3 Refraction in water waves When waves in water travel through water of different depths they change speed. In shallow water the waves slow down; in deeper water they speed up.We can investigate this by changing the depth of the water in a ripple tank.As the water waves slow down, their direction changes due to the change of speed. This is called refraction.Perspex sheet used to change depth of water
4 Why does refraction happen? Imagine a car driving from the road into a muddy field.In the muddy field it slows down as there is more friction.mudroadIf it enters the field at an angle then the front tyres hit the mud at different times.tyre 2tyre 1Tyre 1 hits the mud first and will move more slowly than tyre 2. This causes the car to turn towards the normal.When the car leaves the mud for the road, tyre 1 hits the road before tyre 2 and this causes the car to turn away from the normal.
5 Why does refraction happen? If the car approached the muddy field at an angle of incidence of 0° then both front tyres would hit the mud at the same time.The tyres would have the same speed relative to each other so the direction of the car would not change, it would just slow down.
6 This causes the light to bend or refract. RefractionThe speed of light waves depends on the material they are travelling through.air = fastestglass = slowerdiamond = slowestIf light waves enter a different material (e.g. travelling from glass into air) the speed changes.This causes the light to bend or refract.airglass
8 What happens in refraction: air to glass When light is refracted as it travels from air to glass:angle of incidence > angle of refraction i > ri > rAs the light ray travels from air into glass it moves towards the normal.airglassIn general, when light rays move from a less dense medium (air) to a more dense medium (glass) they ‘bend’ towards the normal.
9 Refraction through a glass box What happens when a light ray passes from glass into air?
10 What happens in refraction: glass to air When light is refracted as it travels from air to glass:angle of incidence < angle of refraction i < rAs the light ray travels from glass into air it moves away from the normal.airglassIn general, when light rays travel from a more dense medium (glass) to a less dense medium (air) they ‘bend’ away from the normal.i < rIf the two surfaces of the block are parallel, then the ray at the start is parallel to the ray at the end.
11 Refraction – angle of incidence What happens to light travelling from air through a glass block when the angle of incidence is 0°? i = 0°When the angle of incidence is 0 the light ray is not deviated from its path.airglassundeviated light ray
13 Travelling through different materials 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 sand?In the water.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 in a more dense medium?Light moves slower through a more dense medium.
14 The speed of light Light travels at 300,000 km/s in a vacuum. As light enters denser media, the speed of light decreases.From this bar chart, which material do you think is denser, glass or water?Glass must be denser than water because light travels more slowly through glass than water.
15 Refractive index = speed of light in air The speed of lightWe can study refraction of light by comparing its speed in air to that in a different material.A number called the refractive index is the ratio of these two speeds:Refractive index = speed of light in airspeed of air in materialExample:The speed of light in air is 300,000,000 m/s, and the speed of light in water is 225,000,000 m/s. What is the refractive index of water?1.33
16 Calculating refractive index The speed of light in air is 300,000,000 m/s.The speed of light in crystal is 150,000,000 m/s. What is the refractive index of crystal?Refractive index = speed of light in airspeed of light in crystalRefractive index = 300,000,000150,000,000Refractive index of crystal = 2.0
17 Refractive index = sin i Snell’s lawi rRefractive index = sin isin rairglassExample:When a ray passes into a glass block, i = 45° and r = 28°. What is the refractive index of the glass?Refractive index = sin 45sin 28Refractive index = 1.5
19 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 refracted, 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?
20 Real and apparent depth The rays of light from a stone get bent (refracted) as they leave the water.Your brain assumes these rays of light have travelled in straight lines.imageYour brain forms an image at the place where it thinks the rays have come from – the stone appears to be higher than it really is.actual location
21 The Archer fishThe Archer fish is a predator that shoots jets of water at insects near the surface of the water, e.g. on a leaf.The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey.image of preyprey locationThe fish does not aim at the refracted image it sees but at a location where it knows the prey to be.
22 Magic coinsPlace 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?
24 ƒ F Refraction and lenses Draw normal lines where the rays enter the air (at 90º to the surface).Work out how the ray is refracted as it enters the lensDraw normal lines (at 90° to the surface) for each ray.Imagine parallel rays of light from a distant object hitting the lens.Work out how the rays are refracted as the leave the lensThe 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).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
25 Convex lensesConvex lenses work by bending (refracting) rays of light to a principal focus. Convex lenses can be used to project or magnify images.The distance from the centre of the lens to the principal focus (F) is called the focal length (ƒ).The thicker the lens, the shorter the focal length.
26 How do light rays pass through lenses Parallel light rays strike a convex lensThey pass through the focal point of the lens.FForm a parallel beam if they pass though the focal point (F).Diverging light raysF
27 Finding the focal length of a lens 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 Ray diagram for an object >2F away The image (l) is formed between F and 2F away from the lens, and is inverted and diminished.
29 Ray diagram for an object 2F away Object at 2FO2FFIThe image (l) is formed at 2F away from the lens, is inverted and the same size.
30 Ray diagram for an object between 2F and F Object between 2F and F awayO2FFThe image (l) is formed further than 2F away from the lens, is inverted and magnified.I
31 Ray diagram for an object at F Object at F awayO2FFThe image (l) is formed at infinity – the rays never meet. This set up is used for searchlights.
32 Ray diagram for an object close than F Object between F and lensO2FFThe virtual image (l) is formed on the same side of the lens as the object. It is the right way up and magnified.
33 Summary of images with a convex lens Object positionImage positionReal or virtualMagnified or diminishedInverted or erect>2Fat 2Fbetween 2F and Fat F–between F and lensbetween F and 2Frealdiminishedinvertedat 2Frealsame sizeinverted> 2Frealmagnifiedinvertedat infinitysame side as objectvirtualmagnifiederect
34 Magnification = height of image The magnification factor of a lens can be calculated by using this equation:Magnification = height of imageheight of object
36 Remember the word: TAGAGA Towards (normal) Air Glass Revision tipRemember the word:TAGAGATowards (normal)AirGlassAway (from normal)
37 Glossaryconvex lens – A lens that brings light rays to a focus.focal length – The distance from the centre of the lens to the principal focus.magnification – The size of the image relative to the size of the original object.refraction – The bending of light when it enters a different material. This happens because light changes direction and travels at a different speed when it enters different materials.refractive index – The ratio of the speed of light in air to the speed of light in another material.