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PHYSICS 3. Monday, 01 September 2014. Refraction. Lesson objectives Understand what is meant by refraction. Be able to describe the refraction of light rays at a rectangular block. Refraction : effects of refraction. Many visual effects are caused by refraction.
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Monday, 01 September 2014 Refraction Lesson objectives • Understand what is meant by refraction. • Be able to describe the refraction of light rays at a rectangular block.
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.
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
Key Terms Incident Ray Refracted Ray Emergent Ray
Along The Normal If a ray of light crosses the boundary between two media, along the normal, it will not be deviated from its path but the speed of the light will still change.
Less Dense To More Dense If a ray of light moves from a less dense medium, such as air, to a more dense medium, such as glass, the ray of light will slow down and be refracted towards the normal. The angle of incidence is greater than the angle of refraction.
More Dense To Less Dense If a ray of light moves from a more dense medium, such as glass, to a less dense medium, such as air, the ray of light will speed up and be refracted away from the normal. The angle of incidence is less than the angle of refraction.
Total Internal Reflection Q1 Q2 .. internal .. .. infrared … reflected .. .. core … dense .. .. pulses .. thousands . Q3 a) True b) False B A C D E
Total Internal Reflection (cont.) Q4 -------- --------- --------- Q5 The critical angle is the angle above which only total internal reflection of light occurs and there is no refraction.
1. When light changes direction as it moves from one medium to another we call this effect what? • Reflection • Refraction • Diffraction • Total internal reflection
2. What happens to the speed of light as it moves from air into glass? • Decreases • Increases • No effect • Decreases and increases
3. 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
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.
Investigating Light 1. Place a block of Perspex on a piece of paper and draw around the block. 2. Shine a ray of light at the Perspex block and mark it on the paper. 3. Mark on the path of the light ray through the glass block. As the ray of light enters the Perspex block it CHANGES DIRECTION. When the ray of light leaves the Perspex block it CHANGES DIRECTION AGAIN. This effect is called REFRACTION. 4. Mark on the path of the light ray as it leaves the block. 5. What do you observe?
Refraction When a light ray is incident upon a Perspex block; several things happen. As the light ray enters the Perspex THE LIGHT RAY SLOWS DOWN. Incident ray Normal The light ray also CHANGES DIRECTION. This is called REFRACTION. The light ray BENDS TOWARDS THE NORMAL inside the block. Refracted ray When the light ray leaves the Perspex it SPEEDS UP AGAIN and is REFRACTED AWAY FROM THE NORMAL. Emergent ray
Density and Refraction DENSITY is the property of a material which tells us HOW MANY PARTICLES are in a CERTAIN VOLUME. Example: LIQUID WATER is MORE DENSE than GASEOUS WATER. When a light ray moves from a LESS DENSE MEDIUM to a MORE DENSE MEDIUM it SLOWS DOWN and REFRACTS TOWARDS THE NORMAL. When a light ray moves from a MORE DENSE MEDIUM to a LESS DENSE MEDIUM it SPEEDS UP and REFRACTS AWAY FROM THE NORMAL. (Less dense) Air Perspex/Glass/Water (More dense) (Less dense) Air
Refraction Refraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. When a pen is placed in water it looks like this: In this case the light rays are slowed down by the water and are _____, causing the pen to look odd. The two mediums in this example are ______ and _______. Words – speed up, water, air, bent, medium
Using Refraction : Lenses Imagine parallel rays of light from a distant object hitting the lens. The distance between the centre of the lens and F is called the focal length []. The lens refracts all the rays to a point called the principal focus [F]. A lens can be thought of as a series of prisms. Work out the direction of the refracted rays using the second refraction rule. 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. F Draw normal lines [at 90° to the surface] for each ray. When light enters a less dense medium [e.g. air], it bends away from the normal. When light enters a more dense medium [e.g. glass], it bends towards the normal. ƒ
Complete the Diagrams Below for Light Entering a Perspex Block A. B.
Experiment • Follow the instruction sheet which shows how to position your rectangular block. • Measure any angles carefully with your protractor. Always measure from the nearest normal. • Once you have your results, complete the analysis.
Wave slows down and bends towards the normal due to entering a more dense medium Wave speeds up and bends away from the normal due to entering a less dense medium Refraction through a glass block: Wave slows down but is not bent as it enters along the normal
Along The Normal If a ray of light crosses the boundary between two media, along the normal, it will not be deviated from its path but the speed of the light will still change.
