cube. He shines a ray of light onto it at an angle of 45°. He measures an angle of refraction of 25°. What might the cube be made from? Use the table on page 93 to help you decide.
People sometimes jump into deep water thinking they will be able to stand on the bottom with their head above water. Use your understanding of the refraction of light to explain why they think this.
2 of 3 (Level 5) (Level 6) (Level 7)
6.4 Bending light Questions
Sophie thinks that the huge diamond she has been given by her fiancé is actually made of glass. She finds a cube of glass and shines a ray of light into it at an angle of incidence of 0°. She then repeats the test with the suspect diamond.
a. How will this help her to discover the truth?
b. How can she improve her test?
Dr Smith has a 1cm thick block of Perspex and a 5cm thick block of Perspex. She shines a ray of light into each one, from air, with an angle of incidence of 75° and then looks at the rays that emerge. She’s hoping to demonstrate refraction to her class. Which block produces a more noticeable result? Explain your answer.
3 of 3 (Level 8) (Level 8)
6.4 Bending light For your notes
When rays of light move from one transparent material to another, the path of the light changes; in other words, the light is refracted.
The effect of refraction can cause optical illusions – for example it makes a swimming pool floor look closer than it is.
When light travels into a denser transparent material, it is refracted towards the normal; when it travels into a less dense transparent material, it is refracted away from the normal.