Understanding Waves, Reflection and Refraction

1. Understanding Waves, Reflection and Refraction Physics Revision

2. In this lesson you will revisit: • The basic principles of waves. • The wave formula. • Reflection. • Refraction and Total Internal Reflection.

3. You need to be able to: • Describe the parts of a wave. • Know the difference between longitudinal and transverse waves. • Use the wave equation. • Explain the processes of reflection, refraction and total internal reflection. • Know how sound waves behave and the uses of ultrasound. • Describe how seismic waves behave and what they tell us about Earth’s structure.

4. Waves : Introduction  = wavelength Frequency is the number of waves that pass a point each second. a = amplitude Crest / Peak  a  Displacement+ Time a  Trough

5. Transverse Waves • Most waves are transverse. • Transverse waves include light and all other electromagnetic waves, ripples on water, waves on strings and a rope being flicked up and down. • The vibrations are at 90º to the direction of travel of the wave.

6. Longitudinal Waves • The only ones that you need to know are sound waves and earthquake p-waves. • The vibrations are along the same direction as the wave is travelling.

7. The Wave Formula V =  x  v   f  x

8. Reflection of Light • It allows us to see objects. • From a rough surface, like paper, light reflects in all directions – DIFFUSE REFLECTION. • When light reflects from an even surface it’s all reflected at the same angle – CLEAR REFLECTION. • Angle of incidence = Angle of reflection

9. Refraction – a property of all waves. • This is when waves change direction as they enter a different substance. • It is caused by the change in speed of the waves. • In water waves slow down in shallower regions. • Sound waves can also be refracted.

10. Air toPerspexangle of incidence > angle of refractioni > r • As the light ray moved from air into perspex it moved towards the normal. • If light rays move from a less dense medium (air) to a more dense medium (perspex) they ‘bend’ towards the normal. i > r

11. Perspex toAirangle of incidence < angle of refractioni < r • As the light ray moved from perspex into air it moved away from the normal. • If light rays move from a more dense medium (perspex) to a less dense medium (air) they ‘bend’ away from the normal. i < r

12. This angle is called the critical angle [ c] Total Internal Reflection  i <  c Refraction  i =  c Critical case  i >  c Total Internal Reflection [TIR] Different materials have different critical angles. Diamond has the lowest at 24º which is why it reflects so much light.

13. Uses of Total Internal Reflection • Binoculars. • Periscopes • Reflectors • Optical Fibres • You will need to know the position of the prisms in binoculars and periscopes.

14. Which wave below is a longitudinal wave? • Ultraviolet • Water • Light • Sound Which wave below is a longitudinal wave?

15. What are the units of frequency? • Metres • Hertz metres • Hertz/metres • Hertz

16. A wave has a wavelength of 6m and a frequency of 5Hz. What is the speed of the wave? • 1.2 m/s • 0.88 m/s • 3.0 m/s • 30 m/s

17. Which of the following is not a light source? • The Sun • A star • Traffic lights • A book

18. Which of the following is not seen by reflected light? • Your hand • Jupiter • Light bulb • The Moon

19. Which of the following is the most dense? • Air • Water • Glass • Lead

20. When light changes direction as it moves from one medium to another we call this effect what? • Reflection • Refraction • Diffraction • Total internal reflection

21. What happens to the speed of light as it moves from air into glass? • Decreases • Increases • No effect • Decreases and increases

22. If a ray of light moves from air to glass parallel to the normal what happens? • No change in direction • It bends away from the normal • It bends towards the normal • It stops