KS4 Waves : Refraction

1. KS4 Waves : Refraction

2. Define refraction Draw ray diagrams showing refraction of light by a glass or Perspex block List and explain everyday examples of refraction By the end of this lesson you should be able to:

3. Animation showing refraction at the air/glass boundary

4. Apparatus: Power pack Ray box Slit Perspex block Protractor Investigating refraction The ray box will get very hot. Be careful when handling it!

5. You will lose marks in an examination when you are drawing a light ray if you do not: 1. Use a ruler. 2. Add an arrow to show direction. Exam tip

6. Draw around the Perspex block on a piece of paper. What to do……

7. Mark on a line known as the NORMAL perpendicular to the surface of the block. The normal For the light ray entering the block and the light ray leaving the block mark each ray with two crosses. Repeat several times with the light ray entering the block at different angles. Draw in the incident ray, emergent ray, remove the block and then join up the two rays.

8. As the light ray moved from air into perspex? As the light ray moved from perspex into air? If the angle of incidence = 0°? What do you notice about the incident ray and the emergent ray? What happened……

9. Results Does the angle of incidence…… If the light ray entered the block parallel to the normal then it travels through undeviated. …affect the angle of refraction? If the incident ray enters the block at an angle to the normal then the direction of the ray changes as it enters and leaves the block, the light ray has been refracted.

10. Measure the angles of incidence and refraction and fill in the table below

11. 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 Air toPerspexangle of incidence > angle of refractioni > r

12. 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 Perspex toAirangle of incidence < angle of refractioni < r

13. When the angle of incidence is 0 the light ray is not deviated from its path. Un-deviated light ray Angle of incidence = 0°

14. Animation to show what happens to a ray of light passing through a rectangular block of glass.

15. Remember the word: TAGAGA Towards (normal) Air Glass Away (from normal) Glass Air Revision tip

16. 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? Fast and slow In the water Light moves more slowly through denser media.

17. The speed of light Light travels at 300 000 km/s in a vacuum, as it enters denser media the speed of light decreases. Perspex must be denser because light travels slower through Perspex than water. Looking at the chart, which do you think is denser, Perspex or water?

18. Refraction : effects of refraction Many visual effects are caused by refraction. This ruler appears bent because the light from one end of the ruler has been diffracted, but light from the other end has travelled in a straight line. Would the ruler appear more or less bent if the water was replaced with glass? More bent, because glass is more dense than water.

19. Refraction : magic coins Place a coin in the bottom of a bowl and clamp an empty cardboard tube so that it points above the coin. Gradually add water to the bowl and watch the coin through the tube float up - can you explain this?

20. Refraction : apparent depth The rays of light from the coin get bent [refracted] as they leave the water. Your eye assumes they have travelled in straight lines. Your brain forms an image at the place where it thinks the rays have come from - the coin appears to be higher than it really is.

21. Animals (including humans) allow for refraction when hunting fish in water. Animals and human hunters The animals do not aim at the fish (it is just the refracted image), instead they aim at a location where they know from experience the fish actually is. image actual location

22. Refractive index We can study refraction of light by comparing its speed in air to that in a medium. A number called the refractive index is the ratio of these two speeds: Refractive index = speed of light in air speed of light in substance Example: The speed of light in air is 300 000 000 m/s, the speed of light in water is 225 000 000 m/s. What is the refractive index of water? 1.33

23. Calculating refractive index 1.0 1.33 2.5 1.5

24. Using refraction : lenses summary  There are two main types of lens: Convex Concave  Convex lenses work by bending [refracting] rays of light to a principal focus.  The distance from the centre of the lens to the principal focus [F] is called the focal length [ƒ].  The image formed by a convex lens is inverted [back-to-front and upside-down].  The thicker the lens, the shorter the focal length[ƒ].

25. Using Refraction : lenses  The lens refracts all the rays to a point called the principal focus [F].  The distance between the centre of the lens and F is called the focal length [].  Imagine parallel rays of light from a distant object hitting the lens.  Draw normal lines where the rays enter the air [at 90º to the surface].  Use the first refraction rule to work out the ray direction.  Work out the direction of the refracted rays using the second refraction rule. F  Draw normal lines [at 90° to the surface] for each ray.  A lens can be thought of as a series of prisms. When light enters a more dense medium [e.g. glass], it bends towards the normal. When light enters a less dense medium [e.g. air], it bends away from the normal. ƒ

26. What do you think happens when… Parallel light rays strike a convex lens? They pass through the focal point of the lens. Form a parallel beam if they pass though the focal point (F). Diverging light rays? F

27. Using Refraction : lenses - finding  Hold the lens in the other hand and move it closer to the screen until a clear image appears. Hold a plain white screen in one hand. Chose a distant object [to get parallel rays of light]. Use a ruler to measure the distance between the lens and the screen - this is the focal length [ƒ]. ƒ

28. Refraction : lenses 1. Find the focal length [ƒ] of your lens. 2F F F 2F 2. Fix the lens to the centre of a metre rule and mark the distances F and 2F either side of the lens. 3. Place the candle >2F away from the lens and move the screen until an image appears and record observations. 4. Repeat for the candle at 2F, between 2F and F, at F and between F and the lens.

29. Results

30. Refraction : lenses 2F F F 2F Object >2F away O I The image [ l ] is formed between F and 2F away from the lens, is inverted and diminished.

31. Refraction : lenses 2F F F 2F Object at 2F O I The image [ l ] is formed at 2F away from the lens, is inverted and the same size.

32. Refraction : lenses 2F F F 2F Object between 2Fand F away O I The image [ l ] is formed further than 2F away from the lens, is inverted and magnified.

33. Refraction : lenses 2F F F 2F Object at F away O The image [ l ] is formed at infinity - the rays never meet [we use this set-up for searchlights].

34. Refraction : lenses 2F F F 2F I Object between F and lens O The VIRTUAL image [ l ] is formed on the same side of the lens as the object, is the right way up and magnified.

35. Results between F and 2F real diminished inverted at 2F real same size inverted real magnified inverted >2F atinfinity same side as object virtual magnified erect

36. Refraction : lenses 2F F F 2F Magnification = Distance from lens to image Distance from object to lens

37. Air Water Glass Lead Which of the following is the most dense? 

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

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

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

41. 2.6 0.5 2.0 1.5 If light travelling through a medium has a speed of 150 000 000 m/s. What is the refractive index of the medium? 

42. Explain what refraction is? Describe what happens to a light ray if it enters a medium of different density at an angle? Describe what happens to a light ray if it enters a medium of different density along the normal? Describe examples of refraction? Draw ray diagrams depicting the refraction of light? Calculate the refractive index for a medium? Can you……